REVIEW

other, as, for example, the telescope, c, moved to the position c', then clearly the bridge will be thrown out of balance, and the galvanometer will be deflected. It will also be clear that the extent of deflection of the galvanometer will depend upon the length of arc included between the two positions of the telescope, c, c', and will be greater as that are increases; so that, with a battery of constant electromotive force, it becomes possible to determine the extent of movement of the telescope, c, by simply observing the indication of the galva

nometer.

It will, of course, be obvious that, as the angle between the positions, c and c', of the telescope increases, the length of the line, A, T, will constantly decrease, while the deflection of the galvanometer will constantly increase; so that the galvanometer indicates ranges starting from infinity when the galvanometer shows no deflection, small ranges being indicated by large deflections of the galvanometer and rice versa. As a matter of convenience, Lieut. Fiske employs for this purpose a galvanometer so constructed that the deflections of the index will be proportional to the differences of potential at the terminals.

It will be clear that by the method just described, the operation of finding the range is reduced to a very easy and rapid process, and, at the same time, greatly simplified as regards apparatus.

Observers stationed at the two telescopes, c and D, align them with the distant object, when a third observer instantly reads the range from the galvanometer, which is provided with a scale suitably marked in linear units, such as yards.

If, however, the angle A, B, T, is not a right angle, then the factor, sin A, B, T, must be taken into consideration in solving the formula, A T = X sin A B T.

A B sin AT B

Or, in other words, the observer at the galvanometer may simply multiply the range indication by the sin A, B, T, numerically expressed, in order to reduce the indicated range to the true range. The angle, A, B, T, is observed directly on the arc, F.

In the foregoing demonstration, it is assumed that the resistance in the circuit remains constant, that is, remains the same as it is when the two telescopes are parallel to each other, and stands in the positions, C, D, touching the middle parts of their arcs. But, as a matter of fact, this resistance does not remain the same when the telescopes move to positions nearer the extremities of the arcs. To illustrate, if the resistance of the circuit is a certain amount with the telescopes in the position, C, D, it will be less when the telescopes are turned in the position, c', D'. Now, the variation of resistance due to this change of position will affect the total resistance in circuit to an extent depending upon its ratio to the resistance of the whole circuit. And, consequently, if that ratio, be made very small, as it easily may be by simply introducing a high resistance in the battery loop at i between the points, A and B, then, inasmuch as the variation in resistance due to change in position of the telescopes may thus be rendered inappreciable, the total resistance of the circuit may be taken as constant ; so that, despite the fact that the angle, A, B, T differs from a right angle, the deflection of the galvanometer, as before, will remain practically constant for any given angle, A, T, B.

It is evident that if the high resistance before mentioned be not put in the battery loop, then the decrease in resistance due to change in position of the telescopes from the middle point of their arcs towards the extremities of the arcs may bear a considerable ratio to the resistance of the whole circuit. And as this decrease in resistance will be attended by corresponding increase in current strength, it follows that proportionately greater deflections of the galvanometer will follow for any given angle, A, T, B; so that, consequently, the ranges indicated by the galvanometer will be less than those which would have been shown had the high resistance been put in the battery loop. Again, if the resistance of the battery loop between A and B is extremely small with relation to the rest of the circuit, the decreasing resistance of the whole circuit, due to change in position of the telescopes, may become very large; and this result may be intensified if the members, a, b, c, d, connecting the arcs, are connected to those ares at points less than 90 degrees from the middle points of those arcs. If, for instance, these wires are connected to the arcs at points 81 degrees removed from the middle

points, and if the resistance in the battery loop were one-tenth of the arc of 81 deg., then when both telescopes were moved to positions 60 deg. from the middle points, the resistance of the whole circuit would then be only about half of what it was when the telescopes were at the middle parts or the positions, C, D. Consequently, for any given relative angular displacement of the telescopes occurring 60 deg. away from the middle points of the arcs, the corresponding deflection of the galvanometer would be about twice as great as if the same relative angular displacement occurred when the telescopes were near the middle points of the arc; so that the range indicated in the latter case would be about half as great as in the former.

But it will be observed that if the telescope, D, for instance, were 60 deg. removed from the central position, the angle, A, B, T, would be 30 deg., or 150 deg., and then its sine would be one-half; so that the range indication for any given angle, A, T, B, would be only one-half of what it would be with the same angle, A, T, B, when the telescope at D' is in its middle position. In other words, the fact of the decreased resistance caused in the circuit as the telescopes move away from the middle position tends to automatically make the very correction for the sine of A, B, T which ought to be introduced, because the telescope no longer stands at 90 deg. to the base-line; and this is found to be the actual occurrence in practice.

In the actual construction of the instrument advantage is taken of this principle, and the resistance of the circuit is made such that the galvanometer shows the correct range, no matter what be the direction of the target.

For getting distances ahead or astern, a range-finder on the bridge is used, while for getting distances on either side, a fore and aft pair is employed.

In what has been said above, the resistance of the galvanometer has been neglected, and it has been assumed that the E.M.F. and internal resistance of the battery, and the resistance of the various contacts remain constant While this is not theoretically true, Lieut. Fiske finds that by using a small storage cell, and by making the contacts carefully, no appreciable error is introduced. The galvanometer is usually secured in the conning tower, where the captain can consult it, but it may be in any suitable place.

Fig. 2 shows the range-finder as actually used on shipboard. Telephones are so secured to the telescopes that the act of putting the eye to the telescope brings the mouth against the transmitter and the ear against the receiver, so that both observers are continuously in communication with each other.

The telephones have been added in the recent instruments because practical work at target practice at sea showed the value of instantaneous communication between the observers. The observers, now, can instantly change their lines of sight from some point of the target which may be enveloped in smoke to another part that may be clear; they can also take the distance of a number of objects in rapid succession. Furthermore, the observers constantly keep each other advised as to whether or not their telescopes are pointing at the target, and thus guard against the danger of reading the galvanometer at a time when the indication might be incorrect by reason of either telescope being temporarily thrown off the target by a lurch of the ship. This is found a great convenience when the sea is heavy, and it makes the indications of the galvanometer as trustworthy when the ship is rolling in a heavy sea, as when she is at anchor in smooth water.

The instruments are made of aluminium, bronze, and iron, and are left exposed on deck without any protection whatever, except that a cover is placed over the telescope when not in use. They require no care except an occasional cleaning, and are not affected by the weather; in fact, salt water is frequently thrown on them when they are exhibited, and no effect whatever is produced. The reason is that the resistance of the whole circuit is only 1 ohm, and the resistance of even salt water is so enormous in comparison that no leak is occasioned.

Fig. 3 shows the range-finder provided with a shield, and fig. 4 the instrument secured for sea.

As regards the practical application of the system, we may observe that it has been in use in the United States Navy for more than a year, having been fitted on board the Chicago and the Baltimore, two first-class armoured cruisers. Two

14

pairs are also in use in the Russian Navy. It has been used during gunnery practice, and it is found that the vibration due to firing has no effect upon the instruments. Careful experiments at sea, moreover, show that the errors of the instrument are insignificant, and the indications absolutely in

stantaneous.

The apparatus can be seen at Messrs. Elliott Brothers, 101, St. Martin's Lane, W.C.

MR. BRUSH'S WINDMILL DYNAMO.

It is difficult to estimate the effect of an invention on existing practices and industries. Occasionally a new invention will appear which will greatly affect a whole range of allied inventions and industries in such a way as to entirely change time-honoured customs, inaugurate new practices, and establish new arts. The commercial development of electricity is a notable example of this.

After Mr. Brush successfully accomplished practical electric illumination by means of are lights, incandescent lighting was quickly brought forward and rapidly perfected. Gas lighting was also improved in various ways. Simultaneously with these, the electric distribution of power was carried forward, and important improvements were made in prime movers for driving dynamos. In this direction much has been done both in steam and water motors. Wind power has been repeatedly suggested for driving dynamos, but the adaptation of the windmill to this use seems to have been a problem fraught with difficulties. Few have dared to grapple with it, for the question not only involved the motive power itself and the dynamo, but also the means of transmitting the power of the wheel to the dynamo, and apparatus for regulating, storing, and utilising the current.

A mill, says the Scientific American, as well as all of the electrical apparatus used in connection with it, and the very complete system by which the results are secured, have been designed and carried out according to the plans of Mr. Charles F. Brush, of Cleveland, Ohio, and under his own personal supervision. As an example of thorough-going engineering work it cannot be excelled.

Every contingency is provided for, and the apparatus, from the huge wheel down to the current regulator, is entirely automatic.

The reader must not suppose that electric lighting by means of power supplied in this way is cheap, because the wind costs nothing. On the contrary, the cost of the plant is so great as to more than offset the cheapness of the motive power. However, there is a great satisfaction in making use of one of nature's most unruly motive agents.

Passing along Euclid Avenue, in the beautiful city of Cleveland, one will notice the magnificent residence of Mr. Brush, behind which, and some distance down the park, may be seen, mounted high on a tower, the immense wheel which drives the electric plant to which we have referred. The tower is rectangular in form, and about 60 feet high. It is mounted on a wrought iron gudgeon 14 inches in diameter, and which extends 8 feet into the solid masonry below the ground level. The gudgeon projects 12 feet above the ground, and enters boxes in the iron frame of the tower, the weight of the tower, which is 80,000 lbs., being borne by a step resting on the top of the gudgeon. The step is secured to a heavy spider fastened to the lower part of the frame of the tower.

In the upper part of the tower is journalled the main wheel shaft. This shaft is 20 feet long and 6 inches in diameter. It is provided with self-oiling boxes 26 inches long, and carries the main pulley, which has a diameter of 8 feet, and a face of 32 inches. The wheel, which is 56 feet in diameter, is secured to the shaft, and is provided with 144 blades, which are twisted like those of screw propellers. The sail surface of the wheel is about 1,800 square feet, the length of the tail which turns the wheel toward the wind is 60 feet, and its width is 20 feet. The mill is made automatic by an auxiliary vane extending from one side, and serving to turn the wheel edgewise to the wind during a heavy gale. The tail may be folded against the tower parallel with the wheel, so as to present the edge of the wheel to the wind when the machinery is not in use. The countershaft arranged below

[JANUARY 2, 1891.

the wheel shaft is 3 inches in diameter, it carries a pulley 16 inches in diameter, with a face of 32 inches, which receives the main belt from the 8-foot pulley on the wheel shaft. This is a double belt 32 inches wide. The countershaft is provided with two driving pulleys each 6 feet in diameter, with a face of 6 inches, and the dynamo is furnished on opposite ends of the armature shaft with pulleys which receive belts from the drive wheels on the countershaft.

The dynamo, which is one of Mr. Brush's own design, is mounted on a vertically sliding support and partially counterbalanced by a weighted lever. The countershaft is suspended from the main shaft by the main belt, and the dynamo is partly suspended from the countershaft by the driving belts. In this way the proper tension of the belts is always secured, the total load on the dynamo belts being 1,200 lbs., and on the main belt 4,200 lbs. The ends of the countershaft are journalled in sliding boxes connected by equalising levers which cause both ends of the shaft to move alike. pulleys are so proportioned that the dynamo makes 50 revolutions to one of the wheel. The speed of the dynamo at full load is 500 revolutions per minute, and its normal capacity at full load is 12,000 watts.

The

The automatic switching devices are arranged so that the dynamo goes into effective action at 330 revolutions a minute, and an automatic regulator is provided which does not permit the electromotive force to run above 90 volts at any speed. The working circuit is arranged to automatically close at 75 volts and open at 70 volts. The brushes on the dynamo are rocked automatically as the load changes. The field of the dynamo is slightly compounded. The current passes from the dynamo to contact shoes of polished and hardened steel carried by a cross bar on the tower, which shoes slide on annular plates surrounding the gudgeon. Conductors extend underground from these plates to the dwelling house. To guard against extraordinary wind pressure, the tower is provided at each of its corners with an arm projecting downwardly and outwardly, and carrying a caster wheel very near but not in contact with the circular rail concentric with the gudgeon. Ordinarily, these caster wheels do not touch the rail, but when the wind is very high, they come into contact with the rail and relieve the gudgeon from further strain.

In the basement of Mr. Brush's house there are 408 secondary battery cells arranged in 12 batteries of 34 cells each; these 12 batteries are charged and discharged in parallel; each cell has a capacity of 100 ampère hours. The jars which contain the elements of the battery are of glass, and each cell has its liquid covered with a layer of "mineral seal" oil, a quarter of an inch thick, which entirely prevents evaporation and spraying, and suppresses all odour.

The house is furnished with 350 incandescent lights, varying from 10 to 50 candle power each. The lamps most commonly used are from 16 to 20 candle power. About 100 incandescent lights are in every day use. In addition to these lights there are two arc lights and three electric motors. It is found, after continued use of this electric plant, that the amount of attention required to keep it in working condition is practically nothing. It has been in constant operation more than two years, and has proved in every respect a complete success.

THE ELECTRO-MAGNET.*

BY PROF. SILVANUS P. THOMPSON, D.Sc., B.A., M.I.E.E

(Concluded from page 786, vol. xxvii.)

HERE upon the lecture table is a Duboscq's arc lamp. In this pattern (fig. 71), one lever, B, which is curved, plays against another, A, which is straight. A similar mechanism is used for equalising the action in the Serrin arc lamp, where one of the springs that holds up the jointed parallellogram frame is applied at the end of a rocking lever to equalise the pull of the regulating electro-magnet. In this lamp there is also introduced the principle of oblique approach; for the armature of the electro-magnet is not allowed to travel straight towards the poles of the magnet, but is pulled up obliquely past it.

* Cantor Lecture. Delivered before the Society of Arts, February, 3rd, 1890.

Another device for equalising the pull was used by Wheatstone in his step-by-step telegraph in 1840. A hole is pierced in the armature, and the end of the core is formed into a projecting cone, which passes through the aperture of the armature, thereby securing a more equable force and a longer range. The same device has reappeared in recent years in the form of electro-magnet used in the Thomson-Houston arc lamp, and in the automatic regulator of the same firm.

POLARISED MECHANISM: USES OF PERMANENT MAGNETS. We must now turn our attention to one class of electro-magnetic mechanism which ought to be carefully distinguished from the rest. It is that class in which, in addition to the ordinary electro-magnet, a permanent magnet is employed. Such an arrangement is generally referred to as a polarised mechanism. The objects for which the permanent magnet is introduced into the mechanism appear to be in different cases quite different. I am not sure whether this is clearly

recognised, or whether a clear distinction has even been drawn between three entirely separate purposes in the use of a permanent magnet in combination with an electro-magnet. The first purpose is to secure unidirectionality of motion; the second is to increase the rapidity of action and of sensitiveness to small currents; the third to augment the mechanical action of the current.

(a) Unidirectionality of Motion.-In an ordinary electro-magnet it does not matter which way the current circulates; no matter whether the pole is north or south, the armature is pulled, and on reversing the current the armature is also pulled. There is a rather curious old experiment which Sturgeon and Henry showed, that if you have an electro-magnet with a big weight hanging on it, and you suddenly reverse the current, you reverse the magnetism, but it still holds the weight up; it does not drop. It has not time to drop before the magnet is charged up again with magnetic lines the other way on. Whichever way the magnetism traverses the ordinary soft iron electro-magnet, the armature is pulled. But if the armature is itself a permanent magnet of steel, it will be pulled when the poles are of one sort, and pushed when the poles are reversed-that is to say, by employing a polarised armature you can secure unidirectionality of motion in correspondence with the current. One immediate application of this fact for telegraphic purposes is that of duplex telegraphy. You can send two messages at the same time and in the same direction to two different sets of instruments, one set having ordinary electromagnets, with a spring behind the armature of soft iron, which will act simply independently of the direction of the current, depending only on its strength and duration; and another set having electromagnets with polarised armatures, which will be affected not by the strength of the current, but by the direction of it. Accordingly, two completely different sets of message may be sent through that line in the same direction at the same time.

Another mode of constructing a polarised device is to attach the cores of the electro-magnet to a steel magnet, which imparts to them an initial magnetisation. Such initially-magnetised electro-magnets were used by Brett in 1848, and by Hjörth in 1850. A patent for a similar device was applied for in 1870 by Sir William Thomson, and refused by the Patent-office. In 1871 S. A. Varley patented an electro-maguet having a core of steel wires united at their ends.

Wheatstone used a polarised apparatus consisting of an electromagnet acting on a magnetised needle. He patented, in fact, in 1845, the use of a needle permanently magnetised to be attracted one way or the other between the poles of an electro-magnet. Sturgeon had described the very same device in the Annals of Electricity in 1840. Gloesner claims to have invented the substitution of permanent magnets for mere armatures in 1842. In using polarised apparatus it is necessary to work, not with a simple current that is turned off and on, but with reversed currents. Sending a current one way will make the moving part move in one direction; reversing the current makes it go over to the other side. The mechanism of that particular kind of electric bell that is used with magneto-electric calling apparatus furnishes an excellent example of a polarised construction. With these bells no battery is used; but there is a little alternatecurrent dynamo, worked by a crank. The alternate currents cause the pivotted armature in the bell to oscillate to right and left alternately, and so throw the little hammer to and fro between the two bells.

(b.) Rapidity and Sensitiveness of Action.-For relay work polarised relays are aften employed, and have been for many years. Here on the table is one of the Post-office pattern of standard relay, having a steel magnet to give magnetism permanently to a little tongue or armature which moves between the poles of an electro-magnet that does the work of receiving the signals. In this particular case the tongue of polarised relay works between two stops, and the range of motion is made very small in order that the apparatus may respond to very small currents. At first sight it is not very apparent why

putting a permanent magnet into a thing should make it any more sensitive. Why should permanent magnetism secure rapidity of working? Without knowing anything more, inventors will tell you that the presence of a permanent magnet increases the rapidity with which it will work. You might suppose that permanent magnetism is something to be avoided in the cores of your working electromagnets, otherwise the armatures would remain stuck to the poles when once they had been attracted up. Residual magnetism would, indeed, hinder the working unless you have so arranged matters that it shall be actually helpful to you. Now for many years it was supposed that permanent magnetism in the electro-magnet was anything but a source of help. It was supposed to be an unmitigated nuisance, to be got rid of by all available means, until, in 1855, Hughes showed us how very advantageous it was to have permanent magnetism in the cores of the electro-magnet. Fig. 51 is the drawing of Hughes's magnet to which I referred in Lecture III. A compound permanent magnet of horse-shoe shape is provided with coils on its pole pieces, and there is a short armature on the top attached to a pivotted lever and a counteracting spring. The function of this arrangement is as follows. That spring is so set as to tend to detach the armature, but the permanent magnet has just enough magnetism to hold the armature on. You can, by screwing up a little screw behind the spring, adjust these two contending forces, so that they are in the nicest possible balance; the armature held on by the magnetism, and the spring just not able to pull it off. If, now, when these two actions are so nearly balanced you send an electric current round the coils, if the electric current goes one way round it just weakens the magnetism enough for the spring to gain the victory, and up goes the armature. This apparatus then acts by letting the armature off when the balance is upset by the electric current; and it is capable of responding to extremely small currents. Of course the armature has to be put on again mechanically, and in Hughes's type-writing telegraph instruments it is put on mechanically between each signal and the next following one. The arrangement constitutes a distinctive piece of electro-magnetic mechanism.

(c.) Augmenting Mechanical Action of Current.-The third purpose of a permanent magnet to secure a greater mechanical action of the varying current is closely bound up with the preceding purpose of securing sensitiveness of action. It is for this purpose that it is used in telephone receivers; it increases the mechanical action of the current, and therefore makes the receiver more sensitive. For a long time this was not at all clear to me, indeed I made experiments to see how far it was due to any variation in the magnetic permeability of iron at different stages of magnetisation, for I found that this had something to do with it, but I was quite sure it was not all. Prof. George Forbes gave me the clue to the true explanation; it lies in the law of traction with which you are now familiar, that the pull between a magnet and its armature is proportional to the square of the number of magnetic lines that come into action. If we take N, the number of magnetic lines that are acting through a given area, then to the square of that the pull will be proportional. If we have a certain number of lines, N, coming permanently to the armature, the pull is proportional to N2. Suppose the magnetism now to be altered-say made a little more; and the increment be called dN; so that the whole number is now N+dN. The pull will now be proportional to the square of that quantity. It is evident that the motion will be proportional to the difference between the former pull and the latter pull. So we will write out the square of N + dN, and the square of N, and take the difference.

Increased pull, proportional to N2 + Nd 2 N + dN2;
Initial pull, proportional to N2

We may neglect the last term, as it is small compared with the other. So we have, finally. that the change of pull is proportional to 2 NdN. The alteration of pull between the initial magnetism and the initial magnetism with the additional magnetism we have given to it, turns out to be proportional, not simply to the change of magnetism, but also to the initial number, N, that goes through it to begin with. The more powerful the pull to begin with, the greater is the change of pull when you produce a small change in the number of magnetic lines. That is why you have this greater sensitiveness of action when using Hughes's electro-magnets, and greater mechanical effect as the result of applying permanent magnetism to the electro-magnets of telephone receivers.

ELECTROMAGNETIC MECHANISM.

There are some other kinds of electro-magnetic mechanism to which I must briefly invite your attention, as forming an important part of this great subject. Of one of these the mention of permanent magnets reminds me.

MOVING COIL IN PERMANENT MAGNETIC FIELD.

A coil traversed by an electric current experiences mechanical forces if it lies in a magnetic field; the force being proportional to the intensity of the field. Of this principle the mechanism of Sir. W. Thomson's siphon recorder is a well-known example. Also those galvonometers which have for their essential part a movable coil suspended between the poles of a permanent magnet, of which the carliest example is that of Robertson (see Encyclopædia Britannica, ed. viii., 1855), and of which Maxwell's suggestion, afterwards realised by d'Arsonval, is the most modern. Siemens has constructed a relay on a similar plan.

MAGNETIC ADHERENCE.

There are a few curious pieces of apparatus devised for increasing adherence electro-magnetically between two things. Here is an old device of Nicklès, who thought he would make a new kind of rolling gear. Whether it was a railway wheel on a line, or whether it was going to be an ordinary wheel gearing, communication of motion was D

16

to be made from one wheel to another, not by cogs or by the mere adherence of ordinary friction, but by magnetic adherence. In fig. 72 there are shown two iron wheels rolling on one another, with a sort

FIG. 72.-NICKLES' MAGNETIC FRICTION GEAR.

FIG. 73.-FORBES'S ELECTRO-MAGNET.

of electro-magnetic jacket around them, consisting of an electric current circulating in a coil, and causing them to attract one another and stick together with magnetic adherence. In Nicklès' little book on the subject there are a great number of devices of this kind described, including a magnetic brake for braking railway waggons, engines, and carriages, appling electro-magnets either to the wheels or else to the line to stop the motion whenever desired. The notion of using an electro-magnetic brake has been revived quite recently in a much better form by Prof. G. Forbes and Mr. Timmis, whose particular form of electro-magnet, shown in fig. 73 is, peculiarly interesting, being a better design than I have ever seen for securing powerful magnetic traction for a given weight of iron and copper. The magnet is a peculiar one; it is represented here as cut away to show the internal construction. There is a sort of horse-shoe made of one grooved rim, the whole circle of coil being laid embedded in the groove. The armature is a ring which is attracted down all round, so that you have an extremely compact magnetic circuit around the copper wire at every point. The magnet part is attached to the frame of the waggon or carriage, and the ring armature is attached to the wheel or to its axis. On switching on the electric current the rim is powerfully pulled, and braked against the polar surface of the electro-magnet.

Forbes's arrangement appears to be certainly the best yet thought of for applying a magnetic brake to the wheels of a railway train. Another, but quite distinct, piece of mechanism depending on electromagnetic adherence is the magnetic clutch employed in Gülcher's arc lamp.

REPULSION MECHANISM.

Then there are a few pieces of mechanism which depend on repulsion. In 1850 a little device was patented by Brown and Williams, consisting, as shown in fig. 74 of an electro-magnet which repelled part of itself. The coil is simply wound on a hollow tube, and inside the coil is a piece, B, of iron, bent as the segment of a cylinder to fit in, going from one end to the other. Another little iron piece, a, also shaped as the segment of a tube, is pivotted in the axis of the coil. When these are magnetised one tends to move away from the other, they being both of the same polarity. Of late there have been many ampère-meters and voltmeters made on this plan of producing repulsion between the parallel cores.

[JANUARY 2, 1891.

De Kabath. Two cores of iron, not quite parallel, pivotted at the bottom, pass up through a tubular coil. When both are magnetised. instead of attracting one another, they open out; they tend to set themselves along the magnetic lines through that tube. The cores being wide open at the bottom tend to open also at the top.

ELECTROMAGNETIC VIBRATORS.

Then there is a large class of mechanisms about which a whole chapter might be written, namely, those in which vibration is maintained electromagnetically. The armature of an electro-magnet is caused to approach and recede alternately with a vibrating motion, the current being automatically cut off and turned on again by a selfacting brake. The electro-magnetic vibrator is one of the cleverest things ever devised. The first vibrating electro-magnetic mechanism ever made was exhibited here in this room in 1824 by its inventor, an Englishman named James Marsh. It consisted of a pendulum vibrating automatically between the poles of a permanent magnet. Later, a number of other vibrating devices were produced by Wagner, Neef, Froment, and others. Most important of all, is the mechanism of the common electric trembling bell, invented by a man whose very name appears to be quite forgotten-John Mirand. How many of the millions of people who use electric bells know the name of the man who invented it? John Mirand, in the year 1850, put the electric bell practically into the same form in which it has been employed from that day to this. The vibrating hammer; the familiar pushbutton; the indicator or annunciator, are all of his devising, and may be seen depicted in the specification of his British patent just as they came from his hand.

Time alone precludes me from dealing minutely with these vibrators, and particularly with the recent work of Mercadier and that of Langdon-Davies, whose researches have put a new aspect on the possibilities of harmonic telegraphy. Langdon-Davies's rate-govenor is the most recent and perfect form of electro-magnetic vibrator.

INDICATOR MOVEMENTS.

Upon the table here are a number of patterns of electric bells, and a number also of the electro-mechanical movements or devices employed in electric bell work, some of which form admirable illustrations of the various principles that I have been laying down. Here is an iron-clad electromagnet; here a tripolar magnet; here a series of pendulum motions of various kinds; here is an example of oblique pull; here is Jensen's indicator, with lateral pull; here is Moseley's indicator, with a coil and plunger, ironclad; here is a clever device in which a disc is drawn up to better the magnetic circuit. Here, again, is Thorpe's semaphore indicator, one of the neatest little pieces of apparatus, with a single central core surrounded by a coil, while a little strip of iron coming round from behind serves to complete the circuit all save a little gap. Over the gap stands that which is to be attracted, a flat disc of iron, which, when it is attracted, unlatches another disc of brass which forthwith falls down. It is an extremely effective, very sensitive, and very inexpensive form of annunciator. The next two are pieces of polarised mechanism having a motion directed to one side or the other, according to the direction of the current. From the back-board projects a small straight electro-magnet. Over it is pivoted a small arched steel magnet, permanently magnetised, to which is attached a small signal lever bearing a red disk. If there is a current flowing one way then the magnet that straddles over the pole of the electro-magnet will be drawn over in one direction. If I now reverse the current, the electro-magnet attracts the other pole of the curved magnet. Hence this mechanism allows of an electrical replacement, without compelling the attendant to walk up to the indicator board. The polarised apparatus for indicators has this advantage, that you can have electrical as distinguished from mechanical replacement.

THE STUDY OF ELECTROMAGNETIC MECHANISM.

The rapid survey of electro-magnetic mechanisms in general has necessarily been very hurried and imperfect. The study of it is just as important to the electric engineer as is the study of mechanical mechanism to the mechanical engineer. Incomplete as is the present treatment of the subject, it may sufficiently indicate to other workers useful lines of progress, and so fitly be appended to these lectures on the electro-magnet. In a very few years we may expect the introduction into all large engineering shops of electro-magnetic tools. On a small scale, for driving dental appliances, electro-magnetic engines have long been used. Large machine tools, electro-magnetically worked, have already begun to make their appearance. Some such were shown at the Crystal Palace, in 1881, by Mr. Latimer Clark; and more recently, Mr. Rowan of Glasgow, has devised a number of more developed forms of electromagnetic tools.

SUPPRESSION OF SPARKING.

It now remains for me to speak briefly of the suppression of sparks. There are some half dozen different ways of trying to get rid of the sparking that occurs in the breaking of an electric circuit whenever there are electro-magnets in that circuit. Many attempts have been made to try and get rid of this evil. For instance, one inventor employs an air blast to blow out the spark just at the moment it occurs. Another causes the spark to occur under a liquid. Another wipes it out with a brush of asbestos cloth that comes immediately behind the wire and rubs out the spark. Another puts on a condenser to try and store up the energy. Another tries to put on a long thin wire or a high resistance of liquid, or something of that kind, to provide an alternate path for the spark, instead of jumping across the air and burning the contacts. There exists some half-score, at any rate, of that kind of device. But there are devices that I have thought it worth while to examine and experiment upon, because they depend merely upon the mode of construction adopted in the building of the electromagnet, and they have each their own qualities. I have here five straight electro-magnets, all wound on bobbins the

JANUARY 2, 1891.]

same size, for which we shall use the same iron core and the same current for all. They are all made, not only with bobbins of the same size, but their coils consist as nearly as possible of the same weight of wire. The first one is wound in the ordinary way; the second one has a sheath of copper wound round the interior of the bobbin before any wire is put on. This was a device I believe, of the late Mr. C. F. Varley, and is also used in the field magnets of Brush dynamos. The function of the copper sheath is to allow induced currents to occur, which will retard the fall of magnetism, and damp down the tendency to spark. The third one is an attempt to carry out that principle still further. This is due to an American of the name of Paine, and has been revived of late years by Dr. Aron of Berlin. After winding each layer of the coil, a sheath of metal foil is interposed so as to kill the induction from layer to layer. The fourth one is the best device hitherto used, namely, that of differential winding, having two coils connected so that the current goes opposite ways. When equal currents flow in both circuits there is no magnetism. If you break the circuit of either of the two wires the core at once becomes magnetised. You get magnetism on breaking, you destroy magnetism on making the circuit; it is just the inverse case to that of the ordinary electro-magnet. There the spark occurs when magnetism disappears, but here since the magnetism disappears when you make the circuit, you do not get any spark at make, because the circuit is already made. You do not get any at break, because at break, there is no magnetism. The fifth and last of these electro-magnets is wound according to a plan devised by Mr. Langdon-Davies, to which I alluded in the middle of this lecture, the bobbin being wound with a number of separate coils in parallel with one another, each layer being a separate wire, the separate ends of all the layers being finally joined up. In this case there are fifteen separate circuits: the time constants of them are different, because, owing to the fact that these coils are of different diameters, the coefficient of self-induction of the outer layers is rather less, and their resistance, because of the larger size, rather greater than those of the inner layers. The result is that instead of the extra currents running out all at the same time, it runs out at different times for these fifteen coils. The total electromotive force of self-induction never rises so high, and it is unable to jump a large air gap, or give the same bright spark as the ordinary electro-magnet would give. We will now experiment with these coils. The differential winding gives absolutely no spark at all, and second in merit comes No. 5, with the multiple wire winding. Third in merit comes the coil with intervening layers of foil. The fourth is that with copper sheath. Last of all, the electro-magnet with ordinary winding.

CONCLUSION.

Now let me conclude by returning to my starting point-the invention of the electro-magnet by William Sturgeon. Naturally you would be glad to see the counterfeit presentment of the features of so remarkable a man, of one so worthy to be remembered amongst distinguished electricians and great inventors. Your disappointment cannot be greater than mine when I tell you that all my efforts to procure a portrait of the deceased inventor have been unavailing. Only this I have been able to learn as the result of numerous inquiries; that an oil painting of him existed a few years ago in the possession of his only daughter, then resident in Manchester, whose address is now, unfortunately, unknown. But if his face must remain unknown to us, we shall none the less proudly concur in honouring the memory of one whose presence once honoured this hall wherein we are met, and whose work has won for him an imperishable name.

Prof. THOMPSON, in replying to the vote of thanks, stated that if any of those present could assist him in discovering what had become of the portrait of William Sturgeon, or in gleaning any particulars with regard to John Mirand, he should be much indebted to them if they would communicate with him.

LEGAL.

Gourand v. Fitzgerald and Others.-We have been requested to publish the following: "Some notices having appeared in your columns in the year 1884, in reference to the affairs of The Consolidated Telephone Construction and Maintenance Company, Limited, in which reference was made to our client, Colonel Gouraud, amongst others, we are instructed to inform you that in consequence of the reports which were referred to, we, in the year 1888, on his behalf commenced proceedings for libel against the parties who were responsible for them.

These proceedings were delayed for some time owing to his serious illness, but the defendants have now given him a satisfactory apology of which we send you a copy herewith, and they have also paid all the costs relating to the action.

"We have their authority for publishing the enclosed apology, which our client will feel obliged if you will insert in your columns, together with this letter.

17

ascertained that, in the hurry of issuing the reports some inaccuracies occurred in them. In the passages referred to we did not intend to convey that you had been guilty of a breach of duty as a director of any of the companies mentioned in them, and we regret if any such incorrect impression should have been produced by our reports. "Your obedient servants,

"C. L. W. FITZGERALD. "A. H. BAKER. "HENRY GREWING."

Dixon & Co. v. South-Western Electric Supply Company. This was an action in the Queen's Bench division of the High Court of Justice, in which the plaintiffs sought to recover damages for an alleged breach of contract to supply electric light to their premises. The precise terms of the agreement was the principal matter in dispute. A draft agreement had been submitted to the plaintiffs, in which Mr. Dixon had made certain alterations, and which he had returned. This, however, had never been signed, and the defendant contended that there was no express agreement between the parties and that the electric energy had only been supplied and received during the respective wills of supplier and consumer, and could be terminated by either without notice. The plaintiffs' case was that they were, under the circumstances, entitled to a reasonable notice; but Mr. Justice Stephen non-suited them, granting, however, a stay of execution pending an appeal,

ELECTRIC LIGHT COMPETITION AT

SOPHIA.

[FROM A CORRESPONDENT.]

THE Bulgarian Direction of Buildings at Sophia some months ago invited a competition for the erection of an electric central station for lighting the palace of the Prince, for the meeting hall of the National Assembly (Sobranje), and the State Printing-office. The advertisement was based on a very elaborate schedule of conditions, prescribing that the installation should be fixed in the printing-office, whence the current was to be conveyed by underground cables to the Palace (a distance of 660 metres), and to the Sobranje (260 metres). For the lamps to be fixed in the various buildings the following conditions were laid down :

I.-For the Palace.

The tenders were to include the entire plant, with accessory apparatus, leads, and reserve battery of accumulators of the capacity of 4,000 candle-hours; further, all the internal appliances in the rooms to be lighted, but not including the buildings, foundations, and all masonry and excavations which the Government Board of Works will execute at its own cost.

In consequence of this advertisement, the following twelve tenders were handed in, with the appended amounts. It is to be noted that the following figures in the various tenders resulted by the supplemental act of the Commission, and, in particular, by the addition of sums for various appliances not tendered.