458

the direct current machine, the copper required is no less than 150 per cent. It is difficult, then, to understand what useful purpose is served by comparisons such as I have alluded to.

5. RELATION OF AIR GAP TO DIAMETER OF ARMATURE AND NUMBER OF POLES IN DIRECT-CURRENT MACHINES.

Having established the proportionality of the volume and diameter, it is easy to find the relation which must exist between the diameter and length of air gap for any particular angle of embrace, in order that sparking may not occur. I need not take up time ringing changes on the several equations, as to obtain the relation now referred to, all that has to be done is to substitute for v in (1) its value in terms of the diameter accordingly as the winding is of the cylinder or drum type, and find the connection between and d. Preserving the same safety factor throughout, it will be found that two-pole dynamos with a mean gap induction of 5,000 C.G.S. units per square centimetre, and pole pieces embracing an angle of 130°, must have-the volume being related to the diameter as described-gap of not less than 036 d if cylinder-wound, or 054 d if drum-wound. As will have been observed, the air gap may diminish as the induction is increased or as the volume is reduced.

But, as is also seen from the equation, the gap required for any particular volume is proportional to the angle of embrace. and if we substitute for two poles a greater number of correspondingly less angular width, working with an increased diameter and volume without a proportionally increased gap is made possible. This is where the advantage of the four, six, or even eight-pole machine comes in. Keeping to two poles, increasing the diameter requires either a proportional increase in the gap, whether the space is required for the conductors and clearance or not, or an increased induction, or a diminished polar angle, or a combination of these. In either case, the magnetising force spent in the gap is increased; and, other things remaining the same, obviously it would be of some advantage to adopt a construction which, while producing no greater tendency to sparking, would admit of the air gap being reduced until its length was just sufficient to accommodate the conductors and allow of the necessary clearance. The work done by an armature of given external dimensions we have seen to be quite independent of the number of poles, and the choice of this number can only be a question, therefore, of structural and working economy.

6. DIMENSIONS OF THE ARMATURE.

It has been observed that the output of an armature is proportional to d Land the induction being the same, the weight of the core for a given number of poles must be proportional to the output, the radial depth increasing directly as the diameter, so that a proportionally increased field may be carried. The number of poles being fixed, the weight of the core for a given output may be taken, therefore, as approximately constant, whatever the ratio of L to d.

As the number of poles is increased, the induction remaining the same, the radial depth of the core is diminished in proportion, and, within the limits of practice, we may make the further assumption that the weight is inversely as the number of poles. The money value of a reduction in the weight of material due to increasing the poles can easily be arrived at.

The power wasted in hysteresis is proportional to the weight of iron magnetised, and to the number of reversals per second. The weight being inversely, and the reversals directly, as the number of poles, the power wasted is for a given output the same; furthermore, as the output is proportional to the speed, we may say that for a given induction the loss in hysteresis is about proportional to the output only, without reference to speed of rotation, weight of core, or number of poles. If it be more important to reduce the loss by hysteresis than to reduce the weight of material, it can, of course, be done. It is a point for the designer to consider.

So much for the core; let us now consider the copper part. Taking Gramme-wound armatures having an interior opening equal to 66, or two-thirds of the core diameter, it is found-the output, speed, and temperature being predetermined-that the ratio of ĺ to d may vary through a considerable range without making any great difference in the weight of copper or efficiency. For instance, the most efficient relation being L it may be varied on the one

d

=

3'

the copper by more than about 10 per cent. Of course the watts wasted in the armature are correspondingly increased, but within the limits of the large variation mentioned, the reduction in the electrical efficiency of the machine is under one-half per cent. In two-pole machines L varies from 5 to 15 d, the normal relation being about L = d. As has been seen, the gap has to be increased in proportion to the diameter, unless a greater number of poles be employed; and the disadvantage of an increased magnetising force would, in machines with only two poles, counterbalance the slight advantage of getting the armature dimensions nearer the best proportion. When the poles are increased, however, the gap may remain fixed, and if the radial depth of the core be correspondingly diminished, the proportions for least copper and highest efficiency are altered: thus in a four-pole d Gramme we can work from L up to L without a greater 3. variation than 10 per cent. in the weight of copper. Observe, this is a question differing altogether from the one which was considered in my former paper. In that case the length and radial depth of the armature over the winding were fixed, the problem then being to find the best relation of copper to iron. Here both radial depth and length of core alter, also the peripheral velocity, though the revotions per minute remain the same. Why the velocity is allowed to ter will be immediately apparent; I only show at the moment that

=

d

=

12

[APRIL 10, 189

the dimensions of the armature may have their relations alterel siderably without making any considerable difference to the we of copper or efficiency.

=

=

To the drum-wound armatures precisely the same reaso applies. Here, speed and temperature being fixed as before, the proportion for two-pole machines is about L = 33 d; but becaus the distance between the bearings which such a relation would ne sitate being inconveniently great, we rarely find L = 2 d exce The latter requires about 5 per cent. more copper than the form while for the usual proportion, L 15 d, 12 per cent. man required than for L 33 d. The variation which can be made w out overstepping the limits of economy has a smaller range in dr than in cylinder machines, owing to the greater relative importa of the end wires, because of their greater length. If the length the core is reduced below L 1.5 d, the copper in machines two poles increases rapidly; but if the number of poles be creased, the length of the end wires being shortened nearly in p portion, the core length may be reduced to a fraction of the diam without sacrifice of copper or efficiency. In a four-pole macia for instance, the same copper is used with L = 5 d as in a two-p one with L 2 d; while for six poles, without increasing 25 d. copper, the relation may be as small as L The eff of adding poles when the diameter is relatively great is not noticeable in Gramme-wound machines, as the end wires are less ti portant.

=

=

FIG. 3.

=

All the above facts go to prove that if there is anything to be gained, as regards the production of the field, by increasing the diameter of the armature and the number of poles, there is nothing considering the armature by itself to be lost by it. It may, in fact be rather an advantage, because the weight of the iron core is re duced. It is true the peripheral velocity is increased; but this does not matter in the least, provided a certain limit is not exceeded. Opinions differ as to what the limit should be, some machines working at 50 feet per second, others at 100, and a few as high as 125. But there is no reason whatever why any properly constructed armature should not run at a periphery speed of 100 feet per second; and provided this velocity is not exceeded, any advantage which may be ob tained by a relatively large armature and increased number of poles should be secured.

7. DIMENSIONS OF THE FIELD MAGNETS.

It will be apparent, from the foregoing considerations, that the employment of two, or more than two poles for direct current machines of moderate dimensions resolves itself mainly into a de

FIG. 4.

liberation regarding the most economical shape to give to the field magnets. As regards the armature, considered by itself, we may say that the choice of dimensions is mostly a matter of convenience, seeing that the amount of copper required and efficiency are for a given output practically unaltered by variations in this respect, while the reduction in the weight of the core due to an increased number of poles is to some extent compensated by the extra expense of larger plates and increased weight of the armature supports. Again, the cost of labour is increased by the larger diameter; but, everything being taken into account, considerations respecting the armature do not influence the design to a very great extent. One thing in favour of increasing the poles, as far as the armature is concerned, must, however, be remembered, and that is the reduction, consequent on a smaller conductor being used, of the losses arising from parasitic currents. We now turn our attention to the magnets.

I have said that for the prevention of sparking it is necessary that, the induction per square centimetre remaining the same, the air gap

PRIL 10, 1891.]

ses proportionally to the diameter, whether the space is necesfor conductors and clearance or not, but that the coefficient by the diameter has to be multiplied to give the length of the necessary to prevent sparking diminishes directly as the pole In comparing the magnetic system of a four or six-pole hine with that of a two-pole one, it is necessary to adopt dimenfor the armature in accordance with the considerations already tioned; hence, if the two-pole armature had a length of core equal ne and a-half times its diameter, in a four-pole one the length d be about half the diameter. The diagrams (figs. 3 and 4) show gross sections of two such machines. The diameter of the fourarmature is 1-4, and its length 5 times that of the two-pole consequently both machines give the same output. The weight he two horseshoe magnets in the four-pole machine comes to er cent. of the weight of the single horseshoe magnet, which pates in this particular case a considerable saving in wrought

In taking the copper weight it is necessary to bear in I that this does not vary simply as the length of the wire if machines are of the same efficiency, but as the square of the th; so in this particular comparison the copper on the two horses would be, roughly, 30 per cent. more than on the single horseassuming the length of the air gaps to be the same. Now, if it possible to reduce the gaps by 12 per cent. or so, the copper ht would be similar in both machines, and we should have credit i certain amount of iron saved in the construction of the fourone, which could be balanced against the increased expenditure labour. If the gap can be reduced by more than 12 per cent.ys retaining the depth of the winding the same-there is a saving opper as well as iron, and it is simply the comparison between

459

case of a two-pole armature of 60 centimetres diameter. Assuming the induction in the gap to be 5,000 C.G.S. lines per square centimetre, and the pole angle 130°, our calculations show that the length of air gap would require to be 3.24 centimetres. This is some 40 per cent. in excess of the requirements of conductors and clearance; so, if the gap is reduced until it just allows of the requisite clearance, the induction must be increased from 5,000 to 7,000. Keeping the output of the machine the same, however, we can reduce the diameter of the armature to 54 centimetres, and work with an induction in the gap of 6,500. Observe, the magnetising force spent in the gap has now been reduced by about 7 per cent., but the total field through the armature has been increased by 15 per cent., and the induction in the armature per square centimetre by more than 25 per cent. If we retain the same section of iron in the fields, it may be assumed then that about the same total magnetising force is required, whether the machine has an armature 60 centimetres diameter, with a gap induction of 5,000 C.G.S. units, or one 54 centimetres diameter, with a gap induction of 6,500. The latter might turn out to be impossible, owing to the greater heating of the armature core; any way, what I am attempting to show now is, that by pressing up the induction with a view of reducing the gap, little, if any, advantage is obtained. But by increasing the number of poles, and reducing the gap in that way, we effect, without increasing the gap induction, a marked economy. As it is unnecessary to go into a mass of figures to prove what each can readily prove for himself from the data already before you, I will just give here the results. If we substitute for the two-pole armature of 60 or 54 centimetres diameter and 90 centimetres long a four-pole one of 84 centimetres diameter and 45 centimetres long, with a gap induction of 5,000, we require for the magnetising

FOR

-ie value of the copper and other materials used and the cost of the bour in the two cases which determines, at least in machines of oderate size, whether two or more poles should be adopted. In getting out the best relation of L to d in the different types of matures, it is assumed, of course, that, the volume being proporonal to the diameter, the depth of the winding remains unaltered, that is the condition which gives uniform rise in temperature. cordingly, for a given output, the layer of copper on the armature ill be of the same depth, whether the machine has two poles and an mature having a length of one and a half times its diameter, or our poles and an armature having a length of only half the diameter. But whether it is possible to reduce the gap to an extent which, with a increased number of poles, will lead to a less costly construction, a question which, for machines of moderate dimensions, must reeive careful consideration in each individual case. The answer lepends upon how much larger the gap has to be to prevent sparking the case of two poles only, than is requisite to accommodate the onductors and allow the necessary clearance. If the difference is onsiderable, it may pay better to add poles, and reduce the gaps that ay, than to do the same thing by diminishing the pole angle or inor both. The question, it appears, is answered

reasing the induction,

à different ways by different people. It is somewhat interesting to lote, for instance, that one engineer, distinguished for the past six fears as an ardent advocate of multipolar machines, has, after relucing from six poles to four, lately arrived at two, while another las jumped straight away from two to six without a halt at the inter

nediate number.

But when we come to machines of a certain size there is undoubtedly gain in employing more than two poles, while very large machines become impossible of construction with two poles only. Consider the

coils while working at the same sparking limit, and with the same efficiency, rather under 70 per cent. of the copper on the two-pole field. When to the saving in copper there is added the saving in iron, there will be, after the extra labour is debited against the four-pole machine, a considerable balance in its favour. But to go beyond four poles in this case would be a mistake. Increasing the poles always results in an actual increase of copper, unless at the same time the power spent in the gap is reduced, and this latter must be effected without reducing the thickness of the armature winding. The economy shown above is simply due to the fact that, in a two-pole machine of the dimensions specified, the gap necessary for the prevention of sparking must be much larger than the conductors require unless pressed up to a high induction. If with four poles the gap is still larger than necessary, we go to six poles, and so on. But when we arrive at a point where increasing the number admits of no reduction in the gap, we go no farther. may be mentioned, however, that even in sizes where a four-pole construction showed no actual economy in first cost, it might still be preferable to the two-pole on account of its symmetrical field and the absence of magnetic pull.

It

Having discussed the design of multipolar machines as influenced by both theory and practice, it remains to conclude this communication by calling attention to some of the ordinary forms of multipolar fields. The double wrought iron horseshoe (fig. 4) is not very frequently used, being rather costly; but a similar machine with magnets of cast iron, lately made by my firm for the National Line ss. America, is shown in fig. 5. The armature core in this case was formed by winding square annealed iron wires on a gun metal flanged cylinder. The winding was of the Gramme type, and the machine was coupled direct to an inverted engine, as shown in the fig. This

460

design, introduced by Gramme in 1869, generally gives place to the arrangement shown in fig. 6, a form designed by the same inventor in 1885, and since adopted for cylinder-wound armatures by many makers, including Mr. Jaspar, in Belgium; Mr. Brown, of the Erlikon Works, in Switzerland; and Messrs. Paterson and Cooper, in England. Lately Mr. Kapp has used the same form for six and eight-pole machines with drum armatures. The magnets and octa

[APRIL 10, 1891

magnetising coils; but in figs. 8 and 9 are shown examples of do magnetic circuits, in which the lines from each pole take two pa through separate coils. Fig. 8 is a type of magnet used by Saut Lemmonier, of Paris, for Gramme-wound armatures, and by Cuer Sautter, of Geneva, for armatures having a Siemens winding as m fied by Thury. The magnetising coils are wound upon the part the system constituting in fig. 7 the yokes, and a greater amoun

FIG. 6.

FIG. 8.

gonal yoke in fig. 6 are of cast iron, in two pieces, the lower limbs, the bottom half of the yoke ring, and bed plate being one casting, and the top limbs and upper part of the yoke being another. Fig. 7 represents a similar field in which the magnet cores are of wrought iron, fitted with cast iron pole pieces, as used by Mr. Kapp in the machines above mentioned. The decision as to whether cast iron or wrought iron should be used is arrived at in a very simple manner

copper is in consequence required. It looks at first sight as if weight of copper were not very different in the two types, but this respect appearances are deceptive, for, as a matter of fact, field of a four-pole machine made to fig. 8 would require about per cent. more copper than if made according to fig. 7. It will observed, however, that the magnet cores and pole pieces, which made throughout of the softest wrought iron, are very light.

by comparing the excess of copper required on one hand with the extra machining required on the other.

It will be observed that in the designs figs. 6 and 7 the yokes are considerably longer than those shown in fig. 4; and the weight of the former, if made of the same material, would be, roughly, twice that of the latter, though even then the complete magnet system would be but 75 per cent. of the weight of fig. 3. The yokes being of cast iron, however, the system really comes about 20 per cent. heavier than fig. 3; the less expensive character of the material compensating, of course, for the increased weight.

All these are examples of single magnetic circuits, where the lines f force from each pole remain undivided in their paths through the

fig. 9 the magnets are a series of wrought iron bars lying paralle the armature, each fitted with a cast pole piece in the middle of length, and having two magnetising coils, one on each side of piece. It is a structure which may be frequently met with, th not precisely in the form shown, and the observations regarding copper made with reference to fig, 8 apply equally here.

The cost of a four-pole machine is approximately represented the cost of a couple of two-pole machines of the same effic which give each half the output at twice the speed; the cost of pole by that of three two-pole machines which give one-third d output at three times the speed and so on For comparis fields must be in both the multipolar machires and two-pole

of the same character-that is, with single or double magnetic circuits.

The fields of alternators may be similarly divided into those having single and those having double magnetic circuits; the former requireing, as in the fields of direct current machines, much less copper than the latter. Amongst those having single magnetic circuits are the machines of Siemens, Ferranti, Mordey, Westinghouse, Elwell-Parker, and Paterson and Cooper. Mr. Kapp possesses the distinction of having the only alternator with double magnetic circuits. The "Phoenix" alternator, shown in fig. 10, possesses some features which may be of interest, as illustrating the way in which the commercial aspect of designing has to be considered. The yoke-ring is of cast iron, but the magnets are of tooled wrought iron, shaped as shown. If the magnets had a breadth equal all the way up to the length of

461

in a degree, sacrificed. The boiler and engine, with the necessary fuel, occupy space, perhaps not in the aggregate of greater cubic measurement than that wanted for accumulators and motor of corresponding power, but space that requires to be of a certain shape and to be in a certain position, so that while the accumulators can be stored out of sight under the seats, in recesses that would be untenanted and useless in a steam launch, the boiler, engine, and fuel occupy a part of the boat that would be available for passengers if it were not for their presence. A steam launch, therefore, to carry an equal number of passengers, must be at least 25 per cent. larger than an electric one. This means a saving of some moment in first cost, and a perpetual saving in working expenses, for there is some 25 per cent. less of useless dead weight to propel wherever the vessel goes. Then the presence of a steam engine in a confined space is not cal

the armature core, they might as well have been of cast iron, for the little advantage consequent on the reduction of copper obtained by narrowing them would not have paid for the extra work in tooling wrought iron. But when we reduce the breadth where the magnetising coils are, as shown, we at once diminish the copper on the fields by 60 per cent., greatly reduce the leakage area, and get a good balance after paying for extra tooling. The machine, as will be seen, has 12 radial magnets, and there are on the armature six flat coils, each equal to three times the width of the magnet cores, and laid on the periphery with a space equal to the core between them.

The length of the paper prohibits me from dealing with many special types of machine, to which, however, all the reasoning here used may be applied without difficulty.

Royal Scottish Society of Arts.

ELECTRICAL NAVIGATION. By A. R. BENNETT, member of the Institution of Electrical Engineers.*

ONE of the chief features of the Edinburgh International Exhibition of 1890 has been the first practical demonstration of electrical navigation in Scotland, a demonstration which is due to the enterprise of the General Electric Power and Traction Company in placing a flotilla of electric launches on the Union Canal.

On the Union Canal electrically propelled boats have, I believe, for the first time in history, plied for hire at twopenny fares, just like so many steamers or omnibuses, and so have been open to everybody. It is gratifying to be able to state that they have met with a very generous measure of support. The terminus at Edinburgh, situated in a side street of not too savoury a character, has militated much against the traffic, while the almost continuous wet weather has naturally deterred people from travelling in open boats; but notwithstanding all adverse circumstances, the launches have constituted one of the most attractive features of the exhibition, and have carried, from May 31st until October 11th, no less than 71,075 paying passengers, besides season ticket holders, officials, and others entitled to travel free. The busiest day was the Edinburgh autumn holiday, when 2,560 passengers availed themselves of the novel mode of

transit.

The first experiment in Scotland in electrical navigation may therefore be set down as a pronounced success, and it is to be hoped that the ball set rolling will not be allowed to rest, and that next year will see electric launches on several of our rivers, lochs, and firths.

Before referring in detail to the flotilla on the canal, I may be permitted, perhaps, to touch briefly on the advantages of electric launches in general.

In designing any boat required to operate safely, economically, and conveniently, several points have to be kept in view. This is espe cially true of passenger boats, because the cargo in that case has not only to be stowed safely and compactly, but comfortably. When steam is the motive power, these two last considerations have to be,

Abstract of paper read at the special meeting held in the Edinburgh International Exhibition, October 13th, 1890.

culated to promote the pleasure or comfort of the passengers. There is a certain amount of smell, of smoke, of dirt, and of noise inseparable from the best steam engine, while the vibration due to the unequal action on the crank shaft-to which motion is imparted by a succession of jerks-is much greater than with an electric boat, the shaft of which receives a smooth, regular, rotary impulse from the motor. Then steam-faithful servant as it is-is not quite devoid of danger, especially when under amateur management. An electric launch might be mismanaged to the extent of rendering it temporarily useless for the purpose of loc motion, but under no circumstances could any injury derived from the machinery or batteries happen to the passengers. I have deemed it desirable to mention this, because it is within my knowledge that persons have avoided the electric launches in Edinburgh, some from fear of receiving shocks, and others out of unnecessary consideration for their watches, which they expected would get magnetised. I need scarcely tell this society that, as the boats are arranged, both fears are entirely illusory.

In other respects the electric boats possess advantages. For instance, one man can steer and work the motor, a feat difficult to perform satisfactorily when furnaces have to be fired and gauges watched. There is no boiler to insure or to be inspected periodically. In fact, while a steam launch takes her power on board unmade, in the form of coal, and manufactures it as she goes along, an electric launch receives her's prepared and ready for use, available at any moment and in any desired quantity by merely turning a tap.

Electric launches are suitable for other purposes than pleasure or carrying passengers. The company has supplied one to the Spanish navy, and when one considers how useful such a vessel must be in connection with a ship of war, it appears strange that the British Admiralty should allow the Spaniard to show them the way in such a matter. Our ironclads are nearly all equipped with the electric light. Their dynamos during the day could charge the accumulators of launches suspended from the davits, so that each ship could let down into the water at any moment several launches with their power ready stored for a six or eight hours' run. The accumulator in the launches, when not needed for propulsion, could supplement, or, in case of need, assist, the lighting of the ship at night; or they could work windlasses, or pumps, or ventilators.

All the launches on the Thames are built of wood; those on the Union Canal are of steel. They are four in number, and are named Theo, Flo, Hilda, and May, after the daughters of Lord Bury, the chairman of the General Electric Power and Traction Company. The hulls were designed by Messrs. Morton and Williamson, Glasgow, and built by Seath and Co., Rutherglen. They measure 40 feet long over all, 6 feet beam, 3 feet 1 inch from gunwale to keel, and draw 2 feet 1 inch of water when empty. Equipped with motor and accumulators, they weigh 3 tons out of water. They are licensed by the Board of Trade to carry 40 passengers, and seat that number easily.

They carry 50 accumulator cells of the E.P.S. boat type, manufactured by the Electrical Construction Corporation, of a capacity af 120 ampère-hours. They require a charging current of from 30 to 40 ampères, and discharge up to 40. The cells have 15 plates cach (7 positive and 8 negative), measuring 63 inches deep (8 inches top, 7 inches bottom), 69 inches deep (8 inches top, 7 inches bottom), and contained in ebonite jars, which, like the plates, are tapered off

on one side, so as to adapt themselves readily to the shape of the boat. The weight of a complete cell is 58 lbs. The cells are ranged along the sides of the launch in two rows of 25, protected by boxes, the lids of which, when covered with cushions, form the passengers' seats. The cells are kept well apart, and stand upon glass insulators, filled with resin oil. This is an altogether unnecessary precaution, however, with ebonite boxes, although a very desirable one with wood. No arrangement of glass insulators could save a current capable of leaking over 13 inches of ebonite. The boxes containing the cells are well ventilated, so that the gas given off from the cells while charging cannot become stored up. Except when charging there are no fumes from the accumulators; then the only emanations are oxygen and hydrogen, which are innocuous.

The accumulator plates are of the grid pattern-that is to say, they are composed of leaden frames, which carry the active pastes in a multitude of small square holes. The exact composition of the pastes used is known only to the makers, but in all probability the positive paste is mostly minium (Pb. O,) and the negative litharge (Pb 0). The plates are said to be formed, after the pastes have been applied to the grids, by giving them a charge for 60 hours in a solution of sulphuric acid, of specific gravity 118. Thereafter the plates are dipped in warm water, scrubbed, and dried.

The solution used with the cells is originally of 118 specific gravity; this is raised to 12 by charging, and decreased to 1.172, when as much work as may prudently be taken out of them has been accomplished. The internal resistance, of course, varies with the specific gravity, but averages 003 of an ohm per cell. After charging, the E.M.F. of the 50 cells is about 107 volts, or 2:14 volts per cell; at the close of a day's work it is 100, or 99 volts, equal to 2, or 1.98 volts per cell.

The motors are of a modified Immisch type, weighing 350 lbs. The Immisch motor has long been noted for its high efficiency and great power, as compared with weight and space occupied, qualities which render it specially serviceable for boat work. The ordinary Immisch motor is also noteworthy for an ingenious arrangement of commutators and collectors, the effect of which is to short-circuit two of the 48 armature coils as they successively reach the point at which they contribute nothing to the result. The effect is to somewhat reduce the resistance in circuit, and to maintain the constancy in direction of the field under the influence of varying loads. But in launch work it has been found better to aim at the greatest possible simplicity, so the motor employed in the Union Canal boats has but one commutator and one collector. In all other respects it resembles the motor which has been placed on the table for your inspection. There are four field magnet coils, having a resistance of 18 ohm, when hot. They are in series with the armature, which is drum wound, has 48 coils, and a resistance of 3 ohm when hot. The commutator has as many segments as there are coils, each coil being coupled to the two segments immediately facing it.

The efficiency of the motor, if run at its most favourable speed, is 85 per cent.; but that speed is too high for the conditions which have to be complied with on the Union Canal, where 4 miles an hour, equal to 510 revolutions, is the limit prescribed by the proprietors, the North British Railway Company, with the object of avoiding injury to the banks. At 4 miles the efficiency is only 75 per cent., which becomes 80 per cent. at 6 miles.

The motors are not, consequently, working to advantage on the canal; they would do much better in open water, like a loch or firth, where speed restrictions were not so stringent. The results, all round, would be better in open water, for the resistance to motion in the narrow and shallow canal is very great.

The direction of motion is changed by reversing the current in the motor armature, which can be done almost instantaneously, by means of the switch.

The current from the accumulators passes through a short length, 14 inch, of leaden wire, No. 10 B.W.G., which fuses at 42 ampères. Injury to the motor from excess of current is thus automatically prevented.

The greatest effort is required when starting the boat from a position of rest, and it happens, fortunately for electric traction generally, that it is in that position that the motor is capable of exerting its maximum force, for when motionless it is not acting as a generator and producing a current that tends to diminish the energy of the working one sent into it from the accumulators. For the first instant there is nothing but the ordinary conductor resistance of the coils to overcome, and a rush of current could occur which would at least fuse the leaden wire were not the precaution taken to automatically interpose a series of resistances when turning the starting lever. The commencement of the movement admits current to the motor through a resistance of about 2 ohms; this is diminished gradually as the lever passes the second and third contacts, and it is not until it reaches the fourth, by which time the armature is in motion and producing a counter current, that the full current is admitted to the motor. It is impossible, therefore, for an ignorant or careless man to injure his machine. There is no sitting on the safety valve possible in an electric launch.

The controlling switch has three levers. The second sets the speed at 6 or 4 miles an hour as required, by joining the 50 accumulators all in series, or in two parallels of 25. In the first case the voltage is 107, and in the second 53, when the batteries are fresh, the resulting current being 34 and 24 ampères respectively.

The third lever is the reversing one, and simply changes the direction of the current in the armature.

Neither the second nor third levers can be shifted whilst the current is flowing. By a mechanical arrangement lever No. 1 locks the other two when it is in the "on" position. It must consequently always be moved to "off" before any variation in the current can be effected, so that the armature is protected from any instantaneous changes.

Sparking at, and consequent burning of, the actual contacts is pre

[APRIL 10, 1891.

vented by extra contacts of iron, which take the spark both at make and break.

The propeller shaft is bolted directly to the armature spindle, that gearing is altogether dispensed with, and loss from friction and annoyance from noise saved, while the space occupied is reduced to a minimum. The shaft passes through an ordinary water-tight gland at the stern, the back and forward end thrust being taken up at the connection with the armature by ball bearings.

Propellers of several patterns have been tried, with the view of finding the most economical form for the special work to be performed on the canal. These experiments have resulted in the selec tion of a two-bladed propeller, made of phosphor-bronze, having a diameter of 19 inches and a pitch of 14 inches. This maintains the speed of 44 miles an hour allowed by the canal authorities, with the least expenditure of current. A three-bladed propeller was tried, bat created too much wash.

As the boats are engaged during the day, the accumulators have to be charged by night. This is effected by means of an Immisch shutwound dynamo giving, at 750 revolutions, 130 volts and 120 amperes which is equal to 15,000 watts or 209 H.P. For charging, the cell are put in series in each boat, and the four boats in parallel. The average output of the dynamo while charging is, however, only 11,528 watts or 154 H.P. Each boat takes, therefore, at the rate of 38 H.P. After a full day's work it requires six hours' charging to bring the specific gravity up from 1.172 to 1.2.

The question of efficiency has engaged my attention, but since this paper was undertaken neither time nor opportunity has allowed of special trials being made, and although the company have kindly placed their records at my disposal, I have not been able to extract therefrom the data necessary to enable any trustworthy light to be thrown on this important point. The loads have varied widely; the daily mileage of each boat estimated only, while the accumulators have furnished current for signal bells and lamps. Each boat has to perform a daily and variable proportion of backing and manoeuvring at full speed, so that careful special experiments are absolutely neces sary to bring out any results of value; and these I hope to obtain the company's permission to make. The makers of the cells cia m for them an output of from 75 per cent. to 80 per cent., and Pri Ayrton has recently shown that, under the most favourable circastances, even as much as from 85 per cent. to 87 per cent, of the energy put into secondary batteries may be got back again. As improvements in manufacture are continually being made, there is therefore good reason to expect that a brilliant future is before the accumulator branch of electric traction.

INDUCTIVE DISTURBANCES IN TELEPHONE CIRCUITS.*

BY J. J. CARTY.

It was not known until 1838 that the earth might be used instead of a return wire in the circuit of a telegraph line. This fact was discovered by Steinheil, while making experiments to determine whether the track of a railroad could be used to complete the circuit instead of a return wire. Steinheil's discovery has been considered as one of the most important in the art of telegraphy, and since his time it has been the almost universal practice to use the earth as a return in telegraph circuits. When, however, the earth return is used with the telephone, it is found that owing to the extrem sensitiveness of that instrument, many foreign currents, which would produce but little effect on telegraph instruments, become a source of serious trouble For this reason, in telephony it is often necessary

to use a complete metallic circuit. Thus it has come to pass that s return to the original practice of employing metallic circuits a in the case of the crudest telegraph instruments, is now regarded a a most important improvement in connection with that most highly developed of telegraph instruments, the telephone.

To obtain complete freedom from inductive disturbances in the telephone, it is necessary not only to use a metallic circuit, but to place the two wires composing the circuit in a special relation to the source of disturbance. One of the methods of so arranging the wits is shown in fig. 1.

FIG. 1.

In this case the two sides of the circuit, L2 and L, are twisted spirally about each other, so that their average distance from t disturbing wire, L', shall be the same. With such a plan, the telephones, a and b, are not affected by induction from the wire 1 This fact has usually been explained by assuming that a current commencing to flow in L', tends to induce two currents in th opposite direction to itself, one in the wire L2, and the other in th

* Read at the 55th meeting of the American Institute of Electron Engineers, New York, March 17th, 1891.