UNE 5, 1891.]

same way as in the Thomson-Houston armature. By iding each one of these three coils into two halves and ding each half diametrically opposite the other, also ording to the example of the Thomson-Houston armae, we finally arrive at a system which allows of driving a -coil motor with three conductors carrying three simple ernating currents. Thus we obtain the three patterns of e-coil, four-coil, and six-coil rotary current motors, fig. 6, ich no doubt will technically be the most important.

FIG. 6.

These three types were therefore subjected to a series of eriments by Messrs. Siemens and Halske, the results of ch are laid down in the following:

An iron ring supplied with a continuous winding was taken m an experimental model, and the winding was divided o three, four and six equal parts respectively. Each part excited by a perfectly constant continuous current, and intensities of these exciting currents were so adjusted as correspond to any phase of the three, four, or six altering currents, in which the resulting field was to be studied. his way, for each phase of rotation, the resulting rotary was made stationary, and the possibility attained of studyits shape by making diagrams of iron filings, and its ensity and direction by corresponding measurements. Thus diagrams, fig. 7, were obtained, which show the actual iation of the rotary field in the three, four and six-coil tor, when, according to the law of the parallelogram, the lting field ought to appear advanced from 30° to 30°.

713

The two concentric circles define the edges of the ring, the radial marks show the ends of the single coils. The arrow gives the direction of the resulting field as constructed according to the law of the parallelogram of forces.

The most interesting results are those shown in fig. 7a giving the rotary field of a three-coil motor. The resulting magnetic axis is mostly curved, except in those cases in which the excitation is quite symmetrical; and even then the lines of force appear contracted at one pole and spread out at the other (fig. 7a, I. First coil 0-0 ampères, second coil: 87 ampères, third coil:- 87 ampères). But as soon as the excitation becomes asymmetrical, we obtain a curved magnetic axis, and besides greatly different intensitics of the two poles (fig. 7a, first coil: +50 ampères, second coil 50 ampères, third coil : 100 ampères), and

so on.

The four-coil motor fig. 7b, gives a considerably better result. The magnetic axis is always straight, and both poles have equal intensities. The six-coil motor shows a still more equal resulting field, fig. 7c.

In order to measure the intensities of the rotary field, a coil was hung in the centre of the ring, in such a way that its magnetic axis was perpendicular to the measured direction of the resulting magnetic axis of the ring. The coil was then excited by a constant continuous current, and was kept in its position by a spring. The torque of the spring served as a measure of the intensity. The horizontal component of the magnetism of the earth could be neglected in these measurements, because the ring surrounding the measuring coil effectually served as a magnetic screen.

The following table gives the results obtained, but the diagrams, figs. 8, 9, 10, contain them in a more convenient form. In the diagrams, figs. 9 and 10, the results obtained are shown in the following way. For each measurement which shows a progress of a thirty-sixth part of one period against the last, and which, therefore, according to the law of the parallelograms, ought to show a progress of the resulting axis of 10°, a radius is drawn, the direction of which defines the actually measured direction of the resulting axis, while its length is proportional to the actually measured intensity of the resulting axis. In the three and six-coil motor these measurements executed over one-third of the whole circum

714

[JUNE 5, 1891.

Four-coil motor.

Measured direction of the resulting axis in degrees.

parallelo

gram.

Measured intensity of the resulting axis.

ference, are given; in the four-coil motor, those over onefourth of the circumference are indicated.

The actually measured values are shown in the upper right hand part of the diagrams, figs. 9 and 10. The other parts

was constant. However, we find the mean intensities in the three, four, and six-coil motor as 117: 127: 136.

The fluctuations of intensity are not appreciable in the six-coil motor. In the four-coil motor those fluctuations reach not quite 13 per cent. of their minimum value. In the three-coil motor the measurements of the intensity are not very valuable, because the resulting axis is mostly bent. The measurements were made in this case by taking the direction of the two poles singly and hanging the measuring coil in the mean direction equidistant from both. In the diagram, fig. 8, therefor, only the mean value of the results so "ound is drawn.

FIG. 10

The fluctuations of the angular velocity of the resulting axis are much larger than the fluctuations of the intensity in all three motors. The three-coil motor is especially interesting from this point of view, because the two poles always move with different velocities. The diagram, fig. shows the movement of the resulting axis during one half of a period. Since the same values theoretically must recur three times during one period, the mean value of the thre corresponding measurements actually executed was three times repeated in the diagram. Thus perfect regularity w obtained, and a good illustration of the actual movement of the axis is arrived at.

Since the velocity of the poles themselves will chiefly ir fluence the armature, it follows that this type is certainly not very advantageous. In the four-coil motor the vars tions of velocity are very much smaller, and still smaller in the six-coil type.

The question arises how far these results would be modified if the inside space of the ring were filled out by an armatore

JUNE 5, 1891.]

and what would be the further modifications if this armature rotated. However, we can assume with a reasonable show of probability that a constant field with a constant velocity would not be changed with regard to its constancy by adding an armature, provided that the saturation of the iron were not thereby essentially altered. Whether irregularities of intensity or velocity would be magnified or compensated by a rotating armature is difficult to decide. At any rate, these experiments show that though the law of the parallelogram of forces as accepted by Mr. Ferraris ought not to be trusted unconditionally, on the other hand the assumption that the resulting field varies as the sum of all ampère windings is still further from the truth.

PECULIARITIES IN THE BEHAVIOUR OF A GALVANOMETER WHEN USED WITH THE THERMOPILE.

ONE of the most recent contributions from the Physical Laboratory of Cornell University, U.S.A., deals with certain peculiarities noticed by E. Merritt in the behaviour of a galvanometer when used with the thermopile.

When the needle is not too thoroughly damped it was observed that on suddenly exposing one face of the pile to some source of radiant heat, the needle is quickly deflected. In a short time, however, the motion becomes less rapid, and in the course of a few seconds the needle comes to rest, and in many cases moves backwards for a short distance. This behaviour is then repeated, and it is only after a long series of such "throws," which gradually becomes less and less marked, that the final steady deflection is reached.

This peculiar motion as observed in the case of a Thomson tripod galvanometer, has been represented graphically by a curve, of which the abscissa of any point shows the time that has elapsed since the beginning of the motion, and the corresponding ordinate is proportional to the deflection of the needle from its position of rest. In the case of galvanometers having needles more nearly "dead beat," the general form of the curve is the same.

Now this phenomenon was noticed six years ago by Violle (ride Annales de Chimie et de Physique, vi. 3., p. 373), while using a thermopile to investigate the radiation of molten platinum and silver, but he offered no explanation. Rubens and Ritter have observed a similar phenomenon with a peculiar form of bolometer, which they used for quantitative measurements of electromagnetic waves (vide Wiedermann's Annalen, vol. xl., p. 63).

Numerous experiments that have been made in the laboratory of the Cornell University with a bolometer of the ordinary type, and with a number of different galvanometers and thermopiles, seem to show that this behaviour is not peculiar to any one instrument, but is always observed when a bolometer or thermopile in circuit with a galvanometer is suddenly exposed to radiant heat.

Merritt first had his attention called to this curious phenomenon in 1888 while engaged in an investigation of the energy of the light from incandescent lamps. In a paper published in 1889 in the American Journal of Science (vide vol. xxxvii, p. 167), entitled "Some Determinations of the Energy of the Light from Incandescent Lamps," he called attention to the fact which at that time rested only upon experimental grounds, that the first throw of the needle, under the circumstances described above, bears a constant ratio to the final deflection, this ratio being independent of the intensity of the radiation to which the pile is exposed. The phenomenon, according to Merritt, appears to be due to the inertia of the needle of the galvanometer, and to the fact that a considerable time elapses after the pile has been exposed to a source of heat before a constant temperature is reached. On account of its inertia the needle is unable to follow immediately the rapidly increasing current that flows when the face of the pile is first exposed. In a short time, however, the continued action of the deflecting force imparts sufficient velocity to carry it, not merely to the position which corresponds to the current then flowing, but to a considerable distance beyond this point. The result is that the motion of the needle is stopped and a retrograde movement begins which continues until the pile has been heated

715

sufficiently to cause another throw forward. This behaviour is then repeated until the temperature of the pile has become constant or until the oscillatory motion of the needle has been destroyed by damping.

Assuming that the heating of the pile takes place in accordance with Newton's law of cooling, and that the electromotive force of the pile throughout the small range of temperatures with which we have to deal is proportional to the difference of temperature between the junctions, Merritt enters into a mathematical argument and deduces the equation of motion of the needle. For this we must refer our readers to the original paper, which appeared in the May number of the American Journal of Science, [3], vol. xli., No. 245, p. 417. Experiments are quoted in support of the mathematical deduction.

We have already noticed that the first throw of the needle bears a constant ratio to the final deflection. And it has been found that the only effect of a change in the intensity of the source of heat is to increase or diminish all the ordinates of the curve in the same proportion, the ratio of any two ordinates remaining the same. If, therefore, the final deflection of the needle is proportional to the quantity of heat received by the pile, the first throw will be proportional to this quantity and may be used in all cases instead of the final deflection.

The importance of this conclusion that the ratio is independent of the deflection (which is justified by experiment), will readily be seen by those who have had occasion to use a thermopile for accurate measurements of radiant heat. Draughts of air and other almost unavoidable sources of temperature variation frequently make the galvanometer quite unsteady, while the extreme delicacy of the instruments that must be used in work of this kind renders them especially susceptible to magnetic disturbances. Many observations are thus made valueless by a change in the zero point of the galvanometer during the three or four minutes required for the needle to come to rest. Only a few seconds are required, however, for the first throw of the needle, and the change in zero point during this time would scarcely ever be sufficient to cause an appreciable error. The use of the first throw in place of the final deflection may, therefore, lead to greater accuracy as well as to saving of time.

It will be observed that the principle underlying this method of taking readings is not confined to the case of the thermopile, but is capable of wide application. The following are suggested as cases in which the method may be employed with especial advantage :

I. All ordinary measurements of radiant heat when the conditions are such as to make the galvanometer unsteady, or when the saving of time is a consideration.

II. For purposes of demonstration in the lecture room, a number of experiments that are usually considered unsuitable for lecture demonstration have been quickly and accurately performed in this way under conditions that would render the use of the ordinary method entirely out of the question.

III. For measurements of heat from a variable source the first throw of the needle will in this case give the amount of radiant energy at the very instant of exposing the pile.

IV. For work with a bolometer or similar instrument under the same conditions that apply in the case of the thermopile.

V. For the measurement of a constant or variable current, when it is desirable to take readings quickly, e.g., the initial value of the current from a cell which is subject to rapid polarisation, may be determined by the first throw of the needle. Other applications of the method will doubtless suggest themselves. Mr. Merritt concludes by expressing the hope that a phenomenon which at first appeared to be merely a matter of curiosity, may thus be made of practical value in the laboratory.

ELECTRICAL SAFETY APPARATUS FOR

MINE CAGES.

DURING the course of a general meeting of the members of the Federated Institution of Mining Engineers, held in the rooms of the Institution of Civil Engineers on Thursday and Friday last, Mr. John Yates (London) read a paper describing an electrical safety apparatus for cages. The following is an abstract of the paper :

The author pointed out that many safety cages had been introduced, and that although their construction was faulty, they had been the means of saving numerous lives. It was, however, found that in quick winding the apparatus was apt to come into action when not required, and that it sometimes did not act when necessary. In his opinion, the conditions which a safety apparatus should fulfil were:-1. It should never fail to act when required; 2. It ought never to come into play except when the rope breaks; 3. It should allow of being tested; 4. It must be simple in construction and require little attention; 5. It should in no way interfere with the ordinary work. These conditions formed the standard to be attained by a good safety cage, and he believed the manner in which this standard might be attained was to use the hauling rope as the means for conveying a current of electricity to four electro-magnets on the cage, each magnet sustaining a gripping cam. This was a general outline of the apparatus. The rope was to all intents and purposes the ordinary hauling rope, except that it had two insulated copper wires in the hemp core. To show that such a rope could satisfactorily transmit current, it was mentioned that a similar rope was in use with Armstrong's electric signalling apparatus, which permitted of signalling taking place between the engine house and the cage whilst the latter was in motion, and which had been applied in many of the collieries in the Durham district.

The insulated copper wires were connected at the cage end to the electro-magnets, and at the winding drum end to two insulated copper rings on the drum axle, these rings being in connection with a battery of a few cells giving a constant current. The electro-magnets were of the horseshoe type, this being considered the most suitable. The arrangement for gripping the guides consisted of four levers, each working independently of the others. One end of each lever was shaped so as to form the gripping cam, whilst the other end was heavily weighted and had a spiral spring attached. The weight and the spring tended to pull down the long arm of the lever and thereby placed the cam in contact with the guide. The electro-magnets were arranged directly over the weights, which they supported, thus keeping the cams out of action. The author considered this apparatus to be exceedingly simple as compared with ordinary safety appliances, which latter necessitated the use of a complicated arrangement of levers, bell cranks, and springs, together with, in many cases, a special suspending frame. It was claimed that, as each lever was complete in itself, the apparatus really consisted of four sets of safety appliances, the failure in the working of one of which would not affect the action of the others; that, as there were four levers, it would appear impossible for the apparatus to fail to act on the breaking of the rope; and that, as a continuous current was obtained, it was very unlikely that the apparatus would come into play when not required, for as long as the rope was intact and the current passed the levers could not come into operation. The apparatus would, the author said, act instantaneously on the fracture of the rope, and it could be tested at any time by breaking the circuit; then the weights, being no longer supported by the magnets, would pull down the long arms of the levers, and the cams would grip the guides, thus stopping the cage. Several methods of gripping the guides were provided. In conclusion, the author stated that the average miner would, after five minutes' inspection, understand the working, that it would only require two or three minutes' attention daily to keep it in proper order, that the cost of applying and maintaining the apparatus would be very small, and that it fulfilled the conditions which an efficient safety apparatus should possess.

The paper was illustrated by wall drawings, and a large model of the apparatus was exhibited.

ANOTHER EARLY GRANULAR CARBON

RHEOSTAT.

By A. M. TANNER.

My published researches concerning the variable resistance of carbon under pressure, show that in 1835, Munck, of Posenschöld, experimented with differently compressed carbon

vder for varying the force of an electrical discharge, and

that in 1861 Beetz placed spongy platinum in a glass tube and compressed it more or less by means of a piston in order to change its conductivity. Hence it appears that neither Du Moncel, Clerac, or Edison first discovered the principle in question, as some of the electrical text-books have it. Another reference is to be added to the list of rheostats or electric current regulators making use of granular or powdered carbon-it is the French patent of Emanuel Rebold, dated March 2nd, 1857, No. 32,415-which shows what is termed a current moderator for induction coils. The sketch here given is taken from the drawing of the

c, copper; & c, granulated carbon; G T, glass tube; s, set screw. patent, and shows a glass tube having copper caps and filled with granular carbon. A screw stem penetrates into the carbon and can be adjusted by a milled nut. After adjustment the rod is held stationary by a guide stem and set screw. The strength of the current is varied by causing the screw rod to penetrate more or less into the granular carbon. It will be seen that Rebold did not propose to obtain & change of conductivity by an endwise pressure upon the carbon mass, but obviously the movement of the screw rod will produce a slight compression in the direction of the walls of the glass tube, and thus the degree of penetration of the rod and the lateral compression of the carbon granules will bring about the desired result, just as is the case in that class of microphones where solid projections on the diaphragm co-operate with granular carbon.

THE BLACKENING OF INCANDESCENT LAMPS.

By LEGH S. POWELL.

THE references which have lately appeared in the ELECTRICAL REVIEW relative to the depreciation in the illuminating power of incandescent lamps through blackening, and its consequent bearing on the economic length of time tha: lamps should be kept in use, have a peculiar interest to thos who have made this department of electric lighting a special study. The tables given on page 621 of your issue for May 15th, show clearly the different degrees of darkening tendency which lamps of different manufacture possess. It is not forgotten that other causes besides blackening contribute to diminish the illuminating power, but in most lamps at least. the one under consideration is the most influential. To the manufacturer of lamps, then, this blackening feature is naturally one of the utmost importance, since in this direction lies one of the principal possibilities for further improvement. Formerly the writer was of opinion that the problem of diminishing this blackening tendency was about on a par with that of attempting to prevent water from boiling when a kettle of it is placed over a fire. But the two cases are scarcely parallel, for it is an undeniable fact that blackening occurs in very different degrees, not only in lamps of different manufacture, but even amongst their own kind. A goo deal of the variation, no doubt, occurs through differences r the temperature of the respective filaments, but this cause is certainly not the only one at work. Much of the difference is attributable to the manner in which the carbon filament he been treated during its manufacture, and this fact raises th very interesting enquiry as to what cause the blackening is due. and wherefore there should be these differences in intensity:

To the writer there appear to be two possible explanations to account for the blackening phenomenon: One is the ge rally accepted view of the vaporisation and sublimation

the glass of carbon, pure and simple; the other, that the obscuring substance may possibly not be pure carbon but a hydro-carbon, or some allied carbon compound.

Probably every lamp manufacturer is familiar with the fact that lamps made with filaments which have been "baked" at a comparatively low temperature, blacken very readily indeed, whilst when made with others which have been raised to a very high temperature, they resist the tendency very much more effectually. Further, that the same results occur in lamps when low or high temperature flashing has been employed. He is also very familiar with the effect of different temperatures on the hardness of the resulting carbon. If then the vaporisation of pure carbon be the true explanation of the darkening of lamp bulbs, it follows that the difference in degree must arise solely from the circumstance that a comparatively loose condition of the carbon is especially favourable to the formation of carbon vapour. But the production of carbon at a low temperature would also be especially favourable to blackening, if the hydro-carbon view is the correct one. The writer can well remember when attending Dr. Frankland's lectures on chemistry, being impressed by that gentleman's assertion that to produce pure carbon by any of the usual methods is almost an impossibility, and that to free it from the ever present combined hydrogen, it is necessary to heat it in chlorine gas for days. That all hydro-carbon compounds are eliminated from the filaments in ordinary use is, therefore, extremely doubtful. In further support of the view that the presence of hydrogen may exert an influence in the feature of blackening, the fact may be cited that filaments of carbon baked at a low temperature become materially harder in process of running lamps made with them, and yet the rate of blackening, instead of diminishing, seems to proceed at an accelerating rate. It would be most interesting if those who have the opportunity would collect and thoroughly examine the composition of the deposit which collects on lamp bulbs, and thus throw valuable light on this interesting subject, and in addition, direct the steps of the pioneering lamp manufacturer into a track which is likely to lead to a material decrease in this unfortunate property of lamps. To carry out the work satisfactorily, it would be necessary to examine the deposit obtained from lamps from various sources, and then, should it prove to be other than pure carbon, to further ascertain, if possible, the treatment the respective filaments have been subjected to. The latter undertaking would, probably, be the more difficult of the two, considering the commercial interests at stake. It may be mentioned that the obscuring film is readily removed from the glass by means of a little warm caustic soda solution. Also the fact that the film is a sufficiently good conductor of electricity, even when faint, to become coated with copper when a piece of the glass is made the cathode in a copper plating bath. It is possibly due to this fact that an incandescent lamp acts as such an excellent condenser.

It is interesting to speculate as to how the obscuring matter, whatever it may be, finds its way from the filament to the glass. It is conceivable that there may be three methods by which the transfer can be effected: Firstly, by the direct formation of a vapour, as in the case of the vaporisation and sublimation of arsenic; secondly, by the solution to saturation of the matter in the residual gas and subsequent precipitation therefrom on the neighbouring cold surface, as instanced by the solution of water in warm air and its re-appearance on a window or other cool surface; thirdly, by the mechanical emission of solid particles. From a consideration of specimens of blackened lamps, it is difficult to determine which of these methods is the true one, or whether two or more methods combine to effect the transfer, because, in vacuo, the conditions of matter are so materially altered in so many essential details from those with which we are ordinarily familiar that arguing by analogy becomes impossible. But whether the transfer is effected by gaseous or solid particles, we may note certain very significant effects. The sharply defined uninterrupted circular line of slightly tinted glass, so distinctly visible by daylight in every well blackened lamp whose -shaped filament is perfectly symmetrical, tells a peculiarly interesting tale. There can be no doubt that this half-coloured line is caused by the limb of the filament nearest the glass acting as a screen to the emissions of the more distant one. In other words, the projected particles of only one of the limbs can strike against

717

the light coloured strip, because just there the further one is hidden, whereas in every other part of the bulb they can, and do, strike from both limbs, and consequently at all these other parts the glass is coloured to twice the depth of the screened portion. Another notable effect is that of the patchy, unequal blackening which occurs in the bulb of a lamp whose filament develops a bright spot, the glass in the neighbourhood of the spot being the darkest part. The same thing happens when the filament is bent so as to stand appreciably out of the centre of the bulb. From these observations it is perfectly clear that the matter, whether in the form of vapour or solid, is in the radiant condition, and that there is no turning of corners for it. The particles fly direct from the filament to the glass and there adhere to it and to one another as firmly as do the particles of copper on the cathode of an electro-plating arrangement; and further, they appear to follow the same law as light, in its property to decrease in intensity with the distance from the issuing point. It naturally follows from this consideration that for a given size of filament, the larger the bulb the less the amount of blackening, other considerations remaining the same.

As regards the remarkably coherent black film, it may be remarked that if it is built up by the accumulation of countless solid particles it would entail that carbon is possessed of far more of the soft, doughy property of lead or wax than one is apt to attribute to it. The principal causes assisting to bring about the cohesion, assuming this view to be correct, are, no doubt, the probable enormous velocity with which the particles travel, and the freedom from any foreign matter acting to interfere with the intimate contact of such particles as they collect.

Referring to the economic length of run that a lamp should have, it seems to the writer that any particular number of hours arrived at by calculation, can only hold good in a general sense, for the reason that lamps do not all blacken equally in the same time. Probably the actual run that lamps will have, in private houses at any rate, will be determined by the inconvenience experienced from an insufficiency of light rather than by any other consideration.

REPORT ON THE ELECTRICAL TRANSPORTATION SYSTEM OF THE NEW ENGLAND PORTELECTRIC COMPANY.

BY FRANKLIN L. POPE

In accordance with the request made by you* through Prof. Dolbear, I have made an examination of the system and experimental plant of the New England Portelectric Company, with a view to an expression of my opinion in reference to the results which may reasonably be expected to be attained in the commercial operation of a plant of this character when constructed on a scale sufficiently extensive to fully develop the capabilities of the invention. To this end I have observed and studied the operation of the existing experimental plant at Dorchester, Mass.,† and have taken notes of the results attained by it in actual operation. It did not seem necessary, for the purposes of the present investigation, to undertake a series of refined tests and measurements. Such a procedure would have consumed considerable time, and have been attended with unnecessary expense, while it could not after all have materially affected the value of the conclusions at which I have arrived. The measurements which have been made, though in a certain sense rough, are, nevertheless, sufficiently accurate and trustworthy to justify and support the conclusions which will be drawn from them.

Description of the Experimental Plant.-The experimental plant at Dorchester comprises an endless track, elevated upon a wooden trestle a few feet above the ground, 2,784 feet in circuit, consisting of one tangent of 588 feet, and another of 576 feet, united at their ends by two curves, one of which is 924 feet long and 282.5 feet radius, and the other 696 feet long and 234-4 feet radius. The track in the first tangent

This report is addressed to Mr. J. T. Williams, President of the New England Portelectric Company.

For illustrations and full description of the experimental plant at Dorchester, see the Electrical World, May 4th, 1889, and October 18th, 1890, see also ELECTRICAL REVIEW, October 31st.