acid, the subsequent formation of hydrogen dioxide will be greater in it than in the plain cell. As far as I could discover, sodium sulphate has little or no action on the acid unless it is added during electrolysis, or to acid which has just been taken from a cell through which a current is passing.

The Pink Colour of the Acid.-It has often been noticed that during charge, particularly with new cells, a pink colour starts from the peroxide plates, and gradually spreads over towards the lead plates, fading away, however, before reaching them. This pink colour was referred to by Mr. Crompton at a meeting of the Institute of Electrical Engineers on 13th December, 1890, and its origin gave rise to some discussion; so as the acid in many of the cells at the Post Office was pink, I tested it by concentrating it down, neutralising with sodium carbonate, and then igniting on platinum foil, and always got the characteristic green of manganese.

However, lest the manganese should have come from some other source than the pink acid. I compared the absorption spectrum of the acid with that of a solution of potassium permanganate of the same shade of pink, and found they both gave the characteristic bands in the green, Fig. 14. I also found that, using two strips of platinum as electrodes in a solution of manganous sulphate, or any two strips of lead in dilute acid, gave the same colour and the same absorption bands, provided the electrodes were sufficiently far apart to prevent reduction by the hydrogen evolved by the negative. This result was important, for it is well known that the pink colour disappears from the acid in a short time if it is taken from the cell, and as persulphuric acid has no action on permanganate, but hydrogen

causes arising in the working than in the manufactur What is required is some substance which can be added the acid to check the formation of the oxidised bodies in which cause sulphating without at the same time injuri the plates in other ways.

Nearly all the " forming" baths which have been intr duced are baths in which hydrogen dioxide would broken up as soon as formed, and perhaps in some modi cation of them the electrolyte of the future will be found though, since the products of the electrolysis of sulphur acid vary with the strength of the acid and the curre density, no hard and fast rule can be laid down for t treatment of cells.

In cells containing acid below density 1,200, in which t proportion of "active oxygen" existing as hydrogen dioxi is high, the addition of 1 per cent. of sodium sulphat or similar substance, is likely to prove beneficial, particular if the work of the cells is intermittent. As the strength the acid is increased, however, and the conditions are mo favourable to the stability of persulphuric acid, le hydrogen dioxide will be produced, and there is mo chance of the alkali released from the sodium sulpha during electrolysis damaging the plates.

Also, Dr. Marshall has succeeded in preparing pu persulphuric acid, and has shown this year that it form salts with the alkalies which are very stable; and what the effect on a cell of the formation of sodium persulphate in would be, is quite unknown. Although the formation peroxides in the acid does not apparently account for th great gassing and sudden loss of charge sometimes observed still we have seen that makers are reverting to Planté process of manufacture, or modifications of it, and we ma

β γ

dioxide decolourises it, this disappearance of the colour shows that the latter is formed.

The Effect of Hydrogen Dioride on the E.M.F. of a Cell.The presence of hydrogen dioxide having been thus proved, both directly and indirectly, its effect on the E.M.F. of the cell was tested. This was done by using strips of lead packed tight into small porous pots, with peroxide of lead to represent the peroxide plates, and using plain strips as the lead plates. A solution of pure sulphuric acid, density 1,180, was used as the electrolyte. The E.M.F. of the couple was taken by the deflection method, and then a drop or two of hydrogen dioxide was added to the acid, which produced a great diminution, or even reversal, of the E.M.F.

The effect of introducing hydrogen dioxide into the body of the peroxide paste was also tried, with a view of reproducing, if possible, the conditions of a cell which is started discharging directly the charge is completed, and in which the "active oxygen" would be accumulated at the positive plate, leaving the lead plate free, and I found that there was a slight increase in E.M.F.

Thus the variations in E.M.F. appear to depend on which plate hydrogen dioxide is formed at. When present at the peroxide plate it causes a rise, but when diffused through the acid and present at the lead plate it causes a lowering of the E.M.F.; and the rise in E.M.F., sometimes noticed on starting the discharge of a cell which has been at rest (mentioned in Prof. Ayrton's paper, J.I.E.E., 1890, p. 572), is probably due to the electrolysis and decomposition of hydrogen dioxide, for, in a cell which has been long idle, practically the whole of the "active oxygen" is due to this body.

CONCLUSIONS.

From the same faults appearing in batteries of such different construction, and judging also from the results of the experiments recorded in this paper, it would appear that the troubles occurring in batteries are due rather to

find that in this case also he was right, and that it is to the electrolyte we must look if we wish to find the means of materially improving the lead reversible battery.

In conclusion, I must thank the firms who have assisted me in the compilation of this paper by supplying informa tion in response to circulars sent out.

MAGNETIC RELUCTANCE.*

BY A. E. KENNELLY.

(Concluded from page 519)

An examination of reluctivity curves naturally suggests the question as to whether there is really a strict linear relationship between H and p. In other words, whether the divergences of the observation curves from geometrical straight lines can be fairly ascribed to errors of observation, allowing for the influences of residual magnetism.

First confining the enquiry to the ascending reluctivity line that which is geometrically consequent upon Frölich's formula and also under special interpretation with Lamont's formula-the agreement of the plotted observations with a straight line between the neighbourhood of the critical H and H = 150 is often so good as to intimate the existence of a definite linear relationship. It is generally to be found, however, that beyond 150 C.G.S. units of H, the line bends downward until all hope of rectilinearity is lost. That is on the simple circuit theory of reluctance. If, however, we introduce an amendment into the definition of reluctance borrowed from the vein or polar theory, the rectilinearity appears to be nearly sustained for a much greater distance. Experimental observations of the relucPaper read before the American Institute of Electrical Engineers, October 27, 1891.

*

tances in circuits of the magnetic metals under powerful magneto-motive forces are yet very scanty, but judging from the results of Ewing and Low the reluctivity of wrought and cast iron on the amended definition appears to be a linear or at least nearly linear function of the force as far as H 25,000 and H 11,000 respectively, the limits of the quoted measurements.

The graphs of these measurements are given in Fig. 10. The observations run from H 3,630 to 11,200 for wrought iron with an isolated observation at H 24,500 in a separate instance, the similar series for cast iron running from 3,900 to 10,610 units of H. The linear relationship is very fairly maintained and the lines prolonged downwards nearly strike the origin. It is to be observed that at the limiting observation for wrought iron its reluctivity is nearly 20 per cent. greater than that of air or of the airpump vacuum.

According to the simple circuit theory, the reluctance is of course the ratio of the magneto-motive force to the flux and the reluctivity this quantity locally reduced to the unit of volume. On the vein theory, however, which as we have seen distinguishes the vein flux from the magnetising flux superposed thereon, the conductivity of a mass of iron is the conductivity of the iron itself added to that of the space it occupies, and consequently applying the vein theory to the magnetic circuit we have the apparent

density at which iron is commonly worked in practice, while at the same time they indicate a limiting value of magnetisation long before that density is reached. Following the vein theory, the polarisation of the iron is then complete and the intensity of magnetisation or magnetic matter per unit cross-section of veins finds its maximum, so that, while the total flux can go on increasing indefinitely, it can only do so by adding to the permeating field flux, the vein flux having reached its full limit. It is not impossible to represent the observed condition of affairs by the simple circuit theory, but the mental picture is not so clear. It would be possible, for instance, to imagine that the molecules of all substances transmitted the stress flux with the same, or almost the same, facility as the ether surrounding them, but that in the magnetic metals they exalted the stress in transmission. Maxwell supposed that the iron molecules were so constructed that they could take part in the ether spin that might constitute the stress, and, if so, by adding to it their momentum of revolution, they could augment its value. There would be then, perhaps, at a certain stress, a speed of revolution which the iron molecules would not exceed, and their reinforcement would be at a maximum, while for stresses enormously greater than this the molecular augmentation would be lost in comparison with the strength of field, and the iron molecules would in the aggregate behave almost like the motionless transmitters of other substances. The consequences of this conception seem more complex, exen if more nearly true.

Turning now to the descending curve of reluctivity between the initial and critical values, closer examination will show that here at least the linear relation is only an

reluctance of the

iron mass as the

joint reluctance of two paths in multiple arc, one through the iron itself and what might be called its metallic reluctance, the other through the reluctance of the space occupied by the iron, and the removal of the iron would leave this latter unaltered The difference between the apparent and metallic reluctance is inappreciable while the latter remains small, that is generally speaking when H is below 150, a limit rarely exceeded, and consequently the question does not present itself under practical conditions, but for large values of H, the difference is considerable and the metallic reluctance approaches the linear relationship with H while the apparent reluctance deviates considerably from it.

These assumptions from the vein theory while they may be convenient are somewhat artificial for they postulate that the space reluctance of a given volume of air is not altered when the volume is occupied by iron. This may not be impossible but it is difficult to imagine any reluctance mechanism of ether that would remain undisturbed by the introduction of a massive substance. On the other hand, while the vein theory imposes this principle not touched upon by the circuital hypothesis, it explains very satisfactorily the fact now apparently beyond dispute, that while iron can be saturated there is no limit yet attained to the flux density that can be made to pass through it. Ewing's results give no limit at the observed flux density of 45,350 C.G.S., nearly three times the flux

FIG. 10.

apparent one. The descent is so steep that on the scale of projection it appears nearly straight, but when magnified it has a distant curvature. Rayleigh* and others have shown that for small degrees of H the permeability commences with a definite steady value, and this being the case it would be impossible for the reluctivity-the reciprocal of that permeability-to be linear towards H. Series of observations covering with sufficient detail the range of H from zero to unity are apparently few, and Fig. 9 gives the plotted values of the reluctivity on an enlarged scale, observed by the writer for a sample ring of Norway iron. The descending line has a marked curvature approximately logarithmic, that would be almost inappreciable, however, on the scale of the other reluctivity diagrams.

Even, however, if we admit that there exists a linear relationship between H and p beyond the critical point, that is to say, if the experimental results justify the belief that Frölich's formula is not merely an empirical one, we are scarcely entitled to attribute to this relationship an intrinsic physical signification. It may enable us to grasp the salient features of the magnetic circuit by the re-establishment of Ohm's law, but the relationship is more likely to be the consequence of a more remote fundamental agency than to be significant of any physical condition resident in metallic reluctivity itself. This is for the reason that the increase of flux under M.M.F. in a magnetic circuit due to the presence of iron is more probably owing to an assisting M.M.F. set up in that iron under stress than to any change in the latter's reluctance, and the removal of the initial source of M.M.F. from the circuit still leaves some M.M.F. active as residual magnetism.

*Phil. Mag., March 1887.

the supply company, and by their men during the day when they are not wanted, as the machinery will be standing. The lights, moreover, can all be switched on and off simultaneously from the central station, thus doing away with the extra expense for gaslighters. Arc lamps would have to be placed farther apart than ordinary gas lamps, the power given out being so much greater. A distance of 200ft. is about correct if the lamps are 20ft. high. If placed lower the glare is too much. They are therefore placed high, and at a good distance apart. I have now come to the application of electricity in such a way as will interest and concern you personally most-namely, the lighting of your own houses by the electric light. Many of you have no doubt looked and longed to have the light in your houses on account of its soft rays, its cleanliness, and its safety against fire risks, but have fought shy of it on account of the expense. It is the general belief that the electric light is expensive. This may have been so a few years ago, but the statement no longer holds good of electric lighting in the present day. When a house is supplied with current for lighting purposes a meter is fixed in the cellar, precisely as with gas, to register the amount of current used. Gas is charged by so much per 1,000 cubic feet, electricity by so much per unit of 1,000 watt-hours. The price per unit varies from 6d. to 8d., and more. To give you an idea of how much a unit is and how much you can get for your money I must tell you that an 8-c.p. incandescent lamp will consume in 30 hours' burning just about one unit, or it costs, roughly, d. an hour per 8-c p. lamp. Gas would be about the same at the rate of 38. 3d. per 1,000 cubic feet, not taking into account any oil and candles used. But it is not only the smaller cost of electricity compared with gas in the actual burning that must be taken into account. When a house is lit by gas a great amount of waste must necessarily occur. At dusk the servant goes round and lights the burners in the rooms all over the house, whether they are wanted for an hour or two or not. In the bedrooms the lights are left low, ready for turning up when dressing for dinner or going to bed, as the case may be, and in the winter months these lights are burning for hours before actually wanted. And why? Because unless it is done there is the bother of lighting it, to save which the gas is allowed to waste for hours. And with gas in the house it is the only way out of the difficulty. But with electric light, where no matches or getting on chairs is necessary, all that is wanted to be done to light up is to simply turn on a switch (a switch was shown and its action explained). This switch can be arranged near the door or in any convenient place easy to reach in the dark. The switching is done in the room, and, when finished, the light is turned off again by the same process. And not alone is the saving in expense enormous, but the great question of health comes into account. Wherever gas, oil, or candles are consumed a considerable amount of carbonic acid gas, a deadly poison, is produced, and I need not tell you how injurious its fumes are to everybody. They not only kill men in time, but silver is tarnished by them, and the ceilings, draperies, and papers are in time made unsightly. To give you an instance as to the health side of the question Mr. Preece, the well-known Post Office electrician, tells us that the electric light which has been introduced into the Savings Bank Department of the General Post Office, has decreased the hours of absence of the staff through sickness at the rate of two hours per head per annum. In other words, this means a saving to the Post Office of £680 per annum. Now the production of their electric light costs them £700 per annum all told. By putting in the electric light, therefore, the working of their staff was increased as by the addition of 200 hands, and, practically, their light only costs them 120 per annum through the decrease in their sick-list. This is an actual fact, on the authority of Mr. Preece, and there is no getting over facts. In schools, churches, public halls, and such places where many people congregate, the health question is most important, and to give you an instance of how it is unnoticed I went into a church last winter and was surprised to find the gas burning during the day, although the electric light was connected up. Ön enquiring the reason, I was told it was done to warm the church. Gas is all very well for warming when the poisonous fumes are carried away up the chimney, but when this is not done they are most injurious to the people inhaling them. The authorities are always anxious to promote sanitary matters in the way of drainage, pure water, and removing the refuse from the dustbins, but the sooner they turn their attention to the way people are deprived of good health by inhaling carbonic acid gas in our homes and crowded halls the better.

As

I mentioned before, our rooms are greatly damaged by gas lighting. In a large London restaurant where electric light is now laid on, I was told that the saving per annum through not having to redecorate the ceilings, and the saving of labour for cleaning the by-gas-tarnished silver, was simply enormous. It is just the same in a smaller way in our homes, and by adopting the electric light we are removing that which means ruination to our goods and chattels, and especially pictures. We introduce cleanliness, we add to our comfort, and, therefore, to our cheerfulness, and there is a total absence of all poisonous vapours. Mr. Hare, the well-known actor, has said that through adopting electric light he finds that life is made worth living, and I think you will agree from what I have said that he is not far wrong in his statement. I have endeavoured to point out to you, therefore, that electric light is a source of increased health and a saving to our purses by its cheapness and cleanliness. But there is another great item in its favour-namely, the absolute safety against fire that its use affords us. You may have read in the newspapers of fires caused by electricity. Perhaps, but very few in England. They mostly happened in America, where up to a recent date no laws were laid down regarding the use of the electric current. Here, however, we have a County Council to look after us, who, although sometimes over-anxious for the safety and welfare of the

public, have certainly forwarded the absolute safety of electric lighting. In America the cables are put up in a careless fashion, and the quality of the insulation is far below the standard and would not be allowed in England. (Here a specimen of Englishmade cable was shown, and the method of insulating it explained.) The insulation should therefore be such that anybody can catch hold of the cables and yet not feel the slightest shock. In America these cables run overhead, and the insulation is often seen hanging down in rags. Naturally, the copper gets exposed, and anybody touching it is very likely to get a shock. In London our cables are now chiefly run underground in iron pipes 3ft. or 4ft. below the road, and are perfectly inaccessible to anybody but the company's inspectors. Fitting up the electric light to houses is now done to such a perfection-I am, of course, speaking of work done by well-known and reliable firms-that all risk of fire is abolished, and fire insurance offices are now giving a reduction in their prices where electric light is used. (Here the lecturer made a short digression on the subject of electric plumbers, and warned his hearers of the consequences of cheap and nasty wiring work.) The only chance of fire occurring through electricity is by reason of a wire overheating, but this is rendered impossible by using safety fuses. (The construction and action of a fuse and cut-out was here explained.) Thus we can protect not only a lamp, but a whole room, a floor, or the whole house. In case of fire through any other cause the fuse would melt in the same way, and all current would be cut off from the house automatically, which with gas is impossible. In a similar way whole streets can be protected, and finally the whole station. You see, therefore, that the electric light is absolutely safe against fire. The next great item in its favour is the easy means it affords us for decorating our homes practically, artistically, and effectively. Mrs. J. E. H. Gordon, wife of the well-known electrical engineer, has written a charming book called "Decorative Electricity," in which she suggests several ways of using the electric light as a means of not only illuminating, but decorating our homes. All rooms are taken into account, and such charming effects are described that I am sure all ladies will vote unanimously for its introduction into their homes without further delay. The candle-power of the lamp can be split up and so arranged that every part of the room is lit up instead of having one big cluster of lamps in the centre of the room. Cosy nooks and corners are well thought out, with a lamp ready to hand and in the very place where it is wanted. (The lecturer here described some of the decorative effects produced in Mrs. Gordon's house.) I have now endeavoured to put before your notice the many ways we can be benefited by the adoption of the electric light. Many people are prejudiced against it because they hear such dreadful accounts. These are exaggerated, to the detriment of the new rival illuminant. To give an instance. A new central station was opened in the vicinity of a milkshop. The proprietor objected to it on the ground that a leak in the mains had turned his milk sour. A butcher made the same complaint in another case, urging that the current escaped and spoilt his meat. The utter absurdity of these statements are apparent. If there is a leak in the system it returns to earth, where it practically came from, and you cannot feel a shock unless you connect both positive and negative poles through the body. You would not notice personally the leak in your house, but the supply company would at once by their instruments, and set to work to remedy it. It can be detected almost to a yard.

I think that a general summing up of the advantages gained by using the electric light will take the following form, which I want you to take home with you for severe reflection. We are introducing a cheaper illuminant, whose properties rather tend to purify than poison the air; we are removing from our midst the source of destruction to our ceilings, walls, pictures, and silver; we are encouraging cleanliness, which is next to godliness; we are furnishing our homes with a light that is excellently adapted for artistic and effective illumination; and we are ensuring our homes against risk of fire.

DISCUSSION.

The Chairman said that if anyone had any questions to ask, Mr. Metzger would be happy to answer them.

Mr. Elliott asked as to the uncertainty of the light, had that been got over? Theatres, for instance, were compelled to keep gas burning as well as electric light to avoid accident or confusion in the event of the latter going out. Then as to the bad effect of arc lights on the eyesight. Could that be overcome? He knew they were all very anxious indeed to have a better and cheaper illuminant than they then had. He should like to see the electric light in Hendon. It might be the means of bringing the price of gas down, or they might leave their old friend altogether if they could get electric light as cheaply.

Mr. Turner asked as to the cost of wiring a house?

Mr. Metzger, in reply, explained that the reason why arc lights must be placed a good height above the ground was in order that they might not affect the eyesight by the intensity of their light. If people used arc lamps in shops they must rot be surprised if their eyesight was affected. Incandescent lamps being of low candle-power, never higher than 200, and averaging eight, did not affect the eyes. As to the lighting of theatres, he did not know whether Mr. Elliott was referring to the past or present illumination of these buildings. Nowadays, the lighting was almost entirely done from electric supply companies' mains. Those theatres which employed their own plant to produce the current had to ensure themselves against all lights going out in the building, and so used gas as well. At Doyly Carte's new theatre, for instance, there was a brand-new plant, but they dare not rely on it, and so had the mains of a supply company taken in

and ready to be switched on at any moment. The cost of wiring a house depended upon the fittings used, and how the rooms were situated. Roughly speaking, under favourable circumstances it would cost about £1 per lamp, including ordinary fittings. (A Speaker wanted to know if he could not give it per yard.) That was very difficult, because corners made a great difference in that case. It was not usual to charge by the yard. The ordinary way was for a wiring contractor to come and look over the house, and quote a price, which might range from 15s. or 20s. per lamp upwards. (A Voice: What is the cost of maintenance? Another Voice: What about the breakage of lamps?) The patents for these lamps, holding up an Edison-Swan, had not expired, but they would do so in the course of the next year or two. Last year they had to pay 5s. each for them. This year they were sold at 3s. 9d. He had reason to believe, however, that when the Edison-Swan patents ran out people would come forward who would be ready to supply that class of lamp for 10d. each. Not only would the lamps be cheaper, but a guarantee would be given for them to run a certain number of hours. With the Edison-Swan lamps, users had to take their chance-they might burn a thousand hours or half an hour. He had tried them himself, and had known one burn several thousand hours, while another lasted only a minute. (A Voice: But they are intended for a thousand.) Yes! (Another Voice: Do you ever test the lamps ?) Of course, and if the EdisonSwan Company sent him one which would not burn, he would return it and obtain another in exchange. In answer to further questions, he explained that it was impossible to test these lamps for a number of hours consecutively, because the result would simply be to shorten their life, or perhaps break them, if the test was very prolonged.

Mr. Warburton, chairman of the Local Board, proposed a vote of thanks to Mr. Metzger for his very interesting lecture. He was sure the audience would be glad to hear from the lecturer whether it would be possible to introduce electric lighting into a scattered district like Hendon, with a reasonable prospect of its proving a pecuniary success.

In reply, Mr. Metzger said the alternate-current system was the best for scattered districts. Hendon district was somewhat longer than it was broad-in fact, it was an oblong. Having roughly sketched a plan on the blackboard, he said that he would suggest that the generating station should be placed near the railway for many reasons. First, because coal could be brought to it cheaply, and secondly, because they would not be liable to be indicted by neighbours as a nuisance on account of vibration. The current would be generated at the moderate pressure of 1,000 volts. (Here Mr. Metzger explained the relation of current to pressure, and the units by which they were denoted; alluding also to the difference between high and low pressure). Two mains would be brought from the station into the centre of the townsay, the Public Hall. Here the 1,000-volt pressure would be converted into the low pressure of 100 volts, electric light companies not being allowed to take 1,000 volts into a house farther than the cellar, because that pressure was thought to be dangerous. The Public Hall would be a sub- or transformer station, and from it mains would be laid under the streets to carry 100 volts pressure. The great advantage of this system, as compared with the low-pressure, was that they could work more economically, and avoid the drop in pressure which occurred with the latter. It was the same as with gas, where the pressure was lowest in houses farthest away from the works. As to the finance question, it was a difficult matter to say what would be the result of adopting electric lighting. If over 3,000 lights were taken up they would work at a profit, but at least this number must be running to make it pay. In a place like Hendon there would be churches, public halls, and schools which would be likely to take the light; and as there were many wealthy people in the place who would be sure to have it as well, he felt quite certain that 3,000 lights would soon be summed up. (A Voice: Of what candle-power?) They would be of 8 c. p. Of course, if any gentleman liked to put a 32-c.p. lamp up in his house, that would be reckoned as four of the 8-c. p. lamps, and so on with the higher power lights. In answer to a question from Mr. Elliott, as to what would be the average cost per mile of the mains, Mr. Metzger said that depended upon the pressure that was put on the cable. He proceeded to explain that in the first instance larger cables than would probably be required for the number of lamps at first taken up would have to be put down, in order that further demands for light might be met without having to take up these mains and lay down larger ones. On the high-pressure system the cable need not be nearly so large as on a low-pressure

one.

(Here he showed a specimen of a 2,000-light cable, and remarked that it would not take up much room in their streets.) Between the station and the Public Hall the cable would be small, because the pressure was high; after that the cables would be slightly increased in size because the pressure was low. (A Voice: Why has that lamp gone out?-alluding to an incandescent lamp over the speaker's head.) Perhaps because the filament the thing which gave the light-had broken. Or, perhaps, from one cause or another, contact at the top of the lamp was broken by contraction or expansion of the parts. If that was so, all that was wanted to make the lamp burn again was to screw it tighter into its socket. In reply to a question from the Chairman as to the size of the 100-volt distributing cables, Mr. Metzger said that this depended upon the number of lights supplied. Electric mains were to a certain extent analogous to water mains. If the latter had to convey water at a very high pressure they were made of small internal diameter, and of stronger material than lowpressure ones. With electric mains for high-pressure work the copper was of small section, but the insulation was of a high class. The extra cost of the insulation, however, would not be so great

as the extra amount of copper that would be required were the mains on a low-pressure system. This was the way they gained by using high pressure.

Mr. Turner having seconded the vote of thanks to Mr. Metzger, the latter gentleman briefly replied, and incidentally mentioned that the weight of the accumulators required to supply current for the lamps in the hall was five tons, a statement which caused considerable amusement. He hoped that the next time he had the pleasure of visiting Hendon they would have the electric light upon a very much larger scale.

Before the meeting separated many of those present examined the fittings, lamps, cut-outs, and cables placed on the platform, the details of each being explained by Mr. Metzger.

INSTITUTION OF ELECTRICAL ENGINEERS.

DISCUSSION ON THE DESCRIPTION OF THE STANDARD VOLT AND AMPERE METER USED AT THE FERRY WORKS, THAMES DITTON. BY CAPTAIN H. R. SANKEY, MEMBER, AND F. V. ANDERSEN, ASSOCIATE. Authorised abstract. (Concluded from page 520.)

Captain Sankey further described the resistance A, used as an intermediary between the standard ohm and the rheostat B, and explained a diagram of it. He showed that there were two mercury contacts at the ends of each strip, and these were in parallel when the strips were in parallel. There were thus 10 contacts in parallel at each end, and calling the resistance of each contact m, and that of each strip s, the resistance in parallel will be 2 m When the strips are in series there are five mercury 10

5

+

contacts in series, and the resistance is 5 s +5 m. The parallel is therefore th of the series resistance, the mercury contacts being eliminated. Recent tests of the resistance, A, when in series, gave 072678, andth of this was 0.0290712. Direct measurement of the parallel resistance gave in two instances 0·029082 and 0·029075. He had tested a Siemens electro-dynamometer by the apparatus, and found its constant to be 28.50. The instrument was then sent to the makers to be recalibrated, the new constant being 28:41. The original constant was 28 67, and hence his determination was equal to the mean of the two given by Messrs. Siemens Bros. Two errors in Table I. of the paper were pointed out. In column 3, line 4, the number 0417 should read 00104, and in column 4, line 3, the number 166 should be replaced by 1.57.

Prof. Ayrton exhibited several diagrams illustrating his previous remarks, and showed several of the latest types of d'Arsonval galvanometers. He expressed his high admiration of the most perfect arrangements provided by Messrs. Willans and Robinson for testing the efficiencies of their steam engines, and he hoped they would not misunderstand him when he made some sugges tions as to how their electrical test apparatus might be modified with advantage. The improvements made in d'Arsonval galvanometers by using curved pole-pieces and freely-suspended coils be illustrated by the calibration curves of two instruments drawn to a very large scale. The deviation from proportionality in the case of the improved form was inappreciable, whilst that of the ordinary type was very pronounced. He also directed attention to the fact that it was not necessary to use a large square sheet of sectional paper for such calibrations, only a narrow diagonal strip cut from the roll at an angle of 45deg. being required. The arrangement of platinoid sheet and d'Arsonval galvanometer used as a standard ampere-meter at the Central Institution, and also the hexagonal multiple scale fitted up at the Acme Works, were illustrated by diagrams, and the secular variation of sensibility of d'Arsonvals were shown by means of tables. Referring to recent forms of instruments he showed two new d'Arsonvals, both provided with adjustable magnetic shunts. One of these had been made by Mr. Pitkin, and the other by Messrs. Nalder Bros. and Co., the coil of the former instrument being wound with "manganin" wire, and that of the latter with platinoid. Both instruments had curved poles and freely-suspended coils. Carpentier's long-range d'Arsonval, an instrument deflecting through 180deg., was exhibited, together with a shunt, by means of which the sensibility could be greatly varied. Speaking of the shape which should be given to the coils of d'Arsonval galvanometers, he called attention to a note on the subject presented to the Physical Society in March, 1890, by Mr. T. Mather, and he exhibited an instrument embodying the improvements there suggested. This galvanometer had a long narrow coil, and no iron core, and, for its winding, was the most sensitive d'Arsonval he (Prof. Ayrton) had yet tested. On the subject of platinoid-sheet rheostats, he said that instead of using a large surface, 6ft. by 3ft. 2in., as Captain Sankey and Mr. Andersen did, it would be better to have two sheets-one for currents, say, from five to 110 amperes, and the other from 50 to 1,100. If the narrow-coil low-resistance instrument mentioned above be used, the former platinoid sheet would require to be about 5in. square and the latter 15in. square; with these the same range of current could be measured as with Captain Sankey's large rheostat, and no temperature corrections whatever need be made to obtain an accuracy of per cent. Further, the expenditure of power when measuring 1,100 amperes would only be 40 watts, whereas the strip described in the paper absorbed about 1,300 watts when measuring the same current. He was glad to see the close agreement in Captain Sankey's measurements of the rheostat