August 29, 1878] 



NATURE 



47T 



made the first set of experiments with the forty-magnet 

 astatic combination, the idea of this arrangement being to 

 make an approximately spherical mass of little magnets all 

 slightly separated from one another, and all previously mag- 

 netised to saturation. As it would have been too difficult 

 to make this entire sphere all of magnets I finished it oft 

 with segments cut from a little wooden sphere. But the half 

 Napierian logarithmic decrement was 0-12095, ^nd we thought 

 this too high. Consequently, in the interval from June 18 to 

 June 23, I took this astatic combination down, and replaced the 

 segments of the wooden sphere by segments of a small leaden 

 hemispherical shell. This diminished the half Napierian lo- 

 garithmic decrement to 0*07825, and with a periodic time of 

 39*5 seconds I obtained most consistent results. But on the 

 other hand the close agreement of the results obtained on June 

 18 and on June 25 leads one to conclude that the wooden seg- 

 ments were quite satisfactory, and that replacing them with the 

 leaden shell was unnecessary. 



The table at the end of this letter gives the value of 

 the most important constants employed. The battery con- 

 sisted of 382 perfectly new Daniell's cells in series, and the 

 galvanometer had a resistance of 20,000 ohms. All resistances 

 were compared with a new German silver wire box recently 

 received from Messrs. Elliott, London. 



The values obtained for v are (as far as I am aware) up to the 

 present time as follows : — 



MM. Weber and Kohlrausch 



Sir W. Thomson 



Prof. Clerk Maxwell 



Professors Ayrton and Perry 

 Velocity of light, M. Foucault 



31 -074 ohmsi 

 28-2 

 28-8 



29*80 ,, 

 29-8 ,, 



During the last twelve months we have been hard at work 

 with the determination of the electromotive force of contact 

 of metals and liquids, using a new apparatus. Some of 

 the results are, we venture to think, most interesting^ — for instance, 

 the electromotive force of contact of hot and cold mercury, no 

 other conductors being in contact with either mercury • the electro- 

 motive force of contact of a cold metal and hot mercury, no 

 third conductor being in contact with either, &c. The deter- 

 mination of the electromotive force of contact of the pairs of 

 constituents of Mr. Latimer Clark's constant mercurous sulphate 

 cell was most laborious, and occupied me weeks, in consequence 

 of the difference of potential that exists between the body of the 

 mercurous sulphate paste, and the layer of water that floats on the 

 surface. However, a forlorn hope kept me hard atit, and I am 

 glad to say at last I was successful in getting good results. We 

 have gone over all the old ground that furnished the basis of our 

 former paper, as well as much new ground. 



Determination of the Number of Electrostatic Units in an Electro- 

 mas netic one. 



The distance between the upper condenser plate and the guard ring was 

 slightly increased by diminishing the size of the plate to avoid the possibility 

 of loss of electricity. 



June 18. — The lower set of needles was weighted with segments cut from 

 a small wooden sphere. 



June 23 and after. — ^The lower set of needles was weighted with segments 

 cut from a small leaden spherical shell. 



Number of magnets in astatic combination 40. 



Number of new Daniell's cells in series 382. 



General Results of some Recent Experiments upon the Coefficient 

 of Friction between 'Surfaces moving at High Velocities, by Douglas 

 Gallon, C.B., D.C.L., F.R.S., &c. — The author of this paper 

 has been recently engaged in making some experiments upon 

 the coefficient of friction when the surfaces in contact move at 

 high velocities, in connection with the action of brakes in .use on 



railways ; and the results which have been arrived at appear to 

 present some interesting features in respect of the laws which 

 govern the coefficient of friction. 



The experiments were made to ascertain the friction between 

 the brake blocks and the wheels of a railway carriage. 



The levers which move the brake blocks were fitted with 

 dynamometers to show first the pressure which was applied to 

 force the blocks against the wheel, and secondly, the force or 

 tangential strain exerted between the wheel and the block when 

 the latter is pressed against the wheel. The dynamometers used 

 were adaptations of Richards's indicators which act by water 

 pressure which transfers the pressure to cylinders fitted with 

 pistons to which a pencil is attached, so as to register the pres- 

 sure over a travelling sheet of paper, as is used with steam indi- 

 cator diagrams. A dynamometer on a similar principle was 

 attached to the draw bar so as to register the force exerted 

 during the experiment in drawing the carriage. 



The speed was also recorded on diagrams by means of the 

 Westinghouse speed indicator, which also acts by water pressure 

 and depends for its action on the speed of revolution of the 

 axles. 



The carriage or van fitted with the apparatus had two pairs of 

 wheels ; one pair of wheels was fitted with brakes whilst the 

 other pair was free. A speed indicator was attached to each 

 pair of wheels, so that the speed of the carriage could be ascer- 

 tained at any time independently of the speed of the braked 

 wheels. 



To check the Westinghouse speed indicator two of Stroudley's 

 speed indicators were also attached to the van, but these do not 

 register automatically. The distribution of the weight of the 

 van between the two pairs of wheels was obtained, as well as the 

 weight of the wheels and axles themselves ; and in order to 

 ascertain the weight thrown on the braked wheels during the 

 progress of the experiment, a dynamometer fitted to the springs 

 of the van showed the weight at every moment carried on the 

 unbraked wheels, from which information it was easy to deduce 

 the weight on the braked wheels. 



The apparatus was designed by Mr. Westinghouse, and con- 

 structed under his supervision by the Brighton Railway Com- 

 pany, through whose assistance these experiments were carried 

 into effect. 



The effect of applying the brake to the wheels is twofold. 

 So long as the wheels to which brakes are applied continue to 

 revolve at the rate of rotation due to the forward movement of 

 the train, the effect of the blocks is to create retardation by 

 the friction between the block and the wheel ; but when 

 the pressure applied to the blocks causes the friction to 

 exceed the adhesion between the wheels and rail, the rotation 

 of the wheels is arrested, and the wheel becomes fixed and 

 slides on the rail, being held in its fixed position by the brake 

 blocks. 



Therefore the experiments give the coefficient of friction — I, 

 between the brake blocks and the wheel, which is equal to 



the tangential force . 

 the pressure applied' 



2, between the wheel and the rail, which is the 



friction of the brake blocks 

 weight upon the wheels 



It has been generally stated that there is no difference in the 

 coefficient of friction observed in the case of bodies at rest, i.e., 

 in a condition of static friction, and the coefficient of friction 

 in the case of moving bodies, i.e., in a condition of kinetic 

 friction ; but Mr. Fleeming Jenkin, in his paper read before the 

 Royal Society in April, 1877, upon the friction between surfaces 

 moving at very low speeds, alludes to the fact that in all cases 

 where a difference in the coefficient of friction is observed 

 between static and kinetic friction, the static friction exceeds 

 the kinetic. 



Coulomb also points out, in his experiments that in the case of 

 static friction, the coefficient of friction increased with the time 

 during which the bodies had been at rest. 



The experiments of Coulomb, Rennie, Morin, and Jenkin, 

 were made with bodies moving at comparatively low velocities. 



The following table shows the mean results obtained from a 

 large number of the experiments made with the apparatus above 

 de.'^cribed, upon the action between the cast-iron brake blocks 

 and the wheels fitted with steel tyres : — 



