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and (electrical) 4.192 × 107 or 779. Gannon.
The result obtained by our students is greater than the mean of these results by just one half per cent. It is probably not generally known that such accuracy can be obtained by the use of such simple apparatus as that here described, and the fact is we think sufficient justification for bringing the result under the notice of the Society.
V. Recent History of the Practical Electrical Units.
A short account of the recent history of the practical electrical units is added for the benefit of those not familiar with the work of the Board of Trade Standards Committee.
The first report of the Electrical Standards Committee, appointed by the Board of Trade at the end of 1890, was presented in July 1891, and contained the following recommendations among others :
"4. That the resistance offered to an unvarying electric current by a column of mercury of a constant cross sectional area of one square millimetre and of a length of 106.3 centimetres, at the temperature of melting ice, may be adopted as one ohm."
"10. That an unvarying current, which when passed through a solution of nitrate of silver in water, in accordance with the specification attached to this Report, deposits silver at the rate of 0.001118 of a gramme per second, may be taken as a currens of one ampere."
At the British Association meeting held at Edinburgh in August 1892 the B.A. Committee on Electrical Standards, in consultation with Dr. von Helmholtz, M. Guilleaume, Professor Carhart, and others, agreed to the following resolutions among others :
"2. That 14-4521 grammes of mercury in the form of a column of uniform cross section 106.3 centimetres in length at 0° C. be the specified column of mercury adopted as the practical unit of resistance."
"That the number 001118 should be adopted as the number of grammes of silver deposited per second from a neutral solution of nitrate of silver by a current of 1 ampere, and
the value 1.434 as the electromotive force in volts of a Clark cell at 15° C."
These decisions agreed with those which had already been laid before the German Government by Dr. Helmholtz and the Curatorium of the Reichsanstalt. It was considered preferable to define the mercury column in terms of length and mass, as the sectional area cannot be directly measured with such accuracy; but the length 106-3 centimetres was adopted so that the sectional area might be, as nearly as could be known, one square millimetre, as this area has been specified in all previous definitions.
In November 1892 a supplementary report was presented to the Board of Trade by its Standards Committee, and included the following resolutions :
"4. That the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14-4521 grammes in mass of a constant cross sectional area, and of a length of 106-3 centimetres, may be adopted as one ohm."
"10. That an unvarying current which, when passed through a solution of nitrate of silver in water, in accordance with the specification attached to this Report, deposits silver at the rate of 0.001118 of a gramme per second, may be taken as a current of one ampere.
"14. That the electrical pressure at a temperature of 15° Centigrade between the poles or electrodes of the voltaic cell known as Clark's cell, prepared in accordance with the specification attached to this Report, may be taken as not differing from a pressure of 1.434 volts by more than one part in one thousand."
At the International Electrical Congress held in August 1893 at Chicago these values were adopted by the delegates as those to be recommended to their several governments as legal units.
The Board of Trade Committee, after having had laid before them the recommendation of the Electrical Congress, which was substantially that contained in their supplementary report of November 1892, presented their final report advising a legalization of the standards as defined, by an Order of Her Majesty in Council; and on August 23rd, 1894, this was effected, and the practical standards we may hope permanently settled.
Of the three units-ohm, ampere, and volt-the two former are defined on the C.G.S. system, and the latter as a secondary unit from the relation of the first two. The numbers given in the above-quoted resolutions as to the ohm and the ampere
are the values which appear to represent most accurately the true volt and the true ampere. In the Report of the B.A. Standards Committee for 1892 is given a table showing the results of nine determinations of the ohm expressed in centimetres of mercury, made between 1882 and 1892; the mean of these results being 106.31, and the average deviation from this mean being 015, or one part in 7000. We may therefore safely say that the ohm, as now defined, does not differ from 109 C.G.S. units of resistance by more than one part in two or three thousand.
As regards the ampere, we have the very careful determination of Lord Rayleigh and Mrs. Sidgwick giving 0011179 gramme of silver per coulomb, that of Kohlrausch 0011183, Gray 001118, and Potier and Pellat 0011192. It is at least probable that the value 001118 adopted by the Board of Trade Committee does not differ from that corresponding to the absolute C.G.S. unit divided by ten by more than one part in a thousand.
Hence it is probable that the number of ergs equivalent to one watt-second does not differ from 107 by more than one part in a thousand; and that being the case it would seem that, considering the great accuracy with which electrical measurements can be made, the electrical method of determining the mechanical equivalent of heat should be the most accurate.
XVI. A Modification of the Ballistic-Galvanometer Method of Determining the Electromagnetic Capacity of a Condenser. By F. WOMACK*.
THE method consists in placing the condenser in parallel with one arm S of a Wheatstone-bridge arrangement of non-inductive resistances. A balance for steady currents having been obtained, the condenser is placed in circuit, and the throw determined due to the depression of the battery-key. The condenser is then thrown out of circuit, and the proportionality of the arms of the bridge disturbed by changing the value of S. The steady deflexion a due to this want of balance is read. From these two readings and the known values of S, the capacity is immediately determined.
(x + y
* Communicated by the Physical Society; read November 23, 1894,
If x, y, z, w denote the integral cyclic currents reckoned from the instant of make,
P(x+y-w) + Ga +R(x+y)=0
P(w-x-y) + Bw
Substituting for z in the second equation and rearranging,
To find the steady current ý, ultimately flowing through S we have the same equations, except that the right-hand side of (ii.) is zero, and , y, w instead of x, y, w.
x= (PB+PR+QR+BR) (PQ+GQ+RQ+PG).
When the balance is disturbed by S becoming S+dS, the
assuming the value of the determinant A to be but little.
affected by the change of S.
X T 2 sin 0/2
x correction for damping
CS2 (PB+PR+QR+BR)(PQ+GQ + RQ+PG)
In the practice of the method, in order to avoid error arising from the altered value of ▲ due to the change of S, readings of the deflexion, a may be taken with equal positive and negative values of dS. Thus an error which otherwise might be as great as 1 in 100 is reduced to 1 in 101.
In order to avoid change of the E.M.F of the battery due to its being for any appreciable time on open circuit, it is desirable to use a reversing-key in the battery branch, and to observe the throw of the galvanometer due to reversal of the sign of the current in S and immediately after observe the deflexion due to the alteration of S.
The method is necessarily subject to the same disadvantages as that of Fleeming Jenkin, in respect of it not giving the instantaneous capacity of the condenser. But measurements of the same condenser taken with different galvanometers, or with a graded moment of inertia of the galvanometer-needle, have led to nearly constant results, the capacity being apparently but little dependent on the period of charging and