172 F. Womack on a Method of Determining the 



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 centi- 

 metres 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 there- 

 fore safely say that the ohm, as now defined, does not differ 

 from 10 9 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 determi- 

 nation of Lord Ra3 r leigh and Mrs. Sidgwick giving '0011179 

 gramme of silver per coulomb, that of Kolilrausch '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 10 7 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 deter- 

 mining the mechanical equivalent of heat should be the most 

 accurate. 



T 



XVI. A Modification of the Ballistic- Galvanometer Method of 

 Determining the Electromagnetic Capacity of a Condenser. 

 By F. Womack*. 



'HE 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 6 

 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. 

 * Communicated by the Physical Society: read November 23, 1894, 



