170 Pro P. Ayrton and Mr. Hay era ft on an Apparatus for 



Rowland (friction) 4' 189 x 10 7 or 778, 



Dieterici (electrical) 4' 232 x 10 7 or 786, 



Micnlescu (friction) 4-186 x 10 7 or 778, 



Griffiths (electrical) 4*198 x 10 7 or 780, 

 Schuster "J 



and V (electrical) 4-192 X 10 7 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 pro- 

 bably 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 elec- 

 trical 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 pre- 

 sented in July 1891, and contained the following recommen- 

 dations 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 centi- 

 metres, 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 

 current 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 resolu- 

 tions 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 



