PHILOSOPHICAL TEANSACTIONS. 
I. On a Standard Voltaic Battery. By Latimer Clark, M.I.C.E. 
Communicated by Sir William Thomson, LL.I)., F.B.S. 
Received June 19, — Read June 19, 1873. 
The object which the author had in view in pursuing the investigations alluded to in 
the following paper was to discover some form of voltaic battery which should have a 
perfectly constant electromotive force, and should maintain a uniform difference of 
electric potential between its poles. This want has been much felt by electricians ; and 
the utility of such an investigation may be best shown by a brief reference to the recent 
history of electrical measurement. 
In September 1861 a paper was read by the author before the British Association for 
the Advancement of Science advocating the adoption of a series of standard units of 
electrical measurement, and pointing out the mutual relations which should exist between 
such units. The subject was independently supported in Committee by Sir William 
Thomson, F.R.S., and the result was the appointment of a “ Committee on Standards of 
Electrical Resistance,” and a grant of money was set aside for the purposes of the 
Committee. 
In 1862 the Committee presented their first Report ; their numbers were then enlarged, 
further sums of money were voted for the continuation of their researches, and further 
Reports were presented in 1863, 1864, 1865, and 1867, after which the Committee was 
dissolved. 
The Committee finally recommended the adoption of a system of natural electromag- 
netic units based on the metre and gramme*, in which the unit current flowing through 
a conductor of unit length exerts the unit force on the unit pole at the unit distance. 
As these units were unfitted in magnitude for practical use, certain multiples have been 
adopted in practice and have received names, and are now in almost universal use among 
electricians. These units are : — 
1. Resistance. — The Ohm, equal to 10 7 absolute electromagnetic metre-gramme units. 
2. Capacity. — The Farad, equal to 10 -7 absolute electromagnetic units. 
3. Potential. — The Volt, equal to 10 5 absolute electromagnetic units. 
* They have since adopted the centimetre-gramme unit. 
MDCCCLXXIV. 
B 
