72 ROYAL SOCIETY OF CANADA 
committee appointed by the British Association undertook to fix the 
standard of electrical resistance. They decided that the practical unit 
should be defined as 10° C. G. S. units in Weber’s absolute system. 
Various coils of wire were constructed and determined in absolute 
measure. Copies of these coils were made and distributed and became 
known as the B. A. unit of resistance. Although this unit was adopted 
in England, on the continent another unit was in vogue which was 
established by Werner von Siemens, and known as the Siemens unit. 
This was equal to the resistance of a column of mercury one metre long 
and one square millimetre in cross section. 
In 1878, Rowland redetermined the value of the B. A. unit by an 
absolute method proposed by Kirchoff, and showed that it was appar- 
ently in error by as much as 1 per cent. 
In 1883, and then a few years later, Glazebrook made some deter- 
minations by the same method used by Rowland and verified his results. 
The mean of his measurements gave a value of the B. A. unit equal to 
98665 < 10° @.°G. S. units. 
This has been subsequently verified, both in Germany and else- 
where, and the unit so defined has been universally adopted as the true 
or international ohm. 
The ampere, equal to 10—1 C. G. 8. units, was determined in ab- 
solute measure by Lord Rayleigh and Mrs. Sedgwick in 1888, who ex- 
pressed it for practical purposes in terms of the electro-chemical equiva- 
lent of silver. Shortly after, the British Association entrusted Glaze- 
brook and Skinner to establish the volt, 18° C. G. S. units, in terms of 
the E. M. F. of the Clark cell. They determined this in terms of the 
true ohm and ampere as defined by Lord Rayleigh and Mrs. Sedgwick’s 
value of the electro-chemical equivalent of silver, and it was found that 
the international volt could be expressed as equal to 42992 of the E. M. 
F. of the Clark cell. 
In measuring the quantity of energy which was required to warm 
the water in my determinations of the mechanical equivalent of heat, 
it was necessary to measure the difference of potential across the ter- 
minals of a platinum wire immersed in the water, and to measure the 
current of electricity flowing through this wire. Instead of measuring 
the latter by the electro-chemical equivalent of silver, I included a 
specially constructed wire resistance coil in the electrical circuit which 
I could standardize by direct comparisons with a large number of certi- 
fied resistances from the electrical standards committee of the British 
Association. A measure of the difference of potential across this re- 
sistance gave at once the value of the current flowing in the circuit. 
The difference of potential on this as well as that on the electric heating 
