SECTION III., 1900 eae] Trans. R. S. C. 
Vi.—WNote on the Relation of the Electrical and Mechanical Units. 
By Howarp T. Barnes, M.A.Sc., D.Se. 
(Communicated by Prof. John Cox and read May 29, 1900.) 
In view of the immense amount of labour expended in establishing 
the international electrical units, volt, ohm and ampere, in absolute 
measure, it is a matter of some interest to compare a determination of 
some physical constant made in terms of these units with a determina- 
tion of the same constant in terms of the mechanical units. Such a 
comparison has already been made between the value of the mechanical 
equivalent of heat as determined by Rowland, with that measured in 
terms of the electrical units by Griffiths, and independently by Schuster 
and Gannon. ‘The values obtained by both these latter investigators 
both show as a result of the comparison with Rowland’s measurements 
an error in one of the electrical constants. 
Quite recently I have been enabled to complete an extensive in- 
vestigation into the value of the mechanical equivalent of heat in terms 
of the electrical units which it has been possible, through the superior 
equipment of the Macdonald Physical Laboratory of McGill University, 
to extend over the entire range of temperature between the freezing 
and boiling points for water, obtaining exceedingly accurate measure- 
ments of the thermal capacity at different temperatures. The method 
which I have used, due to Prof. Callendar, differs essentially from that 
used by both Griffiths and Schuster and Gannon. However, a compari- 
son of my absolute value with that of Rowland has shown a similar error 
in one of the electrical units. In addition to a comparison of the 
mechanical equivalent at one temperature with that of Rowland, it is 
possible, owing to the great range of my experiments, to compare the 
value of the mean mechanical equivalent between 0° and 100° C., with 
the value obtained recently by Reynolds and Moorby by a direct mechani- 
cal method. This comparison also shows that the value which has been 
hitherto assumed for one of the electrical units is in error. The amount 
of the error from the comparison with the experiments of Rowland is 
‘04 per cent, and from the experiments of Reynolds and Moorby is ‘066 
per cent, both in the same direction as regards the unit in question. 
Of the three electrical units the ohm has attracted the largest 
amount of attention, not only in England, but also amongst physicists 
in Germany and America. The first and most important step towards 
the establishment of this unit was made in England in 1861, when a 
Sec. III., 1900. 5. 
