FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
195 
X. The Standard Cells. 
An account of the equipment of the laboratory with regard to standard cells has 
been given by Dr. H. L. Bronson and the present writer in a paper entitled : “Clark 
and Weston Standard Cells,” which was published in ‘The Electrician/ February 10, 
1911. 
During the deflection observations the electromotive force of the two cells P t and 
P 2 which were used in the determination, was repeatedly checked by comparison with 
a set of ten standards of especial constancy and reliability, viz., S 20, M 10, IIP, IV., 
A, B, N, 0, Q 4 , and Q 5 , all of which are discussed in the paper quoted. S 20 and M 10 
were constructed at the National Physical Laboratory, III. and IV. at the Bureau of 
Standards, and A, B, N, O, Q 4 , and Q 5 at McGill. 
Comparisons were made with the Bureau of Standards in 1908 and 1909, and with 
the National Physical Laboratory in 1909 and 1912. It has, therefore, been possible 
to express the mean of our cells in terms of the international mean for the Weston 
normal cell. The electromotive force of each of the cells P x and P 2 at the time of the 
deflection observations was 14 microvolts lower than the mean of the standards 
enumerated above, and the mean of these standards was found to be between 10 and 
20 microvolts lower than the international mean. Calculations based on the exchanges 
with the National Physical Laboratory gave the larger figure, and those based on the 
exchanges with the Bureau of Standards gave the lower figure. We have assumed, 
therefore, that the two cells P x and P 2 were each 30 microvolts lower than the inter¬ 
national mean value for the Weston normal cell. 
XI. Insulation and Magnetic Tests. 
Both systems of coils were wound with double winding so that it was always 
possible to perform a complete test on the insulation. In winter the dry Montreal 
atmosphere rendered insulation precautions unnecessary, but in summer it was found 
convenient to maintain the room at a higher temperature than the surrounding rooms 
and passages, in order to keep the connections and the instruments in the same dry 
condition as in winter. Tests were repeated throughout the investigation, and it was 
found that an insulation resistance of at least four times the necessary magnitude 
could always be maintained. 
The framework of the dynamometer had been tested for magnetic qualities by 
Mr. It. O. King when the instrument was first set up, and found to be quite satis¬ 
factory. The influence of the covers, which were made for the sides and the ends of 
the dynamometer, was determined by comparing deflection observations taken with 
and then without the covers on the instrument. In the latter test air-currents were 
kept out by screens of paper at a distance of about two feet from the dynamometer. 
No difference in the deflections was observed. In the same way the presence of the 
