86 
PROFESSOR HUGH L. CALLENDAR ON 
(17.) Electromotive Force of the Clark Cells. 
The electromotive force of the Clark cells was determined by simultaneous 
observations with the potentiometer of the difference of potential on either of the 
standard manganin resistances while the currents were being measured by the 
electrodynamometer. This observation depended to some extent on the accuracy 
of calibration of the potentiometer, but this was repeatedly tested with results 
consistent to 1 part in 50,000. Owing to the great steadiness of the current, 
there being no silver voltameter in circuit, the potentiometer readings could 
easily be taken to this order of accuracy. The Clark cells were of the hermetically 
sealed type described in the 4 Proceedings of the Royal Society’ for October, 1897, 
and were kept in a well-stirred water-bath at a constant temperature. 
The results of the first series of observations on the absolute value of the E.M.F. oi 
the Clark cell in terms of the ohm, taken with this apparatus during the spring and 
summer of 1898, were sent in by Mr. R. O. King as the report of his third year’s 
work as 1851 Exhibition Scholar. When certain minor corrections are applied for 
the values of the resistance coils and for the higher harmonics in the series repre¬ 
senting the force between the fixed and suspended coils, the result found for the 
E.M.F. of the Clark cell at 15° C is, 
Clark cell = P4334 volts, 
the volt being defined as the potential difference due to a current of one-tenth of a 
C.G.S. unit through a standard ohm. This result was not immediately published, 
because Mr. King hoped to be able to secure an order of agreement higher than 
1 part in 10,000 in the comparison of the small coils with the large coils, which was 
much the most difficult part of the measurements. But it may be questioned 
whether a higher order of accuracy could reasonably be expected in this determi¬ 
nation, and although the value above given was not at the time intended to be final, 
I feel that it may be regarded with considerable confidence on account of the high 
order of accuracy of the individual measurements, and the many new devices which 
were introduced into the design of the apparatus. 
The work of setting up the apparatus and taking the observations was performed 
almost exclusively by Mr. King under my supervision, with occasional assistance 
from other students in taking readings. I was able to assist him personally during 
the vacations in some of the more important measurements, such as the graduation 
of the copper tape on the comparator, the first winding of the large coils, and the 
first comparison of the small coils. I also verified the accuracy of most of the 
adjustments, and the perfection of the insulation, and am satisfied that the whole 
work was most carefully and systematically carried out. Mr. King has since left 
McGill College, but I hope that he may yet find time to work out and publish in 
detail the final results of his observations, which should form a valuable contribution 
