FORCE OF THE WESTON NORMAL CELL IN SEMI-ABSOLUTE VOLTS. 
197 
In conclusion the writer desires to express his gratitude to Mr. R. O. King, M.Sc. 
for providing the fellowship which made this research possible. 
The author is especially indebted to Prof. H. T. Barnes, F.R.S., Director of the 
Macdonald Physics Building, McGill University. The work was undertaken at his 
suggestion, and throughout the investigation he was a constant source of advice, 
personal help and kind encouragement. 
Many thanks are due also to Dr. H. L. Bronson and to Mr. F. H. Day, M.Sc., for 
assistance in taking simultaneous observations. 
APPENDIX. 
Note on the Calibration of the Kelvin-Yarley Slide for Accurate 
Electrical Measurements. 
The Kelvin-Varley slide has been described in many electrical works,* and is well 
known as a valuable and accurate instrument. It has been found that an exceptional 
accuracy can be obtained if refined calibrations are occasionally performed. After 
making several trials of various methods, Prof. Callendar, in connection with his 
research on “Continuous Electrical Calorimetry,”f briefly outlined the one which he 
considered to be the most convenient and accurate. This method with some slight 
modifications was adopted; and the table of corrections given on the next page is 
analogous to the tables on pp. 69 and 70 of the paper quoted. “ n” represents the 
slide reading. 
The validity of these corrections was tested both before and after the use of the 
instrument in the investigation. A comparison of the figures given in this table with 
those obtained by Prof. Callendar ( loc. cit.) for the same instrument in 1894 shows 
that a calibration of this kind must be repeated from time to time in order to maintain 
the same degree of accuracy in the use of the instrument. The slow alterations in 
the coils can attain considerable magnitude and ultimately render old calibrations 
quite useless. The changes are due, usually, to an average effect rather than to 
especial weakness in some particular coil or coils. It is thus possible to get a good 
indication of the condition of the whole box by taking and comparing readings for 
several ratios measured first from one end of the slide and then from the other. At 
the same time it is at once apparent in such a test, if the variation happens to be a 
purely local one. For measurements requiring an accuracy of more than 1 part in 
10,000 we should not rely upon a calibration which is older than a year. 
* Price, ‘Measurement of Eleotrical Resistance,’ p. 106; Fleming, ‘Handbook for the Electrical 
Laboratory and Testing Room,’ vol. 1, p. 273; Munro and Jamieson, ‘Pocket Book of Electrical Rules 
and Tables,’ p. 158; Kempe, ‘Handbook of Electrical Testing,’ p. 219; Aspinall Parr, ‘Practical 
Electrical Testing,’ p. 321, &e., &c. 
t ‘Phil. Trans,’ A, vol. 199, 1902, p. 65. 
