66 
PROFESSOR HUGH L. CALLENDAR ON 
Table I. — Verification of Correction at Middle Point of Slide-Box. 
Date. 
Observers. 
Correction 
(ohms in 50,000). 
February, 1894 . . . . 
C ALLEND AK. 
-3-8 
December 20th, 1894. . . 
Callendar. 
-3-96 
January 29th, 1895 . . . 
King. 
-3-77 
November 24th, 1896. 
Thomson and Stovel. 
-4-2 
February 2nd, 1897 . 
Blair and Macdonald. 
-3-72 
March 4th, 1897 .... 
Pitcher and Edwards. 
-3-50 
April 22nd, 1898 .... 
Stovel. 
-3-43 
January 27th, 1899 . . . 
Barnes. 
-3-95 
Some of the above observations were taken by fourth-year students in the course 
of their work, but in the majority of cases I personally verified the readings and 
results at the time of entry. 
(5.) Me th ocl of Calibration. 
In the calibration of the slide-box, the point of most importance was to determine 
the correction for each reading of the main dial, i.e., at 100 equidistant points of the 
whole range. The vernier dial was so small in comparison that the errors of its 
individual coils were negligible in their efiect on the whole reading, although it was 
necessary at each point to take account of the difference of resistance of the whole 
vernier dial and the pair of coils shunted by it in any position of the slider. 
After several trials of various methods extending over nearly a month, I came to 
the conclusion that the most convenient and accurate method of performing the 
calibration was to determine the relative values of the coils of the main dial in pairs 
by comparison with the 2000 ohms of the vernier dial. The flexible copper cable 
connecting the terminals D and D' was disconnected, and the terminals were 
connected to a galvanometer and to a pair of exactly similar resistances of 2000 ohms 
each forming the ratio arms P and Q of a Wheatstone bridge, the other two arms of 
which were the vernier dial S and any pair of consecutive coils R„ and R„ + 1 of the 
main dial. A battery of two storage cells, selected for constancy, was connected to 
the point between the ratio arms and to the terminals CC'. The deflection d„ of the 
galvanometer corresponding to any setting of the slider was proportional to the 
difference of the sum of the corresponding pair of coils R„ and R„ + 1 of the main dial 
from a unit SP/Q, which was approximately 2000 ohms, and remained constant 
throughout the comparisons. The. value of this deflection was reduced to ohms by 
observing the change of galvanometer deflection s produced by a change of 1 ohm in 
one of the arms. This observation was repeated at intervals during the calibration. 
