68 
PROFESSOR HUGH L. CALLENDAR ON 
galvanometer was adjusted to give a deflection of 167 scale-divisions on reversal for 
a change of 1 ohm in 2000 with a time period of 5 seconds, and remained constant 
to less than one scale-division throughout the test of the whole box, which occupied 
only an hour and a hall. 
The observations and results of the two calibrations for the first half of the slide- 
box are compared in the following table. The first column contains the reading of 
the slider on the main dial. The second column the observed deflection of the 
galvanometer cl in equation (l) reduced to ohms by dividing by s. Since s = 167 in 
the second series, one unit in the second decimal place of d/s corresponds to nearly 
2 scale-divisions deflection observed. The observations were taken to half a scale- 
division, but owing to slight variations of sensitiveness and zero it was not considered 
worth while to work the values of d/s beyond the nearest hundredth of an ohm. 
The next column gives the error c?R in ohms of each separate resistance of the main 
dial in terms of the mean of the whole, deduced from equations (1) by the aid 
of (2) and (3). The fourth column gives the correction dn in ohms to the reading at 
each point. This correction is equal to the sum of the errors of all the coils up 
to the point considered, subject only to a small correction, called the “ vernier- 
correction.” to allow for the fact that the next two coils are shunted by the vernier 
J 
dial. The value of the vernier-correction is given by the following expression— 
Vernier-correction to reading n = — n (‘38 — 3cZR ;i+1 — 3c/R„ +; ,)/400. 
This correction is often negligible when n is small, but sometimes reaches '5 or '6 
of an ohm near the higher readings. The next three columns in the table give the 
corresponding values of the same quantities d/s. eTR, and dn, deduced from the 
readings taken during the second calibration. Comparing the two sets it will be 
observed that the discrepancy very rarely exceeds half an ohm, which is only one 
part in 200,000 of the whole resistance. 
