174 REPORT—1863. 
and if we take values of 6 at small intervals dt and sum them, we shall get 
fidt=C fv dt=Ca, 
where w is the whole distance travelled in the time. 
Hence all we have to do is to observe the deviation at every oscillation, 
and to ascertain the whole number of revolutions during the time of observa- 
tion, and the exact beginning and ending of that time. This was done in the 
following way. 
The coil was made to revolve by means of the driving machine, and its 
velocity was regulated by the governor. While the required velocity was 
being attained, the oscillations of the magnet were reduced within convenient 
limits by means of a quieting bar at a distance. The quieting bar was then 
put in its proper place and the observation commenced. 
One observer, A, took the readings of the scale as seen in the telescope, 
writing down the deviation at the extremity of every oscillation, and thus 
obtaining a reading every 9°6 seconds. 
Another observer, B, with a chronometer, wrote down the times of every 
third stroke of the bell. The times thus found were at intervals of 300 
revolutions. When the observer B noted the time, the observer A made a 
mark on his paper, so that after the experiment the readings of deviation 
could be compared with the readings of the chronometer taken at the same 
time. 
The mean time of revolution between any two times of observation could 
thus be found and compared with the mean deviation between the same 
limits of time, and any portion of an experiment accidentally vitiated could 
be rejected by itself. 
The experiments of each day commenced with a comparison by means of 
an electric balance* between the resistance of the experimental coil and that 
of a German-silver coil (called «June 4”). 
Then a series of readings of the scale was taken to determine the undis- 
turbed position of the magnet. The times of beginning and ending this 
series were noted, and called Times of 1st Zero. 
Then the coil was made to revolve, and readings of deviation and of time 
were taken as already described, and called Ist Spin+. 
Then the direction of rotation was reversed and a second set of readings 
obtained, and called*2nd Spin—. 
Then the undisturbed position was again observed with a note of the time. 
This was called 2nd Zero. 
Lastly, the resistance was compared again with the standard coil. This 
series of experiments was then repeated if there was time. 
From the values of Ist zero and 2nd zero, together with the information 
obtained from the photographic registers at Kew, the true value of the un- 
disturbed reading during the 1st spin and 2nd spin was obtained. The dif- 
ference between this and the actual reading is the deviation 6 due to the elec- 
tric currents. T was got by the chronometer readings. Now let r be the 
resistance of the standard coil at standard temperature, R the resistance of 
the experimental coil during the experiment, then by the comparison of re- 
sistances we find ~ 
R=av, 
where w is the ratio observed by means of the electric balance. But we also 
* Vide Report, 1862, p. 159, and present Appendix, p. 166. 
