720 MR. J. J. THOMSON ON THE NUMBER OF ELECTROSTATIC 
used to drive another of about four times its frequency, and the number of beats per 
second between this driven fork and Lord Rayleigh’s standard fork was counted. 
At the temperature of 15° C. there were 12 beats in 20 seconds between the two 
forks, and the standard fork vibrated more slowly than the other. The standard fork 
makes 128T5 vibrations per second, so that if n be the number of vibrations per 
second of the fork used to drive the commutator, we have 
4nX 20—12=128*15 X 20 
ft= 32T875. 
Hie observations. 
The observations consisted of two parts. The capacity of the movable condenser 
had to be adjusted until it was equal to the capacity of the guard ring condenser. 
This was ascertained by the method described hi Part II. ; and then this adjustable 
condenser was put in the Wheatstone’s bridge as in fig. 1, and the resistances of the 
arms of the bridge adjusted so that the deflection of the galvanometer due to the 
steady current was just balanced by the deflection due to the intermittent current 
arising from the flow of electricity to the condenser when the movable piece P was 
in contact with S. The resistances in the arms A D, B C (fig. 1) were kept constant, 
and the adjustment was effected by altering the resistance in A C. 
The steady current, when it was not balanced by the current arising from the 
charging of the condenser, produced a deflection of the dot of light reflected from 
the mirror of the galvanometer of about 120 scale divisions, and as a fine wire was 
placed before the lamp of the galvanometer and focussed on the scale, readings could 
easily be made to quarter of a division. 
The following are the results of the observations, and it may be worthy of remark 
that, as many of the pieces of apparatus used were required for the ordinary work of 
the laboratory, the whole arrangement had to be taken down and put together again 
between each determination. This must have had the effect of getting rid of a good 
many accidental errors, and taking it into consideration the following numbers seem 
as near together as could be expected for such complex observations. The resistances 
are given in B.A. units. 
