586 PROF. J. J. THOMSON AND MR. O. F. C. SSARLE ON THE RATIO OF THE 
speed of the commutator so efficiently that when the condenser was in action the 
spot of light reflected from the mirror of the galvanometer did not move over more 
than half a millimetre. 
The accuracy of measurement of the number of times the condenser was charged 
per second is thus practically the same as the accuracy of the determination of the 
frequency of the tuning-fork ; this frequency could be determined (see infra) to less 
than one part in 10,000. 
The limit which is practically put on the determination of the electromagnetic 
measure of the capacity of the condenser is that imposed by the galvanometer. With 
the galvanometer we employed, which was one made in the laboratory, having 
about 30,000 turns and a resistance of 17,400 legal ohms, when the resistance of 
the variable arm of the Wheatstone’s bridge was 2500 ohms, an alteration of 2 ohms 
could be detected; thus the measurement of the resistance equivalent to the 
repeatedly charged condenser could be made to one part in 1250 : an error of 
this magnitude would cause an error of one part in 2500 in the value of “ r,” 
and as all the other measurements were more accurate than this, there seems no 
reason why this method should not give as accurate a value of “ v ” as that obtained 
for the ohm. 
The electromagnetic way of measuring the capacity affords us the means of testing 
the accuracy of the corrections applied to the electrostatic measure of the capacity; 
we availed ourselves of this in the case of the correction for the effect of the air 
space between the middle cylinder and the guard-ring; we altered the thickness of 
this air space and found that the effect of this alteration was accurately represented 
by the correction we employed. One great advantage of the method is the ease with 
which the number of times per second the condenser is charged can be altered ; this 
affords a valuable means of detecting any leakage or any effect due to self-induction. 
Calculation of the Electrostatic Measure of the Capacity of the Condenser. 
Descripjtion of the Condenser. —The condenser, which was designed some years ago by 
Lord Rayleigh, is represented in section in fig. 1, and in plan in fig. 2. BHPD is a 
thick ebonite board, placed in an approximately horizontal position ; in tliis board two 
concentric circular grooves are cut. A cylindrical brass ring, HP, whose external 
diameter is about 23 cm., and whose height is about 10 cm., fits into the smaller of 
these grooves. Three pieces of ebonite carefully ground down to the same thickness 
(about 3 mm. in most of the experiments), with V-shaped grooves cut in them to 
increase the distance over which the electricity would have to leak are placed at equal 
ntervals on the top of this ring. On these the brass cylinder FGMN is placed ; this 
cylinder is of exactly the same diameter as the cylindrical ring HP, and is about 60 cm. 
long. The cylinders GFMN and HP are placed so that their axes are coincident. On 
the top of this cylinder three pieces of ebonite similar to those on HP are placed, and 
