90 
MR. G. J. BURCH OH THE TIME-RELATIONS OF THE 
5. Charge. External Resistance = 60,000 ohms. Up. 
Period of a single swing- of the rheotome = ‘77 sec. 
Swings. 
l 
y ■ 
Ratio. 
1 
62-5 
42-5 
3 
51 
31 
1-37 
5 
44 - 4 
22-4 
1-38 
7 
36 
16 
1-40 
9 
31 
11 
1-45 
N.B.—In the last three series of observations the period of the swing of the rheo¬ 
tome was made longer, in order to effect a better distribution of the measured points. 
On plotting the results of identical excursions, the curves were found to coincide with 
those obtained with a swing of '36 sec., proving the accuracy of the method to be 
sufficient for the purpose of these experiments. 
The above experiments indicate— 
(a.) That*the time-relations of charge and discharge are practically alike, as is 
shown also by the analysis of the photographed excursions. 
( b .) That, in this particular instrument, the meniscus moved more quickly away 
from the tip of the capillary than towards it. No such difference could be detected 
in the case of electrometers of ordinary quickness of action, and the phenomenon 
appears to be peculiar to those of great sensitiveness and slow action. In some 
instruments it was reversed, and the mercury would only recede when assisted by 
tapping the stand, though it would advance freely with a small difference of potential. 
(c.) That the change of the length of the acid column during an excursion lias 
a measurable effect upon the time-relations of the movement, which, however, is much 
less than it would be if the normal velocity were conditioned simply by the internal 
electrical resistance of the electrometer. It was found by experiments with more than 
twenty instruments that the effect of an added external resistance in increasing the 
time of “half-charge” was such, that they might be assumed to have an internal 
resistance of the form 
R = r(L + Z), 
where l = the distance of the meniscus at any moment from the tip of the capillary, 
and L is a constant of the particular instrument employed. In practice, L is many 
times greater than the maximum values of l, so that the variation of the speed-ratio 
from this cause is less than 3 per cent, over the entire width of the negatives referred 
to in this paper, and consequently not more thnnUrper cent, in the physiological 
curves to which the method of analysis was applied. 
To sum up :— 
The two causes which modify the time-relations of an excursion, act in opposite 
ways. 
