82 
MR. G. J. BURCH ON THE TIME-RELATIONS OF THE 
General Data with respect to the Time-Relations of an Excursion. 
(1.) Dilution of the acid beyond a certain point was found to lessen the rapidity 
of an excursion without altering the extent of it. This indicated that the velocity of 
the movement was affected by electrical resistance. 
A similar result was also produced by the introduction of 100,000 ohms into the 
external circuit. 
(2.) Shortening the capillary so as to reduce the length of the acid column made 
the instrument act more quickly, but in this case the mechanical friction, as well as 
the electrical resistance, was lessened. 
(3.) The shape of the tube, where it tapers to form the capillary, was found to have 
a marked effect; the motion of the meniscus being much more sluggish if this part 
was made too long. In this case the increase of electrical resistance must be com¬ 
paratively trifling, and the result is to be ascribed mainly to friction and eddy 
currents. 
(4.) The same may be said of the orifice, a clean-cut capillary being usually much 
quicker in its action than a splintered one. 
These things may be taken as indicating the action of both mechanical friction and 
electrical resistance in determining the rate of movement of the meniscus. 
The next step was to ascertain whether there was any latent period before the 
commencement of the rise or fall. This was done by photography. 
A difference of potential was suddenly communicated to the instrument by striking- 
open a short-circuiting key, the end of which projected across the slit upon which the 
image of the mercury column was thrown, and so gave upon the sensitive plate, as it 
passed rapidly behind it, a record of the exact instant at which the current was 
allowed to act upon the electrometer. In no case could the smallest interval be 
detected between the opening of the key and the commencement of the excursion, 
even when the difference of potential employed was very small. 
The converse of this was found to be true under ordinary circumstances, that is to 
say, the meniscus ceases to move the moment the source of electromotive force is 
withdrawn. In other words, the electrometer is practically dead-beat. It has been 
generally held to be perfectly so, but this is not the case. If the instrument is one 
that has been specially designed to act with great rapidity, it will be found, on com 
municating to it a fairly strong charge from a condenser of not more than one-third of 
the capacity of the electrometer, that the meniscus will start suddenly forward, and 
then slowly return, perhaps as much as one-tenth of the distance it has traversed. 
But this overshooting is entirely prevented by the introduction of an external 
resistance of, say, 50,000 ohms. Apparently the reason why this phenomenon has not 
been noticed is, that experimenters have contented themselves with observing the 
effect of suddenly breaking the circuit during an excursion; but in so doing, they 
have introduced an infinite resistance. In the case of the condenser experiment, there 
