EXCURSIONS OF THE CAPILLARY ELECTROMETER. 
87 
established by a number of observations, is that the calibration error has a much 
greater effect upon the rapidity of the movement of the meniscus than upon the scale 
readings. In most cases the sensitiveness of an electrometer increases as the 
meniscus approaches the tip of the capillary, and its rapidity of action becomes less. 
But this calibration error is entirely under the control of the maker of the 
instrument. 
(2.) Changes of Resistance. —The principal seat of electrical resistance is in the 
slender column of dilute acid between the meniscus and the end of the capillary. 
But since the length of this varies continually during an excursion, it follows that the 
resistance must also vary, and with it, the velocity of the movement, which must 
increase as the meniscus approaches the end of the capillary , where the resistance is 
least. This variation was investigated in a series of experiments. 
A special electrometer was prepared, with a very long capillary, of exactly equal 
sensitiveness for a considerable distance from the tip—twice as much as was required 
for the experiments. It was extremely slow in its action, but was otherwise a very 
fine instrument. 
Fig. 2. 
To measure the time-relations of the excursion the following simple and inexpensive 
rheotome was devised. Upon a base-board A, fig. 2, clamped to the bench so as to 
project beyond it, are fixed two upright pieces B, between which the bar C swings until 
it comes in contact, as it assumes a vertical position, with the cross-bar G, making 
connection in so doing between the terminals H and /, with which it is connected by 
a fine wire. As it is necessary that the circuit should be broken on both sides of the 
electrometer, to prevent “ creeping,” a light wire K, balanced by the counterpoise P, 
is attached to C, and moves with it. This wire is bent so as to dip into two pools of 
mercury, in the block of paraffin L, thus making a connection between the terminals 
M and N, about ’01 sec. before the bar C comes in contact with G. The weight A, 
which is a disc of lead weighing a couple of pounds, is attached, in the manner shown, 
to a string D, passing through a loop on the end of the bar C, over the rod on which 
it swings, to a cleat F. The weight E being drawn forward and then let go, carries 
