9S 
MR. G. J. BURCH ON THE TIME-RELATIONS OF THE 
with a rheotome furnished with four adjustable break-keys of the tip-over type. 
The first of these, when struck, opened a short circuit on the primary of A, allowing 
the current to act upon that rheochord. The second broke the battery circuit of A, 
and at the same instant the third in like manner opened a short circuit on the 
primary of the second rheochord B, in which the current circulated in the opposite 
direction. About ’005 sec. later the battery of B was thrown out of action by the 
fourth key, thus checking the excursion produced by the second current. It will be 
observed that the electrometer circuit remained unbroken throughout the experiment. 
Unfortunately the short-circuit contacts were not good, so that a certain amount of 
current passed, rendering it impossible to ascertain the exact value of the differences 
of potential which acted upon the electrometer, in several cases—and, moreover, the 
Daniell’s cells used were not sufficiently steady during the operations, which took 
a long time, to enable all the results to be compared with those previously obtained. 
The experiments were varied by reversing the direction of both currents, and 
sometimes omitting the second. 
No. 306. Fig. 6, Plate 3.—“ Artificial Spike,” produced by two short currents of opposite direction. 
Up, '0055 sec.; subnormal = 50 centims. Down, '0045 sec.; subnormal = 60 centims. 
It will be observed that the effect of the second current was to bring the meniscus 
back almost exactly to its former level, and that both the rise and fall commenced 
and ended suddenly, indicating, as do also the measurements of the subnormal at 
close intervals throughout the curve, that it was produced by the introduction of 
differences of potential which remained constant while they lasted. But the move¬ 
ment was in each case only about one-tenth of the full excursion for that difference of 
potential, and the quantity of electricity received as “ charge,” viewing the electro¬ 
meter as a condenser, is approximately proportionate to the product of the time 
into the difference of potential, or what comes to the same thing, the subnormal, i.e .:— 
'0055 x 50 = '275 for tbe upward movement. 
'0045 x 60 = '270 for tbe downward movement. 
But these “ charges may be measured in another way. The electrical capacity of 
an electrometer is perfectly definite in amount, being conditioned by the shape and 
size of the capillary, and perfectly independent of the difference of potential. The 
quantity received by an instrument from any current is therefore proportional to the 
product of its capacity (which is constant throughout the part of the capillary used) 
into the distance l, through which the meniscus is moved by it. The effect of two 
successive charges of equal quantity, but opposite in sign, must be to leave the 
mercury ultimately at zero, as was the case here. Not only was there this close 
correspondence between the values obtained in these two ways for the “ charge,” but 
the Total Indicated Differences of Potential, calculated from the subnormals, were 
within one per cent, of the values found by comparing the two rheochords emploj^ed, 
with that used for Curve No. 289. 
