DETERMINATION OF THE SPECIFIC HEAT OF WATER. 453 
diminish in value, owing to (1) change in the values of the different resistances in cir- 
cuit, due to change of temperature produced by the passage of the current ; (2) change 
of the resistance of the silver voltameter ; (3) running down of the electromotive 
force of the storage cells. In order to counteract the effects of these changes, a 
number of separate resistances, W,, W., W3, and two switches, s, and s, (fig. 5), were 
placed in series with the remainder of the circuit. W, consisted of coils of platinoid 
wire; W, and W, were platinoid wires of different resistance-values, any number of 
which could be connected in series by means of mercury cups and copper bridges. 
The resistances W,, W., W, enabled the current to be adjusted to the proper value 
when running through the temporary circuit; W, or W, enabled us to make any 
necessary change at the moment of sending the current through the heating coil. 
The switches s, and s, were used to keep the spot of light in the zero position during 
the nine or ten minutes the current was passing through the heating coil. They 
consisted of a stone base, on which were arranged eleven brass studs (shown in figure) 
along a semicircle, having binding screws (not shown) attached to their ends. Over 
these studs slides a movable contact piece attached to an insulating handle. The 
movable arm consists of a number of thin brass plates, jointed together only at the 
centre, and so fitting that they form independent springs which press edgewise on 
the brass studs, thus ensuring good uniform contact. The resistances to be inserted 
are placed in the binding screws attached to the studs (soldered to ensure better 
contact). One switch had platinoid wires of 0°008 ohm, and the other platinoid wires 
of 0006 ohm. These switches gave a much greater range than was found necessary 
in any experiment. When the spot of light was observed to move from the zero 
position, a movement of the arms of these switches brought it back. The spot of 
light was never as much as five small divisions from its position of rest, but even a 
deviation of ten divisions during the whole time of the experiment would have 
produced no appreciable difference in the value of the equivalent. 
The current was only switched on to the principal circuit when it was found to be 
running with fair steadiness through the temporary circuit. After some little trouble, 
caused by leakage, we were able to keep the current constant without any con- 
siderable change in the value of the total resistance of the circuit. 
The electromotive force that is thus measured is the difference of potential at the 
terminals of the heated coil, provided there is no leakage in the wires connecting 
these terminals to the galvanometer. The insulation of these wires was found to be 
7 X 10° ohms, so that the electromotive force at the galvanometer is practically equal 
to the electromotive force at the terminals of the coil. 
Method of carrying out an Experiment. 
Two observers were required to carry out an experiment ; one (A) kept the electro- 
motive force at the ends of the coil constant and compared the Clark cells; the other 
(B) took all the temperature observations, 
