452 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
sent to Messrs. Exuiorr for careful examination—gave a value of 31°32 B.A. units at 
the mean temperature of the equivalent experiments, or 30°90 true ohms, if 
1 B.A. unit = ‘9866 true ohm. It is clear that an approximate value of J will 
be sufficient in this part of the calculations. Any error in the value of J will affect 
the cooling correction to half its value, and as that correction amounts to 1 per cent. 
of the equivalent, an error of 1 per cent. in the assumed value of J would involve an 
error of 1 part in 20,000 of the final value. The reductions had naturally to be 
made before we knew what our own value of J was going to be, so we applied 
provisionally Mr. Grirrrrys’ value of 4194 xX 107, and although our own value 
is slightly lower it would have made no appreciable difference if our value had been 
taken instead. 
The Temporary Coil. 
The only part of the arrangement which has not so far been described is a coil of 
platinoid equal in resistance to the one in the calorimeter, in series with a resistance 
approximately equal to that of the silver voltameter. 
This coil and resistance served for a preliminary adjustment of the circuit. It was 
necessary, in order to secure sufficient accuracy, that a balance of electromotive force 
should be obtained soon after the beginning of each experiment, and this could be 
done by means of this coil, which, for distinction, we shall call the temporary coil. 
The keys which serve to change currents were all made of paraffin blocks, and of very 
simple construction. The blocks had six holes filled with mercury, and copper con- 
nectors completed the circuit. The two ways in which connection could be made may 
be seen in the key marked 4, (fig. 5), where the holes are numbered. In the position 
of the key which we shall call a, the cups 1 and 8 were connected, as well as 2 and 4; 
while, in the position b, 1 was connected to 5 and 2 to 6. The keys k, and k, were 
exactly similar, and we shall speak of the positions a and b of the keys when the 
corresponding connections were made. Thus, k; (a), k, (b) means that the key k; was 
in position a, and key #, in position b, The key &; served to short-circuit the silver 
voltameter in the final period ; a copper connector could be placed for this purpose 
either at @ or b. 
Method of Maintaining a Constant Difference of Potential at the Terminals 
of the Heating Coil. 
The difference of potential at the terminals of the heating coil was kept constant 
by balancing it against the battery of twenty Clark cells in the well-known way. The 
success of this method, when the potential difference had to be kept constant for some 
minutes, depends on the ability of the operator in altering the total resistance of the 
circuit through which the current is passing by small variable amounts. The current 
—which, in our experiments on the heat-equivalent, was about 0°9 ampére-—tends to 
