CONTINUOUS ELECTRIC CALORIMETRY. 
61 
but more seldom practised, would be to observe the difference of potential E on the 
conductor, assuming the resistance as before. The advantage of assuming the 
resistance is that one reading only is required, but, as Rowland pointed out, the 
temperature of the resistance, when the heating current is passing through it, must 
be considerably higher than that of the calorimeter. This would introduce a serious 
error, unless it were possible to use a wire of some material like manganin, in which 
the variation of resistance with temperature could be neglected. 
Griffiths (‘ Phil. Trans.,' A, 1893) adopted the method of balancing the potential 
difference on the conductor against a number of Clark cells in series, and deduced 
the expenditure of energy in watts from the formula W = E~/R, by assuming the 
value of the resistance. He tried manganin to avoid the error of super-heating, but 
found that it was not sufficiently constant. In the end he found himself compelled 
to use platinum for the conductor, but avoided the error due to super-heating by 
measuring the actual excess-temperature of the wire as nearly as possible under the 
conditions of the experiment. 
Schuster (‘ Phil. Trans.,’ A, 1895) adopted the same method of balancing the P.D. 
on the conductor, but did not assume the value of the resistance. Instead of this, 
he measured the time integral of the current with a silver voltameter. This is a 
theoretically perfect and most appropriate method of procedure, but it introduces an 
additional measurement, and limits the accuracy to that attainable with the silver 
voltameter. 
For our method of experiment there were several objections to the use of the 
silver voltameter, which put it practically out of the question. As is well known, 
when the current is first turned through the voltameter, the resistance changes 
considerably for some time. This makes it difficult to keep the P.D. on the 
conductor accurately balanced against the Clark cells unless the whole resistance 
in circuit is large. A. change of this kind in the current at the moment of starting 
the experiment would be a fatal defect in the steady-flow method of calorimetry, 
as it would disturb all the temperature conditions, which must be perfectly steady 
and constant before observations are commenced. Moreover, it happened to be 
most convenient for our purpose to employ currents from 5 to 10 amperes, which 
would require very large voltameters, and could not be continuously regulated 
without constructing special rheostats. In any case regulation by hand would 
involve some discontinuity in the heat-flow, which it was desirable to avoid. We 
found it best not to make any attempt to control the current artificially, but to 
employ very large and constant storage cells, and to compensate the slow rate of 
running down of the current by the running down of the head of water, so 
that the temperature-difference might remain practically constant throughout the 
experiment. 
Besides the above special objections to the use of the silver voltameter, there are 
the general objections : (1) that the voltameter method gives only the time-integral 
