The Heating Coefficients of Rheostats. 
693 
We see that the accuracy of the calculation depends again upon the 
correct assumption of A' or But this quantity depends itself upon the 
temperature, and as the latter is unknown, A' is unknown. For a new design 
there is nothing left but to assume A' and to make several trial calculations 
until A' and 8 correspond. 
When a resistance has been constructed, it is best to find t^ experimen- 
tally, as the calculated value is always too small, except perhaps for freely 
suspended wires with little heat capacity and excellent ventilation. A heat 
run is thus essential. For small rheostats this does not take long ; but for 
large apparatus the time required and the cost of a heat run may be consider- 
able. The time may, however, be reduced as follows : 
We carry out a test and plot the temperature rise against time until the 
curve commences to bend towards the abcisse axis. This time should not 
exceed one or one and a half hours even for the largest rheostats. In fig. 7 
a second curve has been plotted, which gives the ratio 
