The Heatmg Coefficients of Rheostats. 
691 
c?5_rate of generation of heat 
clt thermal capacity 
=c^=^'-^ 
if we assume p and C constant, which is approximately correct for con- 
stantan and manganin, but not for iron, copper, etc. The temperature 
coefficient for iron grids is, however, usually much lower than for ordinary 
iron. 
correct ^cift^erJi- 
Equation 14 holds for bare conductors. For those wound on insulating 
material or embedded in oil, sand, or enamel we have 
Y'i>sl 
_ I^E 
~s(MC) 
where M is the weight and C the specific heat of the materials used. 
Of importance is the case when conductors are immersed in oil, for which 
we have 
dS _ I2E 
^/^~Mc+M„a 
When designing a starter with oil insulation for short duration starting, 
the question arises, How much oil is to be taken into account ? Evidently 
