The Heating Coefficients of Rheostats. 
689 
100° to 117 at 500". The suspension of coils in this manner gives the best 
cooling arrangement; but the mechanical construction is weak, as the 
wires sag. 
In fig. 3 the heating coefficient is given for the same wire, which is now 
wound on porcelain rods 1 in. in diameter and provided with a standard 
Oocis 
COOlO 
\ 
VXQO 
\OOQ 
^00 
screw-thread twelve to the inch. The total cooling surface has been measured 
for the evaluation of A' in the manner indicated in the inset. We see that 
now a much greater cooling surface is required to emit the same amount of 
heat, being 1000, whereas for freely suspended coils it was 150 for the 
same temperature — viz. 100° C. 
