to Reversals of Magnetization in Soft Iron. 211 



an infinitely thin wire through which current is made. 

 Curves in fig. 21 give the rise of temperature ther mo electri- 

 cally determined plotted against time, when a continuous 

 current was sent through the copper coil of only one of the 

 rings ; the other being kept at the temperature of the room. 

 They show how far the equation thus calculated is realized in 

 experiment. 



Expanding v in powers of t, thus 



l+fc-Tj-.-..) 



/ ht 2 W \ 



we see that the time-rate of the rise of temperature at the 

 beginning gives the rate of generation of heat independent of 

 the dissipativity h, as was indicated by the general equa- 

 tion ; and, therefore, whatever the diffusivities of the wooden 

 and iron rings may be, the heating-effect due to current alone 

 would have been eliminated in the beginning, if only the rate 

 of generation of heat had been the same in both rings ; in other 

 words, the curve of temperature growth would have begun 

 tangent to the time-axis, when the same continuous current 

 was sent through both the rings. Experiment revealed, how- 

 ever, that this was not the case ; the curve of difference of 

 temperature growth began with definite rate (see fig. 22), so 

 that there must have been a difference in the rate of supply of 

 heat in the two rings. This is very likely due to the difference in 

 the resistance of wire used in winding the rings, or irregulari- 

 ties in the rate of winding in the neighbourhoods of junctions. 

 Assume, therefore, 



v 1 = q/h(l — e~ ht ') for the wooden ring, 



v 2 = q f /h / (l—e~ h ' t ) ,, iron ring; 

 taking their difference, we have 



v = v x — v 2 , 

 which is observed in the thermoelectric current. From this 

 we have dv 



-=qe- M -q'e- m , 



which shows that at the beginning the temperature rises at 

 the rate q—q' per unit of time ; and, further, the difference of 

 temperature, v, is maximum when 



*=log (£)/(* -ht); 



and, therefore, there is or is not a maximum according as 

 q and h are greater or less than q' and h' correspondingly or 



