NOV. 19, 1921 bridgman: the discontinuity of resistance 457 



reaches the threshold value. It is sufFicient to explain, therefore, 

 the effect of the magnetic field on the temperature of discontinuity. 

 I show in the following that the temperature of a polymorphic tran- 

 sition is altered by a magnetic field, so that again we do not have to 

 invoke any unusual or new connection between the conduction mechan- 

 ism at low temperatures and the magnetic field in order to explain 

 the facts. In addition to the effect on the temperature of discon- 

 tinuity, the magnetic field exerts a large effect on the resistance 

 above the transition point ; we are not here concerned with this effect, 

 but merely with the relation between the field and the discontinuity 

 (or polymorphic transition). 



Since it requires energy to magnetize a body, a simple thermody- 

 namic argument is capable of finding the effect of a magnetic field 

 on a transition temperature. The argument runs precisely like 

 that used in deducing Clapeyron's equation for the connection be- 

 tween an increment of pressure and the change in the temperature of 

 a transition point. The classical Clapeyron's equation is 



dr (c'2 — Vi)dp 



This is obtained by a direct application of the second law of ther- 

 modynamics to a cycle consisting of a transition from phase (1) to 

 phase (2) at temperature t and pressure p, transfer of the substance 

 to temperature t -f dr and pressure p -f- dp, transition here in the 

 reverse direction from phase (2) to phase (1), and transfer of the sub- 

 stance back to the initial temperature and pressure. In the equation 

 above, X is the latent heat absorbed when phase (1) passes to (2), 

 and (i'2 — Vi)dp is the work received by the external forces during 

 the cycle. 



The analysis is precisely similar in the case of a magnetic field 

 except that the work done by the external forces, which in the pre- 

 vious case was work done by the external pressure, now becomes work 

 done by the magnetic forces. The cycle consists of a transition from 

 phase (1) to (2) at temperature r and magnetic field H, transfer of the 

 substance to temperature t -\- dr and field H -\- dH, transition in 

 the reverse direction from (2) to (1), and transfer of the substance 

 back to the initial temperature and field. In Clapeyron's equation 

 as written above we merely have to replace (^2 — Vi)dp by the work 

 done during the cycle on the magnetic forces. Now the work done 

 by the magnetic forces during a change of magnetization is Hdl. 



