CHAP. XIII] TORQUE AND TRACTIVE EFFORT 



243 



in such a way as to keep the total flux constant, no e.m.f. was 

 induced in the winding during the deformation, and consequently 

 there was no interchange of energy between the electric and 

 the magnetic circuit. Thus, the decrease in the stored energy 







Coil 



FIG. 68. A lifting electromagnet. 



is due entirely to the mechanical work performed. Equating 

 the two preceding expressions, we have that 



F/.TT'-iBV^-i/iff 2 



If W is in joules per cu.cm., F', is in joulecens per sq.cm. (see 

 Appendix I), so that in a rational system of units the mechanical 

 stress per unit area is numerically equal to the density of the stored 



