ELECTRIC CIRCUITS 



97 



(d) When the current is 12.5 amperes the energy stored in 

 the magnetic field is 



72 



W = L-pr = 15.1 X 



= 1460 watt-seconds. 



70. Example. Determine the inductance of the circuit in 

 Fig. 71 consisting of two parallel wires A and B carrying equal 

 currents but in opposite directions. 



R = radius of each wire in centimeters. 



D = distance between centres in centimeters. 



dx- 



X 



FIG. 71. Inductance of parallel lines. 



When a current 7 c.g.s. units flows in the wires the field inten- 

 sity at the point P distant x cm. from A and D x cm. from B is 



27 27 



3-C = f- = dynes, by equation 90, 



and the flux density is 



27 



27 



= 3 = - f- 



X L) X 



lines per square centimeter; 



the flux in a section of width dx and length 1 cm. is 



d$ = B dx lines; 

 and the total flux between the wires per centimeter length is 



*D-R nD-R 



*- 



2 I 21 \ 



V+TT ~ 



x D-xJ 



, 



2 log 





