Is i THE MAGNETIC CIRCUIT [AWT. 58 



tional to the square of the current which excites the field. The 

 coefficient of proportionality, which depends only upon the form 

 of the circuit and the position of the exciting m.m.f., is defined 

 as the inductance of the electric circuit. The older name for 

 inductance is the coefficient of self-induction. It is assumed 

 hat the magnetic circuit is excited by only one electric cir- 

 cuit, so that there is no mutual inductance. Thus, by definition 



(104) 

 where the inductance is 



..... (105) 

 or, replacing the summation by an integration, 



n P 2 d(P P ...... (106) 



C 



/o 



Since the permeances in eq. (102a) are expressed in henrys, and 

 the numbers of turns are numerics, the inductance L in the defin- 

 ing eqs. (105), or (106), is also in henrys. If the permeances are 

 measured in millihenrys or in perms, the inductance L is measured 

 in the same units. As a matter of fact, the henry was originally 

 adopted as a unit of inductance, and only later on was applied to 

 permeance. 1 



In some cases it is convenient to replace the actual coil (Fig. 45) 

 by a fictitious coil of an equal inductance, and of the same number of 

 turns, but without partial linkages. Let (P eq be the permeance of 

 the complete linkages of this fictitious coil; then, by definition, 

 eqs. (105) and (106) become 



(106a) 



This expression is used when the permeance of the paths is calcu- 

 lated from the results of experimental measurements of inductance, 

 because in this case it is not possible to separate the partial 

 linkages. Use is made of formula (106a) in chapter XII, in cal- 

 culating the inductance of armature windings. 



1 The use of the henry as a unit of permeance was proposed by Professor 

 Giorgi. See Trans. Intern. Elec. Congress at St. Louis (1904), Vol. 1, p. 136. 

 The connection between inductance and permeance seems to have been first 

 established by Oliver Heaviside; see his Electromagnetic Theory (1894), Vol. 

 1, p. 31. 



