TO Tin: COMMITTKE ON JXKCTlilCAL STANDARDS. 203 



turn ; that is to say, tlie inductance of a coil will be n- times the permeance 

 of the magnetic circuit surrounded by it. 



(32) The difference between inductance and permeance is only one 

 of reckoning. Permeance is webers per ainp^re-turn. Inductance is 

 weber-turns per ampere. 



Summary of the Advantages of this Mode of defining 

 Unit Inductance. 



The special feature of this mode of defining the ' henry ' is that it makes 

 inductance depend on the simple ratio N/C, or weber-turns per ampere, 

 instead of on something more complicated. 



It might possibly be defined as the ratio clN /dC, that is, as pro- 

 portional to the tangent of the slope of the B : H curve ; and such a defini- 

 tion would emphasise its variability ; but certain practical advantages 

 would be lacking, because it would be detached from any connexion with 

 the 2iernieance of the circuit. The N/C ratio on the other hand instantly 

 connects itself with permeance, and represents the slope of the secant 

 drawn from the origin to any point of the B : H curve. It exhibits the 

 variability sufficiently ; making the inductance reach a maximum at the 

 shoulder of the curve, and then slowly decrease as saturation sets in. 



It is sometimes said — but the mode of expression is, to say the least, 

 very inconvenient — that there are three different principles on which to 

 define L, all leading to a different result : viz., numbering them inversely, 

 but giving them in their usual order : — 



(3) Energy . . . W=l LC^ 

 (2) E.M.F . . . E = L dC/dt 

 (1) Total induction ]Sr=LC 



But the real facts to be expressed are not here exhibited. The real 

 facts are ^^^ ]Sr=LC 



(2) E=^dN/dt 

 {?>) dW^Cdi^ 



The essential thing to name is therefore N ; and if 10^ c.g.s. lines or 

 unit tubes be called a ' weber,' or a ' weber-turn,' then a volt is a weber 

 or a weber-turn per second, and a joule is a weber-ampere-turn. 

 Nothiiig can be handier than that ; and a henry can be defined as a 

 weber-turn per ampere. 



Instead of saying as above that there are three ways of defining L, the 

 simplest thing is to say that two of the thi'ee equations as first given above 

 are incorrect, except for the special and in j^ractice comparatively rare case 

 when L is constant. Written out correctly they stand as follows : — 



(1) N=LC 



(2) E=L'^^ -hC'^^ 

 ^ ' dt dt 



(3) W=^ LC^-fiC'c^c/L 



It is then obvious that (2) and (3) are too complicated to base a 

 definition upon, and that the first alone gives a feasible system. 



The fact that L is decidedly not in general constant deprives the 



