TO THE COMMITTEE ON ELECTRICAL STAiNDARDS. 2(ll 



free from intrinsic magneto-motive force or magnetic boundary layers, is 

 the webers through it divided by the gausses between its ends. 



(16) The practical unit of permeance is that of a circuit in whicli a 

 weber is excited by a gauss. Its reciprocal is the unit of reluctance. The 

 practical unit so defined is 10** c.g.s. units. 



Examples. — Tlie permeance of a cubic metre of air to parallel induction 

 from one face to the opposite is 1 microweber per gauss. 



Under circumstances sucli that the permeability of iron is 400 times 

 that of air, the permeance of an iron ring of one decimetre cross section 



and one metre in mean diameter is -^-I- =20r = 100 c.g.s. = again one 



microweber per gauss. 



It is, perhaps, a question whether this amount of permeance could be 

 called ' a microhenry ' without confusion. 



ExpJmiation. — The inductance (or self-induction-coeilicient) of an 

 electric circuit consisting of n turns of wire, so far as it is constant, is 

 defined to be equal to n times the induction produced through it by a 

 current of one ampere in each turn. But the gaussage due to n ampere- 

 turns is ^^''^ or -47^1 ; hence the inductance of a wire coil is •4^r?^- times 



the webers caused by each gauss in the magnetic circuit surrounded by it ; 

 i.e., is •inn''- times the permeance of that circuit considered as constant. 



(17) A coil of wire threading n times a complete magnetic circuit of 

 unit permeance under any given circumstances is said to have --iTTn- units 

 of inductance under those circumstances ; and in general the inductance 

 of a coil of n turns is ■■^izn^- times the permeance (as above defined) of the 

 magnetic solenoid enclosed by it. (The permeance may here be considered 

 variable. ) 



["With the ampere-turn as unit gaussage the ^tt is prefixed similarly to 

 both inductance and permeance, so that only the factor n- is needed to 

 convert one into the other. See below.] 



(18) The c.g.s. unit of inductance is equal to « times the induction excited 

 through a coil per c.g.s. unit of current in every turn of wire ; whereas the 

 practical unit of inductance is n times the webers excited per ampere ; 

 hence the practical unit of inductance is 10^ times the c.g.s. unit. 



The practical unit is called a 'henry.' (It has also been called 

 secohm and quadrant.) 



Exatnjjle.—lf the above iron ring were wound closely with 1000 turns 



of wire, the coil would have a coefficient of self-induction equal to - or 



1^ henry s whenever the permeability of the iron was 400. 



A coil of 20,000 turns of wire, wound closely on the same core, would 

 have an inductance of 1[ Iienrys if it contained air or other non-magnetic 

 substance. 



Alternative mode of definition of inductance on first system. 



In view of one of the above practical methods of measuring induction 

 experimentally, the inductance of coils of wire, both self and mutual, may 

 be defined more directly thus : — 



(19) When of two simple circuits one conveys a current, the other 

 in general has induction caused through it ; and the ratio of the induction 



