200 iiKPORT— 1895. 



or one millihenry per kilometre.] More explicitly it is measured by the 

 webers per unit area divided by the gauss-fall per unit length ; in other 

 words, by the ratio of the weber-density to the gauss-gradient. 



(14) The relative inductivity of a substance as compared with that of 

 empty space (/'//Uo) ™^y be called simply its 'permeability' as at present, 

 and is a mere number. 



(Its electrical analogue is specific inductive capacity (k/k„), as contrasted 

 with absolute electric inductivity (k) ; which latter could be defined in 

 practical units as the ratio of the coulombs displaced per unit area to the 

 volt-gradient. ) 



Third quantity to he nained. 



The third quantity for whose unit a name is required is some form of 

 ratio between the two fundamental quantities whose units are here named 

 after Weber and Gauss respectively. It has been practically decided in 

 America that this unit shall be named after Prof. Henry, of Washington, 

 and that it shall equal 10^ c.g.s. units, being equivalent to the earth- 

 quadrant or secohm ; but the precise mode of definition has not yet been 

 finally agreed upon. 



There are two quantities of the same physical dimensions to which the 

 name is applicable, viz., the coefficient of self or mutual induction of a 

 coil or coils of wire, and the permeance or inverse reluctance of a magnetic 

 circuit. 



The most logical order is to define permeance first, as the ratio of the 

 webers of induction to the exciting gaussage, and then to say that the 

 inductance of a coil of n turns of wii-e is 7i^ or 'Ittw^ or 'AiTxri^ times the 

 permeance of the magnetic circuit wliich it embraces, according to the 

 units of gaussage and current which have been decided on. 



If the units of gaussage and current are both the c.g.s. units, then 47?^^^ 

 is the numerical factor connecting inductance with permeance. 



If the c.g.s. unit of gaussage is adopted along with the ampere-current, 

 then •4?r??.2 is the factor. 



But if the circulation of H due to one ampere-turn is adopted as the 

 practical unit of gaussage, then n'^ is the factor ; and the permeance of a 



cylinder, instead of being simply ^.-, is '^^ ,-• 



The apparent simplicity of this last system has much to recommend it 

 for commercial use, though it will complicate the specification not only of 

 permeance but also of magnetic fields and potentials ; but some incon- 

 venience due to the unfortunate definition of the unit pole, and the only 

 less unfortunate definition of the practical unit of current, cannot be 

 avoided ; and our aim must be to place the inconvenience where least 

 likely to be felt in every-day work. 



First system. 



We will begin with the more logical system, and with general statements 

 which apply to both. 



(15) In a complete magnetic circuit the ratio of the total induction 

 to the corresponding gaussage under specified conditions is called the 

 'permeance' of that circuit under those conditions. It is not in general 



constant. 



Or, the permeance of any solenoidal portion of a magnetic circuit, if 



