TO THE COMMITTEE ON ELECTRICAL STANDARDS. 207 



from permeance, and so on. This is much more easily explained to practical 

 and imscientitic men than the absolute derivations are, and it is the order 

 in which they learn them. 



Yours very truly, 



Francis G. Baily. 



Remarks on the Above (especially on pages 203 and 204). 



According to the proposal of the Chicago Chamber of Delegates, the 

 quantity which we call ' inductance,' and Avhich is to be expressed in 

 ' henrys,' is defined by the equation 



E=L , , for self-induction, 

 at 



or 



dG 

 E=M — , for mutual induction, 

 dt 



both being comprehended in one definition, the inductance L or M being 



dG 

 calculated by dividing E in volts by -3- in amperes per second. This 



Ctu 



implies that L or M is not to be defined as , but as . 



c ""'"'^ ^°^" 7rc 



that the former be called ' the total inductance,' and the latter ' the dif- 

 ferential inductance.' The distinction would be somewliat analogous to 

 the distinction between the ' mean specific heat from 0° to t° ' and the ' true 

 specific heat at t°.' Both total and differential inductance should be ex- 

 pressed in ' henrys,' for they are quantities of the same kind, and when 

 there is no iron, &c., in the field they are equal. 



I think that the above mode of definition, involving as it does no 

 magnitude except current and time, is more readily comprehended than 

 Dr. Lodge's proposed definition, in which the magnitudes involved are 

 current, ilux of induction, and the number of convolutions of the coil 

 through which the flux passes. In the definition proposed by the Chicago 

 delesfates the consideration of the number of convolutions does not 

 enter. 



For a circuit or two circuits not having iron, &c., in the field we may 

 define inductance (in henrys) as the E M.F. (in volts) due to variation of 

 current at unit rate (one ampere per second). When the field is modified 

 by the presence of magnetic material the above will be the definition of 

 ' differential inductance.' 



The ' total inductance ' for any specified strength of current will be 

 the mean A'alue of differential inductance for equal increments of current 

 from zero up to the specified strength. 



I would suggest similar nomenclature in the case of permeability : 



- ^ should be called differential permeability, and ^ total permeability. 



I think names are de.sirable both for ^ and for ^-^. I would suggest 



dR ^ ' ' H 



In some respects ' mean ' or ' average ' would be a more correct designa- 

 tion than ' total ' ; but these words would be liable to be misunderstood as 

 referring to an average taken over the different parts of the body or cir- 

 cuit. ' Total ' is to be understood as standing for ' calculated on totals.' 



