206 REPORT— 1895. 



Gaussage would be about 50 in small transformers, up to 40,000 in 

 large dynamos. The latter could be conveniently reckoned in kilogausses. 

 To make the gauss = 1 ampere-turn appears to have great advantages in 

 practice, and connects it directly with its usual source. 



The idea of permeance is very useful, and the identification of its 

 dimensions with those of inductance is neat. But I think it is liable to 

 cause confusion, for the permeance of the core of a coil will be a different 

 number of henrys from the inductance of its wire. Moreover, the argu- 

 ment as to identical dimensions might equally be applied to the case of 

 amperes and gausses. I would therefore have a new unit strictly connected 

 with the henry, so that inductance =:;'»'- x permeance in a coil of n turns. 



As to the units of permeance : with the above meanings of gauss and 

 weber the permeance of a circuit Avould be 47r^A/10/', as you point out, 

 instead of fiA/l. But I wish to suggest a change in the method of 



reckoning, namely, still to retain the value of the permeance as J^_®^^ 



gausses 



\VG OBI'S 1301" SO cm 

 and permeability as ' ^ ' '' ', therein giving up the convention of 



gausses per cm. 



■ unit permeability of space, and giving it the value r2566 xlO"* unit of 

 permeance for a unit cube. In this way both the troublesome lO"*^ and 

 4-/10 are dealt with in an easily intelligible way. To avoid the high 

 power of 10 it may be measured in micro-units of permeance, so that perme- 

 ability of space and air:='012566 micro-unit of permeance for a unit cube, 

 and permeability of soft iron=up to 25 mici-o-units for a imit cube. Thus we 

 have permeance =A/(//, where /i is to be obtained from tables of its value, 

 which can easily be altered to this method. Inductance then becomes 



weber-turns , webers , 



, or 01^ - z^n~ permeance. 



amperes gausses 



[Of course your phrase ' weber-turns per ampere ' means the .same as the 

 above webers -^ amperes, and does not necessarily mean the weber-tui-ns 

 caused by one ampere.] 



It may be objected that the c.g.s. units of strength of field, unit mag- 

 netic pole and intensity of magnetisation do not bear any simple relation 

 to these practical 'units. This is chiefly important in the use of the mag- 

 neto-metric measurement of iron, and in the measurement of the mechanical 

 form of attraction between two magnetic surfaces in contact. But the 

 expressions are not in reality much complicated ; €.(/., present c.g.s. unit 

 of intensity of magnetisation is given by 4-1=: (/it— /j^) H, where f.i^-=Tper- 

 meability of space = 1, and H^c.g.s. unit of magnetic force. This becomes 

 47rl = (a' — /^('s) H' 10^, where H' is the gauss-gradient in the magnetic 

 substance, and //s:='012566 micro-unit of permeance for a cubic centimetre. 



As the single magnetic pole is unchanged, the force on it will be — strength 

 of pole X gauss gradient x 1'2566 ; butas this is not a calculation of frequent 

 occurrence, except in magnetic surveys, the complication will not be serious. 

 Other magnetic relationships are almost entirely of academic interest only, 

 and would be carried out in c.g.s. units. Also the transition would pre- 

 sent no difficulties to people with a little scientific knowledge. 



I am of opinion also that as the legal volt has no direct connection with 

 induction and velocity of motion, it is not necessary to define the practical 

 units as they are defined absolutely. That is, ohm and ampere are the start- 

 ing points, volt is obtained from them, weber from volt, gauss from ampere, 

 permeance unit from weber and gauss, henry from weber and ampere or 



