502 Profs. Ayrton and Perry on the 



enormous change which he has produced in the construction 

 of dynamos. 



Now the line-integral of the magnetic force round a closed 

 magnetic circuit — called by Mr, Bosanquet the Magneto- 

 motive Force — is the number of ampere-turns on the coils 



through which it threads it way x — . Hence if S 2 A 2 be the 



ampere- turns on the field magnet, 



^S 2 A 3 =2S/3 + ^/3, (9) 



8 being d + t the distance from iron of armature to iron of 

 field-magnet, and of course the length of the tube in air is 

 twice this distance as it goes into and out of the armature. 



If N is the total induction through the iron of the arma- 

 ture,— = /3 there and — =/S in the air-space. 

 a 1 a 2 



If vN is the total induction through the iron of the field- 

 magnet anywhere, then — = /3 there, so that (9) may be 

 written 



10 « 2 «A* 



or N = 12 -; (10) 



2 (d + t) + IS 



a 3 a/u, 



and as N is the induction through the armature produced 



Air 

 bv a magneto-motive force -j. S 2 A 2 , equation 10 leads to 



2(d + t) . . 



■ — 2 ■ being called, by analogy with other physical re- 



a ... Iv 



sistances, the air magnetic resistance, and 2 — the iron 



magnetic resistance of the circuit. The members of the 

 Society will perhaps allow us to put in this way Dr. Hop- 

 kinson's theory, although he himself may object to some of 

 the terms we use. 



Now let it be assumed that there is a value of /3 for the 

 armature iron which it is best to use in all machines when 

 giving their permanent output, v may also be taken as 

 practically constant. We are aware that to both these 

 assumptions exception may be taken, but this will not be 

 found to affect the practical general result which we arrive at. 



