416 Mr. K. H. M. Bosanquet on the Determination 



In the higher part of the range of the machine, say for 

 unit-current (10 amperes), we can assume, without serious 

 error, that the fundamental equation (2) holds. That being 

 so, we can deduce the value of M from the resistance and 

 number of revolutions with which a stable current is pro- 

 duced. 



It will be convenient to assume that all the coils of the 

 armature are gathered up into two coils at right angles. 

 Then, if one be parallel and the other perpendicular to the 

 axis of the magnetic field, the determination of the coefficient 

 of induction at rest affects only one of the coils, or half the 

 armature. 



Then, as to one of these coils, or half the armature : — In a 

 half revolution the same effect is produced as if the exciting 

 current were reversed, and a quantity of electricity passes, 



__2MC 



""Er- 

 in a whole revolution -p passes. 



The same applies to the other half of the armature, so 

 that altogether ^ passes in each revolution. 



If there be n revolutions per second, the quantity passing 

 in n revolutions = the current numerically, and this divides 

 out, so that 



M=— 



Sn 



In an old paper of mine on Practical Electricity (Phil. 

 Mag. xiv. p. 246) I find data from which, by interpolation, 

 I obtain the corresponding values for this machine : — 



n' per minute. E. C. 



840 10 9 x 5-55 1 = 10 amperes. 



Whence M = 10 7 x 4-95. 



The assumption made above is rough, but we cannot 

 specialize further unless the distribution of the field is known. 

 The two coils may then be supposed to be in any required 

 proportion instead of being equal. 



Regarding the coefficient as the product of the number of 

 windings of armature and magnets, and of the magnetic induc- 

 tion common to both, due to unit-current, we can find the com- 

 mon magnetic induction in this case. There are 1700 turns in 

 the armature, half of which count, and 234 turns on each of the 

 four magnets, two of which receive each part of the common 



