508 Profs. Ayrton and Perry on the 



where a 2 is the area of the pole-piece exposed to the armature 

 (increased by the fringe-area, which Dr. Hopkinson estimates 

 to be of a breadth OS 8), and 8 is the distance from iron of 

 armature to iron of pole-piece. 



(17) gives, in fact, the straight part of the characteristic. In 

 well-constructed dynamos the magnetic iron resistance is equal 



2S 

 to the air resistance — when the machine is working at its 



°* 



most permanent output ; and it is only necessary to know ft', 



the induction convenient to use when this is the case, to be 

 able to calculate N and S 2 A 2 for a point on the curved part of 

 the characteristic. Thus 



«i^i- N i- Yo 4gS 2 A 



Manchester dynamo described by Dr. Hopkinson (Trans. Roy. 

 Soc. 1886), 



a 1 = 2205 square centim., 



a 2 = 839*5 „ „ 



8 = 0*8 centim. 



Taking & = 18,460 lines per square centim., we find 



ojft=4-07 xlO 6 . 



Now 



So that N = 660 S 2 A 2 represents the straight part of the 

 characteristic. 



Again, N x = a x ft x = 4-07 x 10 6 , 



and 



^#, = 330 8^2, 



Plotting N = 4'07 x 10 6 and S 2 A 2 = 12330 as the coordinates 

 of a point on squared paper, and plotting also the straight line 

 from the origin N = 660 S 2 A 2 , a man who has seen charac- 

 teristics before, will be able to draw the characteristic of this 

 machine with a fair amount of accuracy, especially if he 

 recollects that the iron is near saturation. Those, however, 

 who have less experience may find the Frohlich, which for any 

 dynamo passes through the point on the straight line which 

 represents N = 10,000 a 1} and the second point just found, 



