INDUCTION COILS. 747 



The ratio of the energy disengaged in the secondary wire to that 

 which is expended in the principal wire is 



A' 2 R' A' 2 R' 2 R / 4 \ 4 N' 2 /3\ 2 N 2 _ 



A 2 R A 2 R 2 R' \3/ N 2 \4/ N' 2 



and the efficiency 



_i6_ 



"25" 



Nevertheless, these conditions assume that the magnetisation of 

 iron is proportional to the magnetising force a hypothesis which is, 

 undoubtedly, far from the truth. 



