65 



from this cause is, however, generally so 

 small that no account is taken of it, ex- 

 cept when the induction in the armature 

 core is pushed beyond 18,000 or 20,000. 

 In alternators a much lower induction 

 already produces sensible heating, and it 

 is for this reason that the maximum in- 

 duction must be fixed at a very low figure. 

 With a perfectly laminated core, the 

 heating is attributed to an effect which 

 Professor J. A. Ewing calls " hysteresis," 

 and which may be described as molecular 

 friction. This scientist has experiment- 

 ally determined the energy required to 

 carry certain samples of iron through a 

 complete cycle of magnetization of vary- 

 ing degree, and the results have been pub- 

 lished in the Philosophical Transactions 

 of the Koyal Society.* They are given 

 in ergs per cubic centimeter ; but as 

 these units are rather inconvenient for 

 application in the workshop or drawing 

 office, the author has, in the following 

 table, which refers to annealed wrought- 

 iron, translated them into watts per ton. 



* Phil. Trans. Royal Society, 18&5, p. 523. 



