460 



Prof. J. Perry. 



[May 12, 



(1+ 2*54 xiO _4 0)/(roiO8 + 5*08 xlO- 4 0). Assuming I to be sin Jet, I 

 give the values of I for various values of h. 



Tc. 



I. 



100 



0-988 sin (Jct- 



- l°-4) 



300 



0-981 sin (Jct- 



- 4°-2) 



600 



-958 sin (Tct- 



- 8°-l) 



1000 



0-912 sin (M- 



-12°-4) 



2000 



0-782 sin (Jct- 



-18°2) 



4000 



0-628 sin (M- 



-18°'l) 



VI. Eddy Currents. — There is eddy current loss of power in all 

 the conducting masses near a transformer. If we assume that the 

 induction per square centimetre |3 is the same everywhere, and if it 

 follows the law {3 (in C.Gr.S. units) = 2<ij sin (ikt+ef), the average 

 power in watts wasted in eddy currents in the iron per cubic centi- 

 metre is 6*25 x 10 _13 r 2 7£ 2 2^ 2 <x 2 ;, if the specific resistance of the iron is 

 taken to be 10 4 . The iron is supposed to be of wire of radius r 

 centimetres. 



The average power wasted in the iron is less at high temperatures, 

 being inversely proportional to the specific resistance of the iron. It is 

 evidently proportional to the square of the effective primary voltage 

 in the unloaded transformer, but, of course, magnetic leakage causes 

 a diminution when the transformer is loaded. In fact, the eddy 

 current loss is always proportional to the square of the effective 

 electromotive force in the secondary circuit. In applying the rule r 

 it is to be remembered that the induction is not uniform in the 

 section of a wire, nor is the average induction in each wire the same 

 for all the wires, and therefore the real loss of power in the iron by 

 eddy currents is always greater than the result of applying the above 

 formula.* If we assume that one secondary circuit closed upon itself 

 represents all the eddy current circuits, assuming the truth of the 

 above formula is the same as assuming no magnetic leakage between 

 the primary and this eddy current circuit. Assuming magnetic 

 leakage gives the more correct result, which is practically that the 

 magnetisation is greater in wires near the surface of the mass of iron 

 than it is internally, and also that the induction is greater near the 

 skin of each wire. 



# [Added April 29, 1892. — I have often urged the importance of the consideration 

 of the want of uniformity of induction in the iron ; but even in February last, when 

 I brought it before a meeting of the Institution of Electrical Engineers, it was- 

 looted upon as unimportant, and I had made no calculations which would enable 

 me to prove its importance. The calculations of Prof. J. J. Thomson concerning the 

 eddy currents in thin plates, published in ' The Electrician' of April 8, 1892, place 

 the master beyond doubt.] 



