184 



rent in each transformer. The energy 

 must therefore not be taken as repre- 

 sented by the product of electromotive 

 force and current, as given in the diagram, 

 but by this product multiplied by the 

 cosine of the angle of lag. On purely 

 theoretical grounds all the transformers 

 now in use were equally good and their 

 efficiency was high. Professor Ayrton 

 had tested some, and found that it was 

 as high as 95 per cent. The difference 

 in the transformers was in their details, 

 such as would secure low first cost, 

 facility of manufacture, ease of changing 

 a coil, freedom from humming, ability to 

 keep cool, and, above all things, absolute 

 safety of insulation. 



CORRESPONDENCE. 



Mr. J. I). F. Andrews, in reference to 

 the author's observations on eddy cur- 

 rents (p. 64), and his inability "to 

 suggest an entirely satisfactory explana- 

 tion for the effect of the iron core in 



