4 THE DESIGN OF STATIC TRANSFORMERS 



scene was an instrument calibrating and experimental depart- 

 ment. Our duties, so far as related to transformers, comprised 

 testing the insulation resistances between primaries niul 

 secondaries and " to frame," of transformers of various sizes 

 and types. We duly reported on the insulation resistance, 

 which, in those early days, was often so low as to be well within 

 the range of a very unsensitive testing set. On transformers 

 of other than our own firm's manufacture we made core-loss 

 measurements, estimated the weights of copper and iron and 

 sometimes ascertained the numbers of turns and other particu- 

 lars of the windings. 



The subject of core loss gradually came prominently to the 

 front and for a long time we were fully occupied in studying the 

 methods and results set down in Swing's " Magnetic Induction 

 in Iron and Other Metals," and in ourselves making ballistic 

 tests of sheet iron and steel. The facilities of our department 

 were placed at the disposal of the company's purchasing agent, 

 who was scouring the world's markets for suitable sheet iron. I 

 well remembered that it was not long before we ascertained that 

 the cheapest grades of material often had (if annealed from a 

 suitably high temperature) the lowest core loss and that 

 plates rolled from Swedish soft charcoal-iron, while they cost 

 much more, were no better as regards core loss than were the 

 cheap grades of sheet steel used for various non-electrical com- 

 mercial purposes, such, if I remember rightly, as shipper's labels. 

 The chief difficulty consisted in obtaining in great quantity 

 material of the good quality of the occasional sample. But 

 even our best results of that time (1890 and 1891) related to 

 materials which were far inferior, as regards core loss, to the 

 low-loss alloys at present (1910) employed, and were also much 

 more prone to " ageing." Contemporaneously with the testing 

 of magnetic materials, we were required to make tests of the 

 core losses of completed transformers, for by this time (1891) 

 our company's designers were becoming keenly alive to the 

 importance of keeping down the core losses and were engaged 



