8 THE DESIGN OF STATIC TRANSFORMERS 



transformers with good effect. It was not at first clearly 

 appreciated that the non-intermixing of the primary and 

 secondary coils was so especially a fault; on the contrary, it 

 was considered that the difficulties were due to the increased 

 saturation of the core accompanying the use of these 125-cycle 

 transformers on the 60-cycle circuits, which were at first 

 usually employed for power purposes. For the same terminal 

 pressure halving the periodicity doubled the flux required in 

 the magnetic circuit. This did not so greatly increase the 

 core loss, since the doubled density was partly offset by the 

 halved periodicity ; in fact, the core loss was only increased 

 by some 35 per cent, by employing the same transformer at 

 the same pressure, but on a circuit of half the periodicity. 

 But these transformers had not been sufficiently liberally 

 designed to permit of this 35 per cent, increase in the core 

 loss, and there was also in some cases the further trouble of 

 greatly increased magnetising current due to the greater 

 saturation. The enunciation at about this time of the one- 

 and-sixth-tenths power law of hysteresis by Dr. C. P. Stein me tz 

 was very opportune, and was of great assistance in transformer 

 design. In placing on the market transformers of lower 

 periodicity for power work, in order to avoid the necessity 

 for new parts throughout, the same general type (the shell 

 type) was retained and the same general proportions of the 

 winding space, but occasion was taken to intermix to a 

 moderate extent the primary and secondary coils, so as to 

 decrease the inductive drop, i.e., so as to obtain closer pressure 

 regulation with inductive loads. 



The incursion into the power field rapidly led to requiring 

 transformers of larger size and introduced great difficulties 

 in the matter of so designing the transformers that they 

 should not overheat. The years 1893 and 1894 saw the 

 extensive commercial introduction of the principle of immersing 

 the transformer in oil. No inconsiderable difficulties were 

 experienced in obtaining a suitable oil, and a great deal of 



