70 



and subtracting in the case of the 

 secondary, the loss of pressure due to 

 the resistance of the coils. In a properly 

 designed transformer, this loss of pressure 

 should not exceed about 1 per cent, of 

 the terminal pressure, so that the maxi- 

 mum difference in the pressure main- 

 tained in the secondary circuit between 

 the lamps, when the apparatus is used 

 for parallel distribution, does not exceed 

 2 per cent, between full load and no load, 

 provided the pressure in the primary 

 circuit is kept constant. Such a small 

 variation in the pressure is well within 

 the limits attainable in direct lighting, 

 whenacompoundwounddvnainoisdrivcn 

 from an engine with a fairly good gov- 

 ernor, and the corresponding variation in 

 the illuminating power of the lamps is 

 sufficiently small to be tolerated. A 

 greater variation, especially if it occur 

 suddenly, would, however, not be per- 

 missible ; and the question, therefore, 

 arises as to what rules should be followed 

 in designing transformers so that they 

 may as nearly as possible be self-regulat- 



