TRANSFORMERS 



175 



the voltage is applied. Under such conditions, the 

 reading of the electrostatic voltmeter or the arcing across 

 the spark gap affords no indication of the insulation 

 strain. Indications which are best learned by experience 

 reveal the character of the insulation under test. 



10 ^0 3O 40 5O 60 7O 6O 9O IOO I/O &O I3O MO /5O l6O 

 Kilo VolLS 



Fig. 52 

 Curve of Arc Distances 



The charging current of a transformer varies with 

 its size and design. The current may be measured by 

 means of an ammeter, placed in the low potential circuit 

 of the testing transformer. It will increase as the voltage 

 applied to the insulation is increased. Inability to obtain 

 the desired potential across the insulation may be due 

 to large electrostatic capacity, or to the inability of the 

 high potential transformer to supply a large capacity 

 current at the voltage desired. A breakdown in the insu- 



