TRANSFORMERS 187 



filled with clean water should be placed in series with 

 the spark gap. This limits the flow of current across the 

 gap at the instant of arcing over and prevents a sudden 

 discharge of the transformer windings. 



The transformer windings act as the plates of a con- 

 denser; if suddenly discharged, or brought to the same 

 potential, adjacent turns may easily short circuit. The 

 same phenomenon occurs when potential is suddenly 

 applied to a transformer. To reach the interior of the 

 windings, the charging current must either follow the 

 conductors, or break down the insulation between 

 adjacent turns. The end coils are therefore all strongly 

 reinforced to prevent short circuits. The general 

 instructions already given for high potential test on air- 

 blast transformers also apply here. 



Double Potential 



On transformers built to operate at 50,000 volts or 

 over, the double potential should be the last test applied, 

 in order to discover if any breakdown has occurred 

 between turns under the high potential test. 



OIL- AND WATER-COOLED TRANSFORMERS 



Oil and water-cooled transformers are identical in 

 construction to the oil-cooled type, except that, instead 

 of being placed in corrugated tanks to radiate the heat, 

 they are placed in smooth tanks and have a cooling coil 

 immersed in the oil to carry away the heat generated 

 by the losses. The cooling coils are usually made of 

 wrought iron pipe made up in coils of proper size by the 

 pipe manufacturers. In special cases, however, where 

 salt water is used for cooling, copper pipes are employed 

 to avoid the action of the salt on the cooling coils. In 

 most transformers these coils are placed in the upper 

 half of the tank, but sometimes the cooling coils are made 

 of flattened brass or copper tubing, placed between the 

 primary and secondary coils. In large water-cooled trans- 

 formers with low secondary voltages ,the secondary wind- 

 ing is made of flattened copper tubing, through which 

 water circulates. 



