TRANSFORMERS 443 



of metal grounded to the cover. The central metal section 

 extends below the oil level of the transformer to prevent corona. 

 This type is also provided with a flexible cable passing through a 

 metal tube extending the length of the bushing and supported at 

 top and bottom, connection being made at 

 the top through a water-tight cap. The 

 central metal tube is surrounded by con- 

 centric insulating cylinders dividing the oil 

 space. The joints between the top and 

 bottom porcelains and metal sections are 

 gasketed and clamped, so that they are 

 absolutely oil-tight. The upper porcelain is 

 surmounted by a heavy glass expansion 

 chamber which is also used as a gauge in 

 filling. 



Oil. Transformers should contain suffi- 

 cient oil to completely immerse the core, 

 windings and cooling coil, and a gauge 

 should be attached to the tank in a con- 

 spicuous place to indicate the oil level, 

 while a valve should be provided at the 

 bottom for drawing off the oil. 



Transformer oils should have good in- 

 sulating properties, a high flash and low 

 viscosity, so that the heat may be readily 

 conducted from the coils and core to the 

 radiating surfaces. The flash and burning 

 points are second in importance only to 

 viscosity, and, in fact, vary together; that 

 is to say, an oil having a high burning-point FlQ 2 80. 155,000-Volt 

 compared with another oil will probably be Compound Filled 

 high in viscosity. It is this property of oil Flange-clamped Por- 

 to resist ignition until it is first heated to a celain Bushing, 

 temperature, known as its fire or burning- 

 point, which enhances its value as an insulating and cooling 

 medium. At a temperature somewhat below the fire or burning- 

 point the oil gives off vapors which, as they come from the surface 

 of the oil, may be ignited in little flashes or puffs of flame. This is 

 known as the flash-point. The oil will not support combustion, 

 however, until these flashes are sustained uninterruptedly, or, in 



