SWITCHING EQUIPMENT 545 



densers, which consist of suspension insulators having an insula- 

 tion equal to that used on the line. Normally the glowers have 

 the appearance of ordinary spherical frosted incandescent lamp 

 bulbs. When, however, there is a proper difference of potential 

 across their terminals they will glow with a reddish hue. When 

 the lines are not in synchronism, the glowers will light up in 

 succession, showing the relative direction of rotation and indi- 

 cating whether the incoming machine is running fast or slow. 

 When synchronism is reached there will be no rotating effect, and 

 one glower will be dark while the other two will glow at about 

 half brilliancy. 



Electrostatic Ground Detectors. (1) They give a constant 

 indication of the condition of the system with respect to grounds 

 which, ii not detected immediately, often result in very serious 

 burnouts or voltage disturbances. 



(2) They are superior to any system of ground detecting which 

 necessitates the plugging of potential transformers and lamps or 

 voltmeters to different phases of a polyphase system ; first, because 

 the polyphase electrostatic ground detector shows, at a glance, 

 whether there is a ground on any phase, while with the other 

 scheme it is necessary to plug the primary side of the transformer 

 to the different phases before the test is completed; and, second, 

 because the electrostatic ground detector is supplied with a scale 

 for reading the severity of the ground while with lamps only an 

 approximate indication is obtained ordinarily, and for high 

 resistance grounds no indication whatever, since the ordinary 125- 

 volt carbon lamp will not glow at much less than 25 volts across 

 its terminals. 



Temperature Indicators. (1) It is of great value to know the 

 temperature of certain parts of generator and transformer wind- 

 ings that are inaccessible for thermometer measurements. An 

 instrument known as the temperature indicator has been pro- 

 duced to determine these temperatures. Copper coils of known 

 resistance are placed in the parts whose temperature it is desired 

 to know. The changes in resistance are shown on the scale of 

 the indicator, which is marked in degrees Centigrade correspond- 

 ing to the changt3 in resistance. The instrument itself is a differ- 

 ential voltmeter with three terminals. The connections are such 

 that one of the moving coil windings is in series with a resistance 

 coil which has a zero temperature coefficient and a resistance equal 



