ALTERNATING CURRENT MEASUREMENTS. 



43 



current correctly unless specially calibrated as explained above, 

 and, moreover, the inductance of the plunger type instrument is 

 always fairly large. Therefore, for two reasons, a plunger type 

 voltmeter should be calibrated with alternating electromotive force 

 which, for the purpose of the calibration, is measured by an elec- 

 tro-dynamometer-voltmeter of negligible inductance, or by an 

 electrostatic voltmeter. 



The Thomson inclined coil instrument of the General Electric 

 Company is a good example of the plunger type. The essential 

 parts of the instrument are shown in Fig. 36. The coil A, 



Fig. 36. 



through which the current flows, is mounted with its axis inclined 

 as shown. A vertical spindle mounted in jewelled bearings and 

 controlled by a hair spring s, passes through the coil. 



A vane of thin sheet iron a is mounted obliquely on the 

 spindle so that when a current flows through the coil the ten- 

 dency is for the vane to set itself parallel to the axis of the coil, 

 thus moving the pointer. 



16. The spark gauge. The high electromotive forces used in 

 " break-down " tests of insulators are usually measured by means 

 of the spark gauge which consists of an adjustable spark gap 

 between needle points. This spark gap is varied until the elec- 

 tromotive force to be measured is barely .able to strike across it 

 in the form of a spark. The length of the spark gap is then 



