258 



ELECTRICAL MEASUREMENTS 



The range of the instrument is up to 10,000 volts, without the 

 use of a condenser multiplier, and up to 30,000 volts if the 

 multiplier is employed. The deflection per volt may be varied 

 by means of weights which are hung on the hook at the lower end 

 of the needle. The oscillation of the needle can be checked by 

 bringing a silk thread into contact with the pointer. 



Fig. 144 shows a form of electrostatic voltmeter made by 

 Siemens and Halske for voltages up to 150,000. 



One electrode, A, is under the base of the glass jar C; this jar 

 is filled with oil and the movable elec- 

 trode B is suspended in it. The restor- 

 ing force is a spiral spring. The pull on 

 the electrode B is transmitted to the 

 pointer by a mechanism which is so 



FIG. 144. {Siemens and Halske high-range electrostatic voltmeter. 



arranged that the upper 70 per cent, of the scale is practically 

 uniformly divided. The use of the oil reduces the risk of an arc 

 forming between the electrodes A and B and permits them to be 

 brought nearer together, thus increasing the force and permitting 

 the instrument to be made smaller. The damping is by the fluid 

 friction of B. The shields D and D' are to protect the instru- 

 ment from the influence of surrounding objects. 



The Westinghouse Co. manufactures the high-range volt- 

 meter 2 shown diagrammatically in Fig. 145. 



