OF POTENTIAL DIFFERENCE 255 



To prevent extraneous electrostatic action, the needle, the 

 magnet and the case are connected together electrically. With 

 this construction, the outside case is at the potential of one 

 side of the line. In later instruments, the shielding is accom- 

 plished by an inner case which is insulated from the outside 

 or protective case. The construction of the instrument is such 

 that accidental contact between the 

 quadrants and the movable system 

 is impossible. 



Instruments of this general design 

 are listed, which give a full-scale de- 

 flection with 7 volts. 



In cases where the voltage is high, 

 above 800 volts, the forces become 

 great enough so that the cylindrical 

 needle may be pivoted on jewelled 

 bearings with its axis horizontal (see 

 Fig. 139). 



The controlling moment is obtained 

 by using a small weight attached to 

 a short arm which projects from the 

 axis. Electrical connection with the 

 needle is made by a very fine wire, 

 wound in a flat spiral. 



Some control over the law of de- 

 flection may be obtained by shaping 

 the quadrants. 



To prevent arcing between the 



needle and the quadrants in event trostatie voltmeter, 

 of a great increase of voltage, a 



spark gap is provided between the terminals and within the 

 case. It is intended that it act when the voltage has risen to 

 twice the full-scale value. Fuses which are enclosed in the re- 

 moval >le terminals are thus blown and the instrument automat- 

 ically taken out of circuit. 



A reference pointer and means for clamping the movable system 

 during transportation are provided. The damping is obtained 

 by having attached to the needle an aluminum sector which 

 moves between the poles of a permanent magnet. All of the 



FIG. 140. Working parts 

 of Kelvin multicellular elec- 



