MEASUREMENT OF POTENTIAL DIFFERENCE 241 



was 0.053 henry. While there was no appreciable error at 16 

 cycles per second, the error in the indication at 3,000 cycles per 

 second was 25+ per cent. 



Fig. 131 shows the general construction of a dynamometer 

 voltmeter. The working parts of the instrument are surrounded 

 by a laminated magnetic shield. Electromagnetic damping is 

 obtained by having the movable system carry a fan-shaped 

 sector of aluminum, which swings between the poles of two small 

 permanent magnets. The shielded voltmeter made by the Wes- 

 ton Instrument Co. is very similar in general design to the 

 wattmeter shown in Fig. 177; it has a very efficient air damper, 

 consisting of two light, symmetrically disposed vanes which are 

 enclosed in carefully finished chambers in the base of the frame- 

 work which supports the coils. The vanes are of exceedingly 

 thin metal stiffened by ribs stamped into them and by the edges 

 which are turned over. The useless leakage to the outside air 

 is reduced to a minimum and the desired degree of damping 

 attained by a suitably designed clearance space between the 

 vanes and the walls of the chamber (see page 71). The mo- 

 ment of inertia of the moving parts of this arrangement is very 

 small. 



Hot-wire Voltmeters. The first instrument particularly 

 adapted to the measurement of alternating potential differences 

 was the Cardew voltmeter, invented by Major Cardew, R. E. 



In this instrument the current was passed through a long thin 

 wire of platinum-silver, and by a suitable mechanism the ex- 

 pansion of this wire, due to its rise of temperature, caused the 

 index to move over the scale. Later designers have been able 

 to improve on Cardew 's arrangement for translating the ex- 

 pansion of the wire into the movement of the index, so that a 

 much shorter wire may be employed, thus rendering the in- 

 strument less cumbersome. 



The ingenious multiplying device used by Hartmann and 

 Braun is shown in principle in Fig. 132. 



The active wire ADB is of platinum-indium, DEC is a very 

 fine phosphor-bronze wire, and EFG is a silk fiber which passes 

 once around the drum F and is drawn taut by the spring S. On 

 the passage of the current, ADB is heated and expands, the 

 slack is taken up by the spring S and the index is thus moved 



16 



