THE MEASUREMENT OF CURRENT 59 



hot-wire principle was the voltmeter invented by Major Cardew, 

 R. E. In this instrument the expansion of a long platinum wire 

 due to the passage of the current caused the pointer to move over 

 the scale. The more recent form of hot-wire instrument made 

 by Hartmann and Braun is shown in Fig. 24. Its distinguishing 

 feature is the exceedingly ingenious method of multiplication by 

 which the small expansion of a short wire is caused to produce a 

 large deflection of the pointer, without the use of levers, knife 

 edges, or gears. 



A and B are lugs carried by a composite metal frame, the length 

 of the iron and brass parts being such that the frame is designed 



FIG. 24. Diagram for Hartmann and Braun hot-wire ammeter. 



to have the same coefficient of expansion as the wire AB. The 

 lug C is carried by a portion of the frame having the same coeffi- 

 cient of expansion as the wire DC. An inextensible cord, EG, 

 passes once around the drum, F, to which the pointer is attached, 

 and is drawn taut by the spring S. 



The current flows through the wire AB, which is heated and 

 expands; the slack is taken up by the spring and the pointer is 

 moved over the scale. The zero reading may be adjusted by 

 the screw a. 



The wire A B is of platinum-iridium. This has a smaller coeffi- 

 cient of expansion but a higher melting point than the platinum- 

 silver wire formerly used. It may thus be worked at a higher 

 temperature and gives less trouble from variations of room 

 temperature. 



For work at ordinary frequencies the range of the indicator 

 is extended by the use of shunts. This is of importance in labora- 



