242 



ELECTRICAL MEASUREMENTS 



over the scale. The vane W moving in the air gap of the per- 

 manent magnet M serves as a damping device. The moving 

 system is carried by insulating studs at A } B and C. These are 



FIG. 132. Diagram for Hartmann & Braun hot-wire voltmeter. 



supported on a back plate constructed of two metals in such a 

 proportion that the net coefficient of expansion is the same as 

 that of the wire, so the effect of changes of room temperature is 

 minimized. The position of the end A of the working wire may 



be altered by turning the screw V and 

 the zero position of the pointer thus 

 adjusted. The coefficient of expansion 

 of the platinum-iridium wire is less 

 than that of the platinum-silver wire 

 formerly used; it can be run, however, 

 at a much higher temperature, the re- 

 sult being a considerable reduction of 

 the zero shift, which is troublesome in 

 hot-wire instruments. 



The working parts of the Roller hot- 

 wire voltmeter are sketched in Fig. 133. 

 The pivot D carries a pulley B and the 

 forked arm DFG. A silk fiber is 

 stretched between F and G. This 



passes around and is made fast to the 

 FIG. 133. Diagram for . 

 Roller hot-wire voltmeter. Pivoted drum E, to which the pointer 



is attached. It is evident that any ro- 

 tation of the drum D will cause the pointer to move over the 

 scale. The wire ABC passes around and is made fast to the 

 pulley B. The current flows through the wire AB since the end 



