62 



ALTERNATING CURRENTS 



axle is mounted the pointer of the instrument, the other end of the 

 silk fibre being fixed to the end of a bent flat steel spring. The silk 



HOT WIRE 



COMPENSATION PLATE 

 PHOSPMOR- BRONZE WIRE 

 DAMPING PLATE 



FIG. 42. Hot-wire Instrument. 



fibre which passes round the pulley is prevented from slipping by a 

 small screw. It is a well-known fact that a very slight amount of 

 elongation in a suspended wire produces a relatively enormous 

 increase of sag. Now, in the instrument under consideration we 

 have a double magnifying arrangement of this description, the 

 elongation of the hot wire being magnified by the phosphor-bronze 

 wire, and the sag of this latter giving a still further magnification. 

 A comparatively small elongation of the hot wire thus results in 

 a large angular displacement of the pointer. 



It is evident that if the hot wire were mounted on a plate whose 

 temperature-coefficient of expansion is different from that of the wire 

 itself, the instrument would act as a thermometer, and its zero would 

 change with any change of temperature in its surroundings. In order 

 to prevent this effect, and render the readings independent of the 

 surroundings, the plate supporting the wire (marked " compensation 

 plate " in Fig. 42) is made of bronze, of a composition which gives 

 a temperature coefficient of expansion nearly equal to that of the 

 wire. 



