ELECTRICITY METERS 



513 



being carried on, the demand indicator is an instrument of in- 

 creasing importance and is being rapidly developed. 



The Wright Maximum-demand Indicator. 13 This indicator, 

 which was the first maximum-demand instrument, may be looked 

 upon as a registering differential thermometer, one bulb of which 



FIG. 300. Wright demand indicator. 



is heated by the passage of the current through a suitable heater 

 coil which closely surrounds it. It registers the maximum current. 



The internal appearance of the indicator as made in the smaller 

 sizes, up to and including 25 amp., is shown in Fig. 300. 



The essential working parts are the indicator tube, with its 

 attached index tube i z , the scale s, and the heater strips C. 



