ELECTRICITY METERS 



515 



If a current is now sent through the heater strips, the air in bi is 

 heated and expands, causing the liquid to flow slowly into the 

 index tube iz which is in front of the graduated scale; the flow 

 will continue until the permanent state of temperature corre- 

 sponding to that particular current is reached. 



Owing to the heat capacity of the strips, the glass bulb, etc., 

 and the poor thermal conductivity of the glass, the response of the 

 indicator to the increase of current is sluggish; this lag of the 

 reading behind the increase of current is essential to the success- 

 ful operation of any such device, for it must not take cognizance 



Time in Minutes 



FIG. 301. Showing characteristics of Wright demand indicator. 



of currents which last for only a very short time. The indication 

 desired is that due to the sustained maximum. Fig. 301 shows 

 this gradual increase of reading when the current is kept constant. 

 It is intended that approximately 90 per cent, of the full-load 

 registration be accomplished in 4 min. and the entire registra- 

 tion in about 40 min. 



It will be noted from Fig. 301 that the rise to a fair approxima- 

 tion to the final reading, say 85 per cent of it, occurs more 

 abruptly when the indicator is worked at about its full capacity 

 than when it is lightly loaded. Fig. 302 illustrates this point; a 

 50- and a 100-amp. indicator were tested in series at 45 amp. 



