518 



ELECTRICAL MEASUREMENTS 



on each side of the circuit; in this case the average of the two 

 readings is used. Obviously there is no guarantee that the two 

 maxima occur simultaneously. The cheapness of the Wright 

 demand indicator permits its use with the small consumer. In 

 practice, immediately after the reading of the instrument has 

 been taken, it is reset by the reader employed by the supply 

 company. No trace of the indication remains, and no opportun- 

 ity exists for its subsequent verification in case of a dispute. 



General Electric Type W Watt Demand Indicator. This 

 device is made for use on alternating-current circuits and for 

 polyphase work only; it is essentially a polyphase indicating watt- 

 meter of the induction type, which is provided with an exceed- 

 ingly strong electromagnetic damping system, so that its response 

 to variations of the load is rendered very slow. The indications 



are given on a dial which is 

 provided with two pointers, one 

 of which indicates the load (sub- 

 ject to the time lag of the in- 

 strument); the other shows the 

 sustained maximum to which the 

 load has risen. Fig. 305 shows, 

 in diagram, the essential features 

 of the instrument. Wi and Wt 

 are the two wattmeter elements 

 which are essential to the meas- 

 urement of power in the ordi- 

 nary polyphase systems, for, as 

 is usual in such measurements, 

 the " two- wattmeter method" is 

 here employed; DI is the disc in 

 which currents are induced by the elements Wi and W^; these 

 currents react with the magnetic fields set up by Wi and Wz and 

 cause the indication of the instrument. The disc is made of brass 

 in order that the effect of temperature changes may be minimized, 

 since the electrical resistance of alloys like brass varies much less 

 with changes of temperature than does that of pure metals, such 

 as copper. 



The controlling spring against which the movable system de- 

 flects is at S. In reality three springs are used in series in order 



-Dial 



M M M M 

 FIG. 305. Diagram for General 

 Electric type W watt demand 

 indicator. 



