522 ELECTRICAL MEASUREMENTS 



of spaces in this length is determined. Each one of the spaces 

 corresponds to 100 revolutions. Thus the maximum number of 

 revolutions made by the watt-hour meter in ^ hour is found. 

 The ordinary formula for the watt-hour meter is 



NXKX 3,600 



Kllowatts= tx 1,000 - 



where N is the number of revolutions of the meter disc occurring 

 in t sec. and K is the disc constant of the meter. The kilowatts 

 demand corresponding to one space on the paper tape, that is, 

 to 100 revolutions, if they occurred in J^ hour would then be 



100 X K X 3,600 



30 X 60 X 1,000 " 



If instead of one space in J^-hour, there be any other number, 

 the above is simply multiplied by the number of spaces. To 



FIG. 309. Record from Ingalls relay demand indicator. 



illustrate : take the tape shown in Fig. 309 and assume that the 

 disc constant, K, of the watt-hour meter is 25. Then the kilo- 

 watts demand is given by 0.2 X 25 X (maximum number of 

 spaces in % hour). 



Where the marks are closest together there are eight spaces in 

 % hour, so the demand is 



0.2 X 25 X 8.0 = 40 kw. 



It will be noticed that the device gives information of value 

 other than the maximum demand, for it tells just how the cus- 

 tomer's load varies and gives the hour at which the maximum 

 demand is reached. This may or may not be at the time of the 

 peak of the load on the station. 



The accuracy of this device depends : first, on the accuracy of 

 the watt-hour meter to which it is applied; second, on the rate 

 of the clock mechanism. 



