234 ELEMENTS OF ELECTRICAL ENGINEERING. 



discounts provided the station management wishes to increase the 

 general demand for current, which will be the case so long as the 

 demand is less than can be met by the station. When the de- 

 mand reaches the full capacity of the station the scale of dis- 

 counts should be rearranged so as to be less favorable to larger 

 users than before, unless it is possible at once to secure funds for 

 increasing the size of the station. 



Case II. If there are several distinct classes of customers 

 differing as to their daily regime in the use of current, then [on 

 the assumption that the total cost of operation of a station may 

 be approximately separated into two parts, namely, a constant 

 part A which depends upon the size (that is the capacity) of the 

 station, and a variable part B which is proportional to the station 

 load] the proper basis upon which to establish an equitable 

 schedule of prices is as follows : 



(a) Each customer should be charged so much per year on 

 the rate at which he uses electrical energy during the period of 

 excessive station load. This charge should of course be subject 

 to a scale of discounts and the income from it should cover the 

 constant part A of the total cost of operating the plant, with a 

 margin of profit. 



(b) Each customer should be charged, in addition to (a), a 

 certain amount per kilowatt-hour for the total electrical energy 

 supplied to him. This charge should also be subject to a scale 

 of discounts, and the income from this charge should cover the 

 variable part B of the total cost of operating the plant, with a 

 margin of profit. 



There are three practicable methods for carrying out this 

 system of charging, as follows : 



I. By classification of rates. The usual method is to classify 

 customers according to their daily regime in the use of current, 

 and to establish a distinct rate per kilowatt-hour for each class. 

 For example, there are three more or less distinct classes of 

 service in most electric plants, namely, (a) domestic lighting 

 service with its negligible demand for current during the day and 



