236 ELEMENTS OF ELECTRICAL ENGINEERING. 



book-keeping represents a corresponding expense to the station, 

 and this expense justifies a certain fixed charge against each cus- 

 tomer for the above-mentioned service. This fixed charge is 

 usually made in the form of a minimum monthly charge. 



Fluctuating and intermittent service. If the demand for cur- 

 rent by each of the customers of a station varies in a purely erra- 

 tic way from hour to hour and from day to day, the resultant 

 demand of a large number of customers will approximate very 

 nearly to a perfectly uniform demand day and night. If there 

 are several distinct classes of customers and if the demand for 

 current by a customer in either class departs in a purely erratic 

 way from a certain mean daily regime which is characteristic of 

 that class, then the resultant demand of a large number of custo- 

 mers will be very nearly the same as if each customer were to 

 use current in strict accordance with the mean regime of his class. 

 In both of these cases the system of charging outlined above 

 under cases I. and II. is applicable when the station is large and 

 the number of individual customers great. 



In the case of a small station with a small number of individual 

 customers, erratic variations in daily regime by individuals be- 

 comes a serious matter, inasmuch as the resultant demand shows 

 considerable erratic variation. In such a case any service, such 

 as motor service for elevators, which is peculiarly subject to wide 

 variations should be charged for at a high rate which should be 

 determined partly by a consideration of the mean daily regime 

 and partly by a consideration of the amplitude of the departures 

 from this mean. There is no very satisfactory method for deter- 

 mining an equitable charge in this case, in fact a small station is 

 justified in making the charge for elevator service, for example, 

 prohibitory. 



Advantages of the meter system as compared with the flat rate 

 system. The equitable distribution of charges which is rea- 

 lized by the use of the meter system leads to an economical use 

 of current and enables a station of given capacity to supply cur- 

 rent to an increased number of customers. 



