PRACTICAL OPERATION OF DYNAMOS. 235 



its excessive demand for current from 5 or 6 or 7 o'clock to 10 

 or 1 1 o'clock P. M., (b) store and factory lighting service with 

 its almost uniform demand for current from 3 or 4 o'clock to 6 

 or 8 o'clock P. M., and (c) motive power service with a nearly 

 uniform demand for current from 7 A. M. to 6 P. M. Customers 

 of the first class should be charged at a high rate, those of the 

 second class at a lower rate, and those of the third class at a still 

 lower rate. It is to be understood that these statements refer to 

 differences in scheduled selling price and not to differences in dis- 

 counts ; the same scale of discounts may in fact be applied to 

 all. This system of class rates is exemplified by the scale of 

 prices which are given in Art. 98. 



2. By the use of the two-rate meter. The two-rate meter is 

 an ordinary watt-hour meter with two sets of dials, and a clock 

 which throws into gear one set of dials during the period of ex- 

 cessive station load, between 7 and 10 P. M. for example, and 

 the other set of dials during the remainder of the 24 hours of 

 each day. When this meter is used the customer is charged at 

 a high rate per kilowatt-hour for the energy indicated by the first 

 set of dials, and at a low rate for the energy indicated by the 

 second set of dials. 



3. By the use of the maximum demand meter. The maximum 

 demand meter is a device, generally an ammeter in its essential 

 features, of which the indicator, which is too sluggish to be 

 affected by a momentary short-circuit, is left standing at the point 

 to which it is carried by the greatest current used by the customer 

 during the month. In the use of this device the monthly charge 

 to the customer is so much for the maximum rate and so much 

 per kilowatt-hour for the energy indicated by an ordinary watt- 

 hour meter. 



The minimum charge. There is a certain amount of service 

 rendered to a customer by the mere fact that the current from a 

 central station is at his disposal at any time night or day ; on the 

 other hand the interest on the cost of the watt-hour meter, the 

 cost of its maintenance, and a portion of the cost of the station 



