PRACTICAL OPERATION OF DYNAMOS. 233 



to consumers at a certain price per month for each lamp installed. 

 The 1 6 candle-power incandescent lamp is the unit generally em- 

 ployed, and the price in many cases is fifty cents per month per 

 lamp, with a scale of discounts increasing with the number of 

 lamps. This system of charging is called the flat rate system. 

 The use of this system simplifies the station book-keeping and it 

 avoids the cost of meters, but it is very unsatisfactory inasmuch 

 as a wasteful customer pays no more than an economical one. 



2. The meter system. The selling of electrical energy by the 

 kilowatt -hour as indicated by a watt-hour meter is almost univer- 

 sal in large electric plants, and it is coming to be the rule among 

 small plants also. It may seem that the use of the watt-hour 

 meter would, like the use of the gas meter, insure an equitable 

 system of charging customers, but it is not so. In supplying gas 

 for lighting, a large storage * reservoir makes the gas generating 

 plant independent of the irregular consumption of the gas. In 

 an electric plant, on the other hand, the electrical energy must, in 

 most cases, be generated as used and the capacity of the plant 

 must be sufficient to meet the maximum demand for current. 

 That is to say, the size and cost of the plant is determined by the 

 maximum demand, so that the greater value of the electrical 

 energy at the time of maximum demand must be considered in 

 any equitable schedule of prices. Two simple cases will serve 

 as a basis for the discussion of prices as follows : 



Case L If the demand for current by each customer of a 

 central station were uniform day and night, or if all the customers 

 were to use current according to one fixed daily regime, f then it 

 would be entirely equitable to charge a uniform price per kilowatt- 

 hour, inasmuch as the same fraction of the total energy supplied 

 to each would be supplied during the time of maximum demand. 

 This uniform schedule of prices should be subject to a scale of 



* The use of the storage battery for equalizing the load of an electric station is dis- 

 cussed in a subsequent chapter. 



f Imagine a curve of which the abscissas represent the hours of the day and the 

 ordinates the current used by a customer. This curve represents the " daily regime " 

 of the customer. 



