ELECTRICITY METERS 511 



two potentiometers to measure /i + 7 2 and 7 2 when the galva- 

 nometer is balanced, the multiplying factor can be very accurately 

 determined in the laboratory. 



In a particular case the capacity of the test meter used is 40 

 amp. and there are two sets of shunts and auxiliary resistances, R, 

 mounted on the same base, the ratings being 1,000 amp. and 2,000 

 amp. (see Fig. 299). The voltage drop in the shunts at full load 

 is 100 millivolts, and in the resistor R it is 400 millivolts. The 

 adjustable resistor, r, is a strip of Boker metal, the effective length 

 of which can be altered by the use of screw clamps. To obtain a 

 fine adjustment a carbon compression rheostat is placed in paral- 

 lel with the strip. 



DEMAND INDICATORS 



The business of supplying electrical energy is peculiar because, 

 broadly speaking, the product to be sold cannot be stored. It 

 must be used as generated and the supply company must stand 

 ready to furnish its product to customers at any hour. 



The demand of the individual consumer for the company's 

 product passes through a fairly well-defined daily and seasonal 

 variation, naturally being the greatest when the days are the 

 shortest. It is necessary to install generating machinery of suf- 

 ficient capacity to carry safely the greatest aggregate demand, 

 or the peak of the load as it is called, and provide a sufficient 

 reserve. This means that machinery, representing a consider- 

 able investment, must stand idle for a larger portion of the time. 

 This peculiarity of the business has led electrical companies to 

 divide the cost of supplying their consumers into two parts, 

 " fixed costs," which are independent of the amount of the prod- 

 uct delivered to the consumer, and " running costs," which 

 depend directly upon the amount of energy delivered. 



On account of the large amount of time during which a por- 

 tion of the machinery and the distribution system is idle, the 

 " fixed costs" are large and efforts have been made to establish 

 systems of rates which are in accordance with Hopkinson's 

 maxim that "the charge for a service rendered should bear some 

 relation to the cost of rendering it." 



The investment necessary in order that a company may stand 

 ready to supply any group of consumers is dependent on the 



