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660 MOTION PICTURE HANDBOOK
if you used 75 amperes for a period of, say, two or three minutes, the instrument would take no account of the extra consumption, but if you used that 75 amperes for a period of five minutes or more, then the indicator would get into action and register 75 amperes, so that when the power company's man came around he would know you had used that amount of current at some time for a period of five minutes or more.
The Wright Demand Indicator is installed near the regular meter, and consists of a U-shaped tube containing sulphuric acid. When connected in circuit the current which is used passes through a coil near one leg of the U-shaped tube, the same being in effect an air chamber the bottom end of which is corked by the mercury. This coil is made of wire calculated to carry a certain definite number of amperes, and so long as the current does not exceed that amount, the coil does not become heated beyond a certain point, but if there is current consumed in excess the coil heats in exact proportion to the excess of current and the heat thus generated expands the air inside the leg of the U tube. This heating and expansion of the air is calculated to consume a period of five minutes, and its effect is to force the liquid up the other leg of the tube and over into an extension chamber, the quantity of liquid forced over being in exact proportion to the degree of heat generated in the coil, and therefore to the amperage used. Having once been forced over, the liquid will remain there, and thus the power company has an indisputable and permanent register of the highest amperage you have used for a period of more than five minutes. At the end of the month a reading is taken of the "demand indicator," and if the column of liquid which has been forced into the measuring tube is beyond a certain amount, the station charges a certain extra amount for extra load. After the reading has been taken, the indicator is unfastened and the tube tilted until the liquid runs back out of the measuring tube into the U tube, whereupon the indicator is again ready to begin operations. The reason for this maximum demand charge is logical and simple when it is once understood.
If a customer ordinarily uses 5,500 watts, or approximately 7 1/3 h. p., the power company supplying him must provide that amount of plant capacity for that particular customer. Allowing for losses in generation, transmission of the current, etc., this means about 10 h. p. in boilers, engines, generators, transmission lines and transformers, in order finally to deliver 7 1/3 h. p. to the customer. It costs real money