522 AGRICULTURAL ENGINEERING 



pass through the lamp. The product of the volts by the 

 amperes gives the electric power in watts, the watt being the 

 unit of power. Thus for the lamp just referred to, the current 

 consumption would be 110xJ/2, or 55 watts. One horsepower 

 is equal to 746 watts. The output of dynamos or generators 

 is rated in kilowatts, or units of 1000 watts. Electricity is 

 purchased by the kilowatt-hour, which is electric current at 

 the rate of one kilowatt continued for one hour. One kilo- 

 watt equals 1.34 horsepower; thus to drive a one-kilowatt 

 dynamo, a 1J^- or 2-horsepower engine is provided, as some 

 power is lost in the friction of the dynamo itself. 



Selection of the Plant. In deciding upon a plant one of 

 the first questions that arises is the matter of the voltage at 

 which the plant is to be operated. Electric light plants are 

 now made to furnish current at 30, 60, 110, or even higher 

 voltage. The common voltages are 30 and 110. The 

 lower voltages have some advantages; viz., (1) first cost of 

 the storage battery is lower; (2) the battery has fewer parts; 

 (3) it can be used better with low candle-power lamps; 

 and (4) the lamps, having shorter filaments, are stronger. 



The disadvantage of a low voltage lies primarily in the 

 fact that it is not standard with any lighting plants and is 

 inconvenient to procure lamps and other fixtures for it. 

 There is a decided saving with high voltage, however, in 

 connection with the wiring, especially if the current is to be 

 transmitted far, since the size of wire required to furnish a 

 given light with electricity varies inversely with the voltage. 

 In other words, a wire will transmit twice as much electricity 

 through a given size at 110 volts as at 55 volts. 



If the maximum number of 25-watt lamps in service at 

 one time does not exceed 20, or the demands upon the dynamo 

 from miscellaneous sources such as motors, flat iron, etc., 

 does not exceed 500 watts, a one-half kilowatt generator 

 may be used. A one-horsepower gasoline engine will furnish 



