Take a typical instance where such a plant would be feasible: Farmer Brown has a five horsepower gasoline engine--an ordinary farm engine for which he paid probably $75 or $100. Electric light furnished direct from such an engine would be intolerable because of its constant flickering. This five horsepower engine is installed in the milk room of the dairy, and is belted to a countershaft. This countershaft is belted to the vacuum pump for the milking machine, and to the separator, and to a water pump, any one of which may be thrown into service by means of a tight-and-loose pulley. This countershaft is also belted to a small dynamo, which runs whenever the engine is running. The milking machine, the separator, and the water pump require that the gasoline engine be run on the average three hours each day. The dynamo is connected by wires to the house storage battery through a properly designed switchboard. The "brains" of this switchboard is a little automatic device (called a regulator or a circuit breaker), which opens and shuts according to the amount of current stored in the battery and the strength of the current from the generator. When the battery is "full," this regulator is "open" and permits no current to flow. Then the dynamo is running idle, and the amount of power it absorbs from the gasoline engine is negligible. When the "level" of electricity in the battery falls, due to drawing current for light, the regulator is "shut," that is, the dynamo and battery are connected, and current flows into the battery. These automatic instruments go still farther in their brainy work. They do not permit the dynamo to charge the battery when the voltage falls below a fixed point, due to the engine slowing down; neither do they permit the dynamo current to flow when the voltage gets too high due to sudden speeding up of the engine. Necessarily, an instrument which will take care of a battery in this way, is intricate in construction. That is not an argument against it however. A watch is intricate, but so long as we continue to wind it at stated intervals, it keeps time. So with this storage battery plant: so long as Farmer Brown starts his engine to do his farm chores every day, his by-product of electricity is stored automatically.