Electric power is to be had through motors. A direct current dynamo and a direct current motor are identical in construction. That is, a motor becomes a generator if belted to power; and a generator becomes a motor, if connected to electric mains. This is best illustrated by citing the instance of a trans-continental railroad which crosses the Bitter Root Mountains by means of electric power. Running 200 miles up a 2 per cent grade, it is drawn by its motors. Coasting 200 miles down the 2 per cent grade on the other side of the mountains, its motors become generators. They act as brakes, and at the same time they pump the power of the coasting weight of this train back into the wires to help a train coming up the other side of the mountains. [Illustration: Connections of shunt motor and starting rheostat] Just as there are three types of direct current generators, so there are three types of direct current motors: _series_, _shunt_, and _compound_, with features already explained in the case of generators. Motors are rated by horsepower, and generators are rated by kilowatts. Thus a one kilowatt generator has a capacity of 1,000 watts; as a motor, it would be rated as 1000/746 horsepower, or 1.34 horsepower. Their efficiency varies with their size, ranging from 40 to 60 per cent in very small motors, and up to 95 per cent in very large ones. The following table may be taken as a guide in calculating the power required by motors, on 110-volt circuits: 1/4 Horsepower 2-1/2 amperes, or 275 watts 1/2 hp 4-1/2 amperes, or 500 watts 1 hp 9 amperes, or 990 watts 2 hp 17 amperes, or 1.97 kilowatts 3 hp 26 amperes, or 2.86 kilowatts 5 hp 40 amperes, or 4.40 kilowatts 7-1/2 hp 60 amperes, or 6.60 kilowatts 10 hp 76 amperes, or 8.36 kilowatts 15 hp 112 amperes, or 12.32 kilowatts An electric motor, in operation, actually generates electricity, which it pushes back into the line as a counter-electromotive-force. The strength of this counter force, in volts, depends on the motor's speed, the same as if it were running as a dynamo. For this reason, when a motor is started, and before it comes up to speed, there would be a rush of current from the line, with nothing to hold it back, and the motor would be burned out unless some means were provided to protect it for the moment. This is done by means of a starting rheostat, similar to the regulating rheostat on the dynamo switchboard. This resistance box is connected in "series" with the armature, in the case of shunt and compound motors; and with the entire motor circuit in the case of a series machine.