As with the dynamo, one of the marvels of the motor is its efficiency. In perfect machines, ninety to ninety-five per cent of the electrical energy supplied can be converted into mechanical energy. For this reason it has become a competitor with, and even successor of, steam in countless cases, and especially where water-power can be commanded. A prime motor, in the shape of a water-wheel, may be made to drive scores of secondary motors in places hundreds of miles away. The power developed by the waterfall at Lauffen, Germany, is transmitted one hundred miles to Frankfort, with a loss of only twenty-five per cent of the original horse-power. [Illustration: THE GOLDEN CANDLESTICK.] In its adaptation for practical use, the motor, like the dynamo, assumes all sizes and embraces a host of ingenious devices, yet its power and usefulness always centre around, or are contained in, its two efficient parts, its armatures and fields of force. We have seen how in the dynamo the armatures became the source of induced currents by being made to cut the fields of force of electro-magnets. Now, a dynamo can be made to work in an opposite way; that is, by making the magnetic fields of force rotate in front of the coils or armatures. In the motor, the field of force is mostly established by the current directly from the dynamo. This current passes also through the armature, which begins to rotate, owing to the force of the field upon it. This rotation of the armature through the field of force produces in the armature conductors an electro-motive force, which is the measure of the power of the motor, be the same great or small. VI. “AND THERE WAS LIGHT.” [Illustration: ANCIENT LAMP.]