Andrew Bernknopf, age 14, of Pittsburgh, Pa., for his question:

HOW DOES AN ELECTRIC GENERATOR WORK?

Even the mightiest electric generator has only two main parts. Some sort of energy is needed to keep one of the parts turning, while the other stays still.    Its duty is to change mechanical energy into the energy of electric current.

This astounding magic is called electromagnetic induction—though nobody can explain exactly     why or how it works.

In the 1830s young Michael Faraday cleaned up the lab to pay for his college education. Between lectures he pondered the relationship between magnetism and electric current. While experimenting with a magnet and copper disk, he learned how to generate a small current  and invented the dynamo.

Faraday's little dynamo had a hand crank to turn a copper disk so that it cut through the lines of force surrounding a small magnet.  It generated a current through a loop or circuit of copper wire. A huge modern generator uses the same basic principle to send a mighty surge of electricity through its power lines.

Part of the mystery lies in the invisible force lines that loop outside a magnet from pole to pole. An armature of metal is used to cut through these magnet lines again and again. For reasons unknown, this sends the surging current through the wire circuit attached to the generator.

As a rule, the armature is an iron core inside miles of coiled copper. The armature or the mighty magnet may be spun around to cut through the magnetic force lines. The generator uses some kind of mechanical energy to keep it spinning. This may be a steam engine or a waterfall. Faraday used muscle power to turn a hand crank.

The generator does not create energy but uses mechanical energy to induce electrical current through a wire circuit. It does this by generating voltage, a pushing power that sets electrons jogging all through the wires. The energy in the current is the motion of zillions of electrons.

Faraday's little dynamo sent electrons marching in one direction through his wire circuit. This was direct current, or DC. Our big generators spin and reverse in half cycles, inducing electrons to jog back and forth. This is AC, or alternating current  which jogs to and fro at so many frequencies per second. The power is increased with bigger armatures, more powerful magnets and faster spinning.