Fig. 281.—A magneto.
Fig. 282.—A shuttle armature.
Fig. 283.—The induced current has a field which opposes the motion of the magnet. The heavy line represents the direction of the induced current.
299. Lenz's Law.—While one is turning the armature of a magneto if the two wires leading from its coil are connected, forming what is called a "short circuit," the difficulty of turning the armature is at once increased. If now the circuit is broken, the armature turns as easily as at first. The increased difficulty in turning the armature is due to the current produced in the coil. This current sets up a magnetic field of its own that opposes the field from the steel magnets. This opposition makes it necessary for work to be done to keep up the motion of the coil when a current is passing through it. This fact is called Lenz's Law. It may be expressed as follows: Whenever a current is induced by the relative motion of a magnetic field and a conductor, the direction of the induced current is always such as to set up a magnetic field that opposes the motion. Lenz's Law follows from the principle of conservation of energy, that energy can be produced only from an expenditure of other energy. Now since an electric current possesses energy, such a current can be produced only by doing mechanical work or by expending some other form of energy. To illustrate Lenz's Law, suppose that the north-seeking pole of a bar magnet be inserted in a closed coil of wire. (See Fig. 283.) The current induced in the coil has a direction such that its lines of force will pass within the coil so as to oppose the field of the bar magnet, when the north pole of the magnet is inserted so as to point to the left. That is, the north pole of the helix is at the right. Applying the right-hand rule to the coil, its current will then be counter clockwise. On withdrawing the magnet, the current reverses, becoming clockwise with its field passing to the left within the coil.
A striking illustration of the opposition offered by the field of the induced current to that of the inducing field is afforded by taking a strong electromagnet (see Fig. 284) and suspending a sheet of copper so as to swing freely between the poles. When no current flows through the magnet the sheet swings easily for some time. When, however, the coils are magnetized, the copper sheet has induced within it, currents that set up magnetic fields strongly opposing the motion, the swinging being stopped almost instantly. The principle is applied in good ammeters and voltmeters to prevent the swinging of the needle when deflected. The current induced in the metal form on which is wound the galvanometer coil is sufficient to make the needle practically "dead beat."
Fig. 284.—The magnetic field stops the swinging of the sheet of copper.
300. The Magneto and the Dynamo.—Magnetos are used to develop small currents, such as are used for telephone signals, and for operating the sparking devices of gasoline engines. They are therefore found in automobiles containing gasoline motors. The most important device for producing electric currents by electromagnetic induction, however, is the dynamo. It is employed whenever large currents are desired. The principle of this device is similar to that of the magneto except that it contains an electromagnet for producing the magnetic field. Since the electromagnet can develop a much stronger field than a permanent magnet, the dynamo can produce a higher E.M.F. and a much larger current than the magneto.
