Page:Elementary Text-book of Physics (Anthony, 1897).djvu/384

370 apparatus which are of great importance not only for laboratory use but in the arts.

The telephonic receiver consists essentially of a bar magnet around one end of which is carried a coil of fine insulated wire. In front of this coil is placed a thin plate of soft iron. When the coils of two such instruments are joined in circuit by conducting wires, any disturbance of the iron diaphragm in front of one coil will change the magnetic field near it, and a current will be set up in the circuit. The strength of the magnet in the other instrument will be altered by this current, and the diaphragm in front of it will move. When the diaphragm of the first instrument, or transmitter, is set in motion by sound waves due to the voice, the induced currents, and the consequent movements of the diaphragm of the second instrument, or receiver, are such that the words spoken into the one can be recognized by a listener at the other.

Other transmitters are generally used, in which the diaphragm presses upon a small button of carbon. A current is passed from a battery through the diaphragm, the carbon button, and the rest of the circuit, including the receiver. When the diaphragm moves, it presses upon the carbon button, and alters the resistance of the circuit at the point of contact. This change in resistance gives rise to a change in the current, and the diaphragm of the receiver is moved. The telephone serves in the laboratory as a most delicate means of detecting rapid changes of current in a circuit.

The various forms of magneto-electrical and dynamo-electrifcal machines are too numerous and too complicated for description. In all of them an arrangement of conductors, usually called the armature, is moved in a powerful magnetic field, and a suitable arrangement is made by which the currents thus induced may be led off and utilized in an outside circuit. The magnetic field is sometimes established by permanent magnets, and the machine is called a magneto-machine. In most cases, however, the circuit containing the armature also contains the coils of the electromagnets to which the magnetic field is due. When the armature rotates, a current starts in it, at first due to the residual magnetism of some part of