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One pole of the magneto-electric machine is connected by means of a rod or wire to the machine frame upon which it is to be used, and the other pole to the electromagnet in the ordinary way of conductivity of current, which means stretching the wire from one to the other.
In order to send out continuous waves in the air instead of damped waves as with a flat steel spring you can use an electric driven tuning fork, see C, in which an electromagnet is fixed on the inside of the prongs and when this is energized by a battery current the vibrations of the prongs of the fork are kept going, or are sustained, as shown in the diagram at D.
The screw is now reversed gently and slowly until the moment the rattle ceases, and then the reading is taken. Here the sense of hearing is brought into play. This is also the case when the electric contact is used. This is so arranged that the instant of touching of the point of screw, a, completes the electric circuit, in which an electromagnet of short thick wire is placed.
This is done by the ingenious device of Hipp, shown in figure 90, where M is the electromagnet, P the iron bob, from which projects a wire bearing a light vane B of mica in the shape of a butterfly.
This consists of an electromagnet, T, operated by a key, K, and separate battery. The armature lever, L, is long, pivoted in the centre, and is bent over at the end.
The principle of this instrument is the same as that of the old Kelvin instrument; the clockwork attached to it unrolls a strip of paper wound on a roller; at intervals the needle of the instrument is depressed by an electromagnet and makes a dot on the moving paper. The relative position of these dots forms the record.
The diaphragm of the phonograph mouthpiece is actuated by an electromagnet in the same way as that of an ordinary telephone receiver, and in this manner a record of the message spoken from a distance can be obtained and turned into sound at will. Evidently such a process is reversible, and the phonograph can send a message to the distant receiver.
In practice the electromagnet usually takes the form shown in figure 48, where the poles are two bobbins or solenoids of wire 61 having straight cores of iron which are united by an iron bar B, and A is the armature. Such an electromagnet is a more powerful device than a swinging needle, and better able to actuate a mechanism.
This device is known as an electromagnet, and the charging and discharging of such a magnet may, of course, be repeated indefinitely. Inasmuch as a magnet has the power of attracting to itself pieces of iron or steel, the basic importance of an electromagnet in telegraphy will be at once apparent when we consider the sounder, whose clicks are familiar to every ear.
In cases where it is desirable to reproduce the signals in the local circuit with the same variations in strength as they are received by the relay, the Edison carbon pressure relay does the work. The poles of the electromagnet in the local circuit are hollowed out and filled up with carbon disks or powdered plumbago.
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