while the other key gives it a negative charge. One terminal of the galvanometer G' is connected to the condenser C' while the other terminal is connected to earth.

709. Siphon Recorder.-The instrument now commonly used for receiving cable messages is the siphon recorder devised for the purpose by Lord Kelvin. It is a galvanometer of the type which later became known as the D'Arsonval form. A coil of wire hangs between the poles of a powerful magnet, and through this coil the cable currents pass, causing it to turn. Attached to the suspended coil is a fine capillary tube of glass shaped like a siphon, one end of which dips into a little cup of ink. The other end of the siphon tube just touches a strip of paper which is carried along by clockwork. As the coil turns the siphon moves to and fro across the paper, tracing a wavy line as the paper moves along. An automatic jarring apparatus prevents the friction between the paper and point of the siphon from interfering with the free motion of the coil.

ELECTROMAGNETIC INDUCTION.

710. Faraday's Discovery.-The year 1831 was made memorable by the discovery of electromagnetic induction by Michael Faraday, then professor in the Royal Institution in London. In seeking to find some action of an electric current on a neighboring conductor Faraday having placed a coil of wire carrying an electric current upon another coil which was connected to a galvanometer, found that if the electric current was interrupted or broken there was a sudden deflection of the galvanometer lasting only for an instant, and when the battery connection was made again there was an equal deflection but in the opposite direction. But the steady flow of current in one coil had no effect whatever upon the other.

These momentary currents are called induced or secondary currents, while the battery current by which they are produced is called the primary current. The corresponding coils of wire are known as the primary and secondary coils.

711. Induction by a Moving or Varying Current.-Faraday also showed that when a coil carrying a current is moved either toward or away from another coil connected to a galvanometer, an induced current is set up.

Such an arrangement as shown in figure 414 may be used,

where the primary coil A has a current flowing through it from the battery and the secondary coil B is joined to the galvanometer. If the coil A is either pushed down inside of the coil B or withdrawn from it, an induced current is obtained which flows around B in the opposite direction to the current in A when the two are pushed together, but in the same direction. as in A when the coils are drawn apart.

If while the coil A is inside coil B the current in A is made weaker, an induced current is set up the same as though A were

being withdrawn. But when the current in A is strengthened the effect is as though the coils were moved closer together.

712. Induction by Magnets. Since a coil of wire carrying a current is surrounded by a magnetic field, it may be supposed that a magnet will produce a similar effect, and experiment shows this to be the case. When a bar magnet is thrust into a coil of wire connected in circuit with a galvanometer there is an instantaneous swing of the needle of the galvanometer, but the needle at once returns to its zero position and remains there so long as the magnet is held at rest; when it is withdrawn from the coil. there is another instantaneous deflection opposite to the first.

If the experiment is.repeated with the magnet reversed, the deflections are opposite to those previously obtained.

713. General Condition of Induction.-In general an induced current is set up in a coil whenever there is a change in the number of lines of magnetic force passing through the coil. This condition is illustrated in each of the three modes of producing induced currents just described. When the two coils of Faraday's first experiment are placed in the relation shown in figure 415 so that the lines of force due to the primary coil P instead of passing through the secondary coil pass on each side.

of it, there is no induced current in the secondary coil. So also there is no induction when a magnet is brought up to the coil in the position shown in the upper diagram of figure 416 or when the plane of the coil is parallel to the magnet as shown in the coil C on the right of the magnet in the lower diagram, but when the coil is at right angles to the magnet as in the left-hand coil D there will be an induced current when the magnet is brought up or taken away, because more lines of force of the magnet pass downward through the coil when it is near the magnet than when it is at a distance. (See $502 on number of lines of force.)

714. Induction by Earth's Field.-The inductive effect of the earth's magnetism may be easily observed by means of a coil

of large area and many turns of wire connected with a suitable galvanometer.

If such a coil is held with its plane perpendicular to the lines of the earth's magnetic force as at A, figure 417, the maximum number of lines of force will pass through it. If it is now turned quickly into the position B parallel to the lines of force, where none pass through it, there is an induced current because of the change in the number of lines of force through the coil. If the coil, instead of being turned half-way, is turned completely over, its position relative to the lines of force is exactly reversed and the inductive effect is twice as great as when it was turned half-way over. When the coil in any position is rotated about an axis OX parallel to the lines of force of the field, there is no induction since no change takes place in the number of lines of force passing through it.

FIG. 417.-Coil in earth field.

When the coil is laid flat on a table and slipped about from one place to another there is no induction, even if the table is tipped so that its top is at right angles to the lines of force,

FIG. 418.-Faraday's disc.

because the same number of lines of force pass through the coil wherever it is, since the field is uniform.

715. Faraday's Disc.-The following experiment due to Faraday shows that when a conductor moves across the lines of force of a magnetic field an induced electromotive force is developed.

A copper disc is mounted on an axis so that it can rotate between the poles of a horseshoe magnet, the axis of the disc being parallel to the lines of force. The edge of the disc dips into a mercury trough connected to one end of a low-resistance galvanometer circuit, the other end of which is put in contact with the axle of the disc.

On rotating the disc in the direction of the arrow a current is set up in the direction shown in the figure, the strength of which is proportional to the speed of revolution of the disc. If the disc is rotated in the opposite direction the current is reversed.

This experiment shows that each radial strip of the disc, as it cuts across the lines of force of the magnetic field, is the seat of an electromotive force which is found to be proportional to the number of lines of force cut across per second, for it is proportional both to the speed of rotation of the disc and to the strength of the magnetic field.

716. Electromotive Force of Induction. The general principle illustrated by the preceding experiment may be thus stated:

While a conductor is moving across the lines of force of a magnetic field it is the seat of an electromotive force the amount of which depends on the rate at which the lines of force are being cut across. The unit of electromotive force in the C. G. S. electromagnetic system is so chosen that the electromotive force of induction measured in these units is numerically equal to the number of lines of force, or unit tubes, cut across per second by the conductor; or

where E is the electromotive force induced in a conductor which is cutting across lines of force at the rate of N lines in t seconds, all the quantities being in C. G. S. units.

717. The Volt.-The unit of electromotive force defined as in the preceding paragraph is far too small for convenient use. in practice; therefore the unit of electromotive force in the practical system, which is called the volt in honor of Volta, is made equal to one hundred million-or 108—C. G. S. units, because that power of 10 gives the volt a value near to the electromotive forces of ordinary battery cells.

Hence, to find the electromotive force of induction in volts, the value found in C. G. S. units must be divided by 108.

718. Illustration.-For example, suppose a straight conductor A B one meter long (Fig. 419), is moved in the direction of the large arrow at the rate of 3 meters per second, and suppose it is in a magnetic field of strength 0.5 (about as strong as the earth's field) in which the lines of