charge of a condenser may thus be measured by discharging it through a ballistic galvanometer (see Art. 4186). The needle must not be damped.

219. Methods of Damping: Aperiodic Galvanometers. To prevent the needle from swinging to and fro for a long time devices are used to damp the motion. These are:

(a) Air Damping. A light vane attached to needle beats against the air and damps the motion. In mirror instruments the mirror itself damps, particularly if confined in a narrow chamber.

(b) Oil Damping. - A vane dips into oil.

(c) Magnetic Damping. If the needle swings close to or inside a mass of copper, it will soon come to rest by reason of the eddy-currents (Art. 457) induced in the copper. Eddy-currents damp the motion of the suspended coil in instruments of that class.

The period of swing can be reduced by diminishing the weight and leverage of the moving parts so as to lessen their moment of inertia. It can also be lessened (at the expense of the sensitiveness of the instrument) by increasing the controlling forces. An instrument so well damped as to come to rest without getting up a periodic swing is called an aperiodic or dead-beat instru

ment.

220. Voltmeters, or Potential Galvanometers. If any galvanometer be constructed with a very long thin wire of high resistance as its coil, very little current will flow through it, but what little current flows will be exactly proportional to the potential difference that may be applied to the two ends of its circuit. Such a galvanometer, suitably provided with a scale, will indicate the number of volts between its terminals. Many forms of voltmeter-galvanometers exist, but they all agree in the essential of having a coil of a high resistance- sometimes several thousand ohms. The suspended-coil galvanometers described in Art. 216 make excellent voltmeters.

Weston's voltmeter, largely used in America, is of this class, the coil being delicately pivoted, and controlled by a spiral spring. Any sensitive mirror galvanometer can be used as a voltmeter by simply adding externally to its circuit a resistance sufficiently great. There are also other voltmeters that depend on electrostatic actions; they are a species of electrometer and are described in Art. 290. Cardew's voltmeter (see Art. 430) differs from the above class of instrument, and consists of a long thin platinum wire of high resistance, which expands by heating when it is connected across a circuit. All voltmeters are placed as shunts across between the two points the potential difference of which is to be measured. They are never joined up in circuit as amperemeters are.

221. Amperemeters, or Ammeters. A galvanometer graduated so that its index reads directly on the scale the number of amperes (Art. 207) flowing through the coil is called an Amperemeter. Such instruments were introduced in form for industrial use in 1879 by Ayrton and Perry. Many other forms were subsequently invented. In Ayrton and Perry's instruments (Fig. 127), which are portable and "dead-beat" in action, the needle, which is oval in shape, is placed between the poles of a powerful permanent magnet to control its direction and make it independent of the earth's magnetism. By a peculiar shaping of the pole-pieces, needle, and coils, the angular deflexions are proportional to the strength of the deflecting current. These amperemeters are made with short coils of very low resistance and few turns of wire. Ayrton and Perry also arranged voltmeters (Art. 220) in a similar form, but with long coils of high resistance.

T T'

Fig. 127.

Among the innumerable forms of amperemeter in

P

commerce there are a number in which there is neither magnet nor iron, but which depend upon the mutual force between a fixed and a movable coil traversed by the current. These are dealt with in Art. 394, and are suitable for alternate currents as well as continuous currents. Of this kind are Siemens' electrodynamometer and the Kelvin balances.

Fig. 128.

Other instruments depend upon the magnetic properties of iron under the influence of the current. Of this class are the Schuckert instruments represented in Fig. 128. An index pivoted in the axis of an open coil carries a light strip of soft iron seen endways at B. Another strip A is fixed within the coil. The current flowing round the coil magnetizes these strips and they repel one another. Gravity is here the controlling force.

LESSON XVIII. - Currents produced by Induction

222. Faraday's Discovery. In 1831 Faraday discovered that currents can be induced in a closed circuit by moving magnets near it, or by moving the circuit across the magnetic field; and he followed up this discovery by finding that a current whose strength is changing may induce a secondary current in a closed circuit near it. Such currents, whether generated by magnets or by other currents, are known as Induction Currents. And the action of a magnet or current in producing such induced currents is termed electromagnetic (or magneto-electric) induction,* or simply induction. Upon

*The student must not confuse this electromagnetic induction with the phenomenon of the electrostatic induction of one charge of electricity by another charge, as explained in Lesson III., and which has nothing to do with currents. Formerly, before the identity of the electricity derived from different sources was understood (Art. 246), electricity derived thus

this principle are based the modern dynamo machines for generating electric currents mechanically, as well as induction coils, alternate-current transformers, and other appliances.

223. Induction of Currents by Magnets. If a coil of insulated wire be connected in circuit with a sufficiently delicate galvanometer, and a magnet be inserted rapidly into the hollow of the coil (as in Fig. 129), à momentary current is observed

to flow round the circuit while the magnet is being moved into the coil. So long as the magnet lies motionless in the coil it induces no currents. But if it be rapidly pulled out of the coil another momentary current will be observed to flow, and in the opposite direction to the former. The induced current caused by inserting the magnet is an inverse current, or is in the opposite direction to that which would magnetize the Fig. 129. magnet with its existing polarity. The induced current caused by withdrawing the magnet is a direct current.

Precisely the same effect is produced if the coil be moved towards the magnet as if the magnet were moved toward the coil. The more rapid the motion is, the stronger are the induced currents.

The magnet does not grow any weaker by being so used, for the real source of the electrical energy generated is the mechanical energy spent in the motion.

mous.

from the motion of magnets was termed magneto-electricity. For most purposes the adjectives magneto-electric and electro-magnetic are synonyThe production of electricity from magnetism, and of magnetism from electricity, are, it is true, two distinct operations; but both are included in the branch of science denominated Electromagnetics.

If the circuit is not closed, no currents are produced; but the relative motion of coil and magnet will still set up electromotive-forces, tending to produce currents.

Faraday discovered these effects to be connected with the magnetic field surrounding the magnet. He showed that no effect was produced unless the circuit cut across the invisible magnetic lines of the magnet.

Faraday

224. Induction of Currents by Currents. also showed that the approach or recession of a current might induce a current in a closed circuit near it. This may be conveniently shown as an experiment by the apparatus of Fig. 130.

A coil of insulated wire P is connected in circuit with a battery B of two or three cells, and a key K to turn the

beurrent on or off. A second coil S, entirely unconnected swiththe first, is joined up with wires to a sensitive galvanometer G. We know (Art. 202) that a coil of wire oingwhich a current is circulating acts like a magnet. badowe find that if while the current is flowing in P, the coil is suddenly moved up toward S, a momentary torrent will be induced in S. If P is suddenly moved away from another momentary current will be observed

The second circuit. The first of these two momentary currents is an inverse" one, while the second one is