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Galvanometer.

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instead of a single one, there are several circuits, provided they are insulated, the action becomes still more multiplied, and the deflection of the needle increases; or, what is the same thing, a much feebler current will produce deflection.

As the directive action of the earth continually tends to keep the needle in the magnetic meridian, and thus opposes the action of the current, the effect of the latter is increased by using an astatic system of two needles as shown in fig. 370. The action of the earth on the needle is then very feeble, and, further, the actions of the current on the two needles become accumulated. In fact, the action of the circuit, from the direction of the current indicated by the arrows, tends to deflect the north pole of the lower needle towards the west. The upper needle, a'b', is subjected to the action

AA

of two contrary currents, mn and qp, but as the first is nearer, its action preponderates. Now this current, passing below the needle, evidently tends to turn the pole, a', towards the east, and, consequently, the pole, b', towards the west; that is to say, in the same direction as the pole, a, of the other needle.

From these principles it will be easy to understand the theory of the multiplier. The apparatus, represented in fig. 371, consists of a thick brass plate resting on levelling screws; on this is a copper frame, on which is coiled a great number of turns of wire covered with silk. The two ends terminate in binding screws, n and m. Above the frame is a graduated circle, with a central slit parallel to the direction in which the wire is coiled. By means of a very fine filament of silk, an astatic system is suspended; it consists of two needles, ab and a'b', one above the scale, and the other within the circuit itself.

In using the instrument it is so adjusted that the needles, and also the slit, are in the magnetic meridian.

459a. Uses of the galvanometer.-To show, by means of the multiplier, the electricity developed in chemical actions, for instance in the action of acids on metals, two platinum wires may be attached to the binding screws, m and n. Then one of them is plunged in very dilute sulphuric acid, and the other placed in contact with a piece of zinc held in the hand, which is dipped in the liquid. An immediate deflection is observed, which

indicates the existence of a current; and from the direction which the north pole of each needle assumes, it is seen that the direction of the current is that indicated by the arrows. From which we may conclude, in accordance with the explanation given as to the origin of electricity in the simple voltaic circuit, that the acid is positively electrified and the zinc negatively.

The length and diameter of the wire vary with the purpose for which the galvanometer is intended. For one which is to be used in observing the currents due to chemical actions, a wire about millimeter in diameter, and making about 800 turns, is well

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Magnetisation by Electrical Currents.

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adapted. Those for thermo-electric currents, which have low intensity, require a thicker and shorter wire, for example, thirty turns of a wire millimeter in diameter. For very delicate experiments, as in physiological investigations, galvanometers with as many as 30,000 turns have been used.

460. Magnetisation by electrical currents. From the influence which currents exert upon magnets, turning the north pole to the left and the south pole to the right, it is natural to think, that by acting upon magnetic substances in the natural state the currents would tend to separate the two magnetic fluids. In fact, when a wire traversed by a current is immersed in iron filings, they adhere to it in large quantities, but become detached as soon as the current ceases, while there is no action or any other non-magnetic metal.

The action of currents on magnetic substances is well seen in an experiment due to Ampère, which consists in coiling an insulated copper wire round an unmagnetised steel bar. If a current be

Fig. 372.

passed through the wire, even for a short time, the bar becomes strongly magnetised. The same effect is produced with a bar of soft iron, but in this case the magnetisation is temporary; when the current ceases, the iron, which is destitute of coercive force, reverts instantaneously to the natural state; and, if in this experiment, we imagine an observer floating in the direction of the current, the north pole is always on his left hand.

If the discharge of a Leyden jar be transmitted through the wire by connecting one end with the outer coating, and the other with the inner coating, the bar is also magnetised. Hence both voltaic and frictional electricity can be used for magnetising.

CHAPTER X.

ELECTRODYNAMICS.

461. Reciprocal action of currents on currents.-Ampère did not restrict himself to trying the action of magnets and of the earth upon movable currents; he went further, and was led to the important discovery, that electrical currents act on each other as do magnets; and he thus created an entirely new branch of physics, to which the name electrodynamics has been given. The actions which currents exert on each other are different according as they are parallel or angular.

I. Two currents which are parallel, but in contrary directions, repel each other.

II. Two currents, parallel and in the same direction, attract each other.

To verify these laws use is made of the apparatus represented in fig. 373. On a wooden support are fixed two brass columns, A and

B, joined at the top by a wooden cross-piece. In the centre of this is a brass binding screw, a, and below this a mercury cup. In this is placed an iron pivot which joins the end of a copper wire. This wire is

coiled in the manner represented in the figure, terminating in a mercury cup on the base of the apparatus. It thus forms a circuit movable about the pivot.

That being premised, the circuit is arranged in the plane of the two columns, as shown in fig. 373, and the current from a Bunsen's

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Solenoids.

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battery is passed through it to the foot of the column, A; it passes thence by a copper wire to a binding screw, a; thence into the cup, ao, traverses the entire movable circuit in the direction of the arrows, reaches the cup, C, whence, by a copper strip, it reaches the foot of the column, B, rises in this, and ultimately returns to the battery. When the current passes, the circuit moves away from the columns, and, after a few oscillations, comes to rest crosswise to its original position; thus showing that the ascending current in the columns and the descending current in the circuit repel each other, thereby proving the first law.

The second law may be established by means of the same apparatus, replacing the movable circuit depicted in fig. 373 by another so arranged that the current is ascending both in the columns and in the two branches of the circuit. When the movable circuit is displaced, and the current is passed, the latter returns briskly towards the columns.

Law of angular currents.—In the case of two angular currents, one fixed and the other movable, Ampère found that there was attraction when both the currents moved towards, or both away from, the apex of the angle; and that repulsion took place when,, one current moving towards the apex the other moved away from it.

SOLENOIDS.

462. Structure of a solenoid.—A solenoid is a system of equal and parallel circular currents formed of the same piece of covered copper wire, and coiled in the

form of a helix or spiral, as represented in fig. 374. A solenoid, however, is only complete when part of the wire, BC, passes in the direction of the axis in the interior of the helix. With this arrangement, when the circuit is suspended in the mer

a

Fig. 374.

cury cups, ab, of the apparatus (fig. 373), and a current is passed through, it is directed by the earth exactly as if it were a magnetic needle. If the solenoid be removed it will, after a few oscillations, return, so that its axis is in the magnetic meridian. Further, it will be found that, in the lower half of the coils of which the solenoid consists, the direction of the current is from