CHAP. IV.

GALVANOMETER-MULTIPLIERS.

Electro-magnetic Galvanometers.

WE have pointed out, in the First Part of this work, that we may make use of the calorific and chemical properties of the voltaic current in order to measure its intensity; and we have also given certain details of galvanometers, founded upon these properties. But the action exercised by a current upon a magnetised needle has furnished a means in every respect far superior to the preceding, for determining the existence and appreciating the force of an electric current. This action, in fact, is the only one that is general, that is to say, which always, and in all cases, accompanies the presence of dynamic electricity, whatever be the nature of the circuit and the feebleness of this electricity; whilst the other actions occur only when the circuit contains a conductor capable of manifesting them, and when the current is endowed with a certain energy. Furthermore, the electro-magnetic effect of the current is instantaneous, whilst the electro-chemical effect must necessarily endure for a certain time, in order to be appreciated; and, though the calorific action is equally instantaneous, it possesses the inconvenience of not giving the direction of the current, whilst this direction is indicated in the most prompt and decided manner by the direction of the deviation of the magnetised needle.

We have seen that a conductor traversed by a current placed above a needle, but very near to it, and parallel to its axis, makes this needle deviate to the east or to the west, according as it is moving in a direction from north to south, or from south to north. If it is below, it makes it deviate to the east, when it is moving in the direction from south to north, and to the west when it is moving in the direction from north to south. It follows from this, that, if the conductor that trans

mits the current, passing first above the needle, is bent so as to return below, and so to form two parallel branches, between

Fig. 132.

which the needle is suspended (Fig. 132.), the current, that traverses the upper branch, tends to make the needle deviate in the same direction as the current that traverses the lower one, precisely because it has in the former a contrary direction to what it has in the latter. By thus arranging the wire by which the current is transmitted, we obtain an action upon the needle twice as powerful as if, being retained rectilinear, instead of being bent, it had acted only above or below. But, instead of bending it once only, we may bend it twice, which doubles the effect; three times, which trebles it; in a word, we can cause the wire to make a very great number of convolutions, and can so multiply by a considerable quantity the action of the current upon the magnetised needle. It follows from this that a very feeble current, whose action would be scarcely sensible if the wire by which it is transmitted made but one convolution, is able to exert a very marked action when the number of convolutions becomes considerable. This apparatus has therefore been named the galvanometer-multiplier. It is also sometimes called, and rightly so, the Rheometermeasurer of a current. It is to a German philosopher, M. Schweigger, that we owe the original idea of the galvanometermultiplier.

In order to construct it, we take the precaution to employ a copper wire covered with silk, so that the different convolutions may be juxtaposed and superposed, without there being a direct metallic communication from one convolution to the other, and so that the current may thus traverse the wire in all its length. We coil the wire round a wooden or metal frame, solidly fixed upon a stand, and which leaves between

its lower and upper surface the smallest possible space; it is

Fig. 133.

in the interior of this space that the magnetised needle is suspended: the two ends of the wire, which are carefully deprived of the silk that covers them, serve to place the galvanometer, that is to say, the wire of the instrument, in the circuit (Fig. 133.). At the moment when a circuit is thus

closed, providing that a current is propagated in it, we see the needle move; the direction in which it moves indicates the direction of the current, the presence of which is detected by this movement; and the number of degrees, or the size of the arc of deviation, enables us to appreciate its intensity.

Nobili's Galvanometer-multiplier.

In order to increase the sensibility of the galvonometermultiplier, M. Nobili conceived the ingenious idea of neutralising the directive force of terrestrial magnetism, which tends, in opposition to the action of the current, to maintain the needle in its normal direction, or to bring it back to it, by employing, instead of a single magnetised needle, two needles that are fixed parallel to each other, the reverse poles facing each other at the extremities of a small rod of straw or metal, which passes through their centres of gravity. One of the needles is placed within the frame and the other without; so that, as may be easily seen, the deviation which the current tends to impress upon the former accords with that which it produces upon the latter, the position of the poles being in the one the reverse of what it is in the other; for, if they had their homogeneous poles turned on the same side, they would on the contrary deviate in opposite directions, the one being above and the other below. Thus, there follows from the addition of this second needle, a considerable increase in the sensibility of the apparatus. It is necessary that the two

needles should be as similar as possible; they are suspended to a cocoon filament fixed to the upper extremity of the small stem by which they are united. If they had exactly the same magnetic force, this system would be astatic;-that is to say, would not experience any directive action on the part of the earth, and would remain in equilibrium in all azimuths. It would be a difficult matter to obtain this result: moreover, it is necessary that the system of two needles should have a slight directive force, in order to be able to assume a determinate position; and that, consequently, one of the needles should have a little more powerful magnetism than the other.

We must be careful, in the construction of the galvanometer, to choose a copper wire as much deprived of iron as possible, which is not always easy; in this respect copper, not brass, is preferable. With regard to the dimensions of the wire, they depend upon the kind of circuit into which it is to be introduced. If it is a circuit whose conductibility is imperfect, which contains liquids, for example, it is advantageous to have a long and consequently a fine wire, in order that its convolutions may be as near as possible to the needle; in fact, the introduction into the circuit of such a wire as this does not

Fig. 134.

sensibly modify its conductibility. But, if the circuit is a good conductor, all metallic, for example, the current would be too much enfeebled by the addition of a long and fine wire, and more would be lost by this cause of weakness than would be gained by the increase of sensibility of the apparatus, resulting from a more considerable number of convolutions. It would be better in this case to employ a shorter wire and one of larger diameter. We shall have occasion, in a more general

manner, to treat upon this question, which is connected with

the propagation of dynamic electricity, in the Fourth Part of this work. For the present, we shall confine ourselves to describing Nobili's galvanometer-multiplier (Fig. 134.), remarking that, according to the purpose to which it is proposed to be applied, the dimensions and the number of the convolutions of the wire must be varied; and, if we have not beforehand a very decided object in view, it is well always to have two instruments at command, one with a short wire, the other with a long one.

in.

First, the following are the dimensions of the frame:-The width is 1 inch, and its length 13 in. The opening in the upper part is in. The external height between the two horizontal cheeks is in., and the interior length between the two vertical sides is 2 It is within this interior and vacant space, 2 in. long by in. high, that the lower needle moves. The upper needle is situated above the frame, but as near as possible to the wires that are coiled around it. The silkcovered wire is less than 3 in. in diameter; and it makes 800 convolutions around the frame. The needles are two ordinary sewing needles 1 in. in length, and magnetised to saturation. They are placed parallel to each other, with the reverse poles facing, at a distance of in., and are fixed at each of the extremities of two small copper wires twisted upon each other. The suspension thread is a doubled thread of the cocoon, from 4 to 6 inches in length. The frame is placed upon a movable support, which enables us to give it all possible positions in respect to the needles. With this view it is fixed upon a pivot, which turns upon its axis by means of a wheel and pinion, that is made to move at pleasure, in one direction or another, by means of an external knob. In order that this movement of the frame may be executed, it is necessary that the two ends of the silk-covered wire should have sufficient length from the point where they quit the frame, and that to which they are fixed upon the frame of the instrument.

The cocoon filament is suspended to a bracket stem, and by means of a small drum moved by a wheel, the two-needle system may be made to descend or to ascend by insensible