GALVANISM 595 strument, because it is kept at right angles to the axis of the coil through which the current passes. A horizontal, graduated circle, contain- ing a declination needle, is fixed within a vertical circle, the two turning on a vertical axis which passes through the centre of a lower station- ary, horizontal, graduated circle, an index being used to measure the arc of revolution. A stout copper wire, covered with silk, is passed one or more times around the rim of the vertical cir- cle, according to the strength of the current which is to be measured. For weaker currents the coils are increased. In using the instru- ment the plane of the vertical circle is placed in the plane of the magnetic meridian. The needle and index will then each stand at 0, respectively on the upper and lower horizontal circles. If a current is now sent through the wire, the needle will be deflected ; and if the vertical circle is rotated till the needle lies in its plane, and therefore again points to 0, the deflection will be marked by the index on the lower circle. The deflecting force of a current acting at right angles to the axis of the needle exactly balances the magnetic force of the earth, which is proportional to the sine of the angle which the needle makes with the mag- netic meridian. An instrument called a differ- ential galvanometer is sometimes used to mea- sure at the same time the difference in strength of two currents. For this purpose two sepa- rate coils of the same sized wire are passed an equal number of times around the same needle. When two currents are sent in contrary direc- tions through the coils, the amount of deflection produced will indicate the difference in strength between them. Sir William Thomson's mirror galvanometer (fig. 20) measures a delicate gal- vanic current with more precision than any other instrument that has been invented. A magnet is suspended within a coil of wire which varies in size and length according to the size and length of the conductor through which the current has already passed. If it has passed through long circuits containing bad conductors, the coil should be long and of fine wire, because the current will have been so much weakened that a fine long wire is now FIG. 20. Sir "William Thomson's Mirror Galvanometer. sufficient to conduct it, and therefore it may be used to induce a considerable magnetic force. The coil is placed within the cylinder mounted upon the rectangular box shown in the figure, and to one side of the magnet suspended within it there is attached a mirror which reflects a ray of light upon a horizontal graduated screen in front of it, and behind which there is placed a lamp which sends a ray of light through an orifice. A slight deflection of the magnet, which together with the mirror weighs only a few grains, gives the reflected ray a wide range over the graduated screen. A bar mag- net, S, placed in the magnetic meridian, is used to counteract the earth's magnetism and there- by increase the delicacy of the instrument. Another bar magnet, T, perpendicular to the magnetic meridian, is used to adjust the instru- ment to zero when no current is passing. An instrument called a rheostat, invented by ^ylleatstone for the purpose of comparing re- sistances, is represented in fig. 21. Two cylin- ders of equal diameters turning upon their axes are held in a frame. One of them, A, is of metal, and the other, B, of some non-conduc- tor, as vulcanite or baked wood. There is a spiral groove in the non-conducting cylinder in which a wire, connected with the binding screw 0, is wound for an indefinite distance, FIG. 21. Kheostat. and then transferred to the other cylinder and wound upon it to its further end. By turning the crank connected with one of the cylinders the wire may be all transferred from one to
