The most sensitive galvanometer as yet constructed is the reflecting galvanometer of Sir William Thomson, a diagram of which is shown at Fig. 84. A small piece of magnetised steel watch spring, 3/8ths of an inch long, is fastened with shellac on the back of a little round concave mirror, and of about the size of a fourpenny piece. This is suspended by a piece of unspun silk thread in the centre of a coil of many hundred turns of fine copper wire insulated with silk, and well protected between the turns with varnish. The two ends of the coils are soldered to terminal screws _a_, _b_, so that any conducting wire can be joined up to it as required. The little mirror hangs in the middle of its coil, with the magnet lying horizontally. By means of a lamp _L_ placed behind the screen, the light of which passes through a slit _M_, and is thrown on the face of the mirror, a spot of light is reflected on the scale _N_. When a current passes through the coil, the little magnet is deflected, and since the magnet is attached to the mirror, which is very light, both are deflected as forming one body, and the spot of light moves accordingly along the scale _N_. A powerful steel magnet _S_ is placed above the coil, and can be moved up or down, whereby the directive force of the earth may be increased or weakened. This magnet _S_ is used to steady the spot of light, which otherwise would shake about, and there would be no certainty about the measurement. A second magnet _T_ is placed perpendicular to the magnetic meridian, to adjust the zero of the instrument, i.e., to bring back the spot of light to a fiducial mark at the centre of the scale when no current is passing. This instrument should only be used at important stations, and when special tests of a delicate nature are required to be applied. _Astatic Galvanometer._--An astatic galvanometer is that in connection with which an astatic needle is employed, by the use of which the sensitiveness of a galvanometer is greatly increased. An astatic needle is a combination of magnetised needles _with their poles turned opposite ways_. At Fig. 85 a diagram of such an instrument is shown. Two magnets _D_ and _C_ are joined, with the north pole of one over the south pole of the other, forming one suspended system. In the ordinary form of astatic galvanometer the needles _D_ and _C_ are about two inches long, and are each covered by a coil, these latter being so joined that the current must circulate in opposite directions round the two so as to deflect both magnets similarly. The deflection of the needles _D_ and _C_ is observed by means of a pointer or glass needle _A_, _B_, rigidly connected with the astatic system by a prolongation of the brass rod connecting the needles _D_ and _C_. The coils are flat and of the shape indicated in Fig. 85, and are also made in two halves, placed side by side with just sufficient space between them to allow the rod to hang freely.