The galvanometer is fixed on a graduated slate disc, round which the platinum wire is stretched. Underneath the slate disc three resistance coils of the value of 10, 100, and 1000 Siemens' units are wound on a hollow wooden block, which protrudes at one side, and on the projection carries the terminals for the reception of the leading wires from the battery and unknown resistance. The adoption of three different resistance coils enables the measuring of large as well as small resistances with sufficient accuracy. [Illustration: GALVANOMETERS FOR TESTING. PLATE XXII] The whole instrument is mounted on a wooden disc, which is supported by three levelling screws, so that it may be turned round its axle. On the same axle a lever is placed which bears at its end an upright arm, carrying a contact roller. This roller is pressed against the platinum wire round the edge of the slate disc by means of a spring acting on the upright arm, and forms the junction between the _A_ and _B_ resistances of a Wheatstone's bridge, which resistances are formed by the platinum wire on either side of the contact roller, one of the three resistance coils forming the third resistance of the bridge. _G_ is the galvanometer, _k_ a milled head from which the needles are suspended, and by turning _k_ they can be raised or lowered, _m_ is the head of a screw which arrests or frees the needle when in motion. _h__{1}, _h__{2}, _h__{3}, _h__{4}, are the terminals of the respective ends of the three resistance coils, viz., 10, 100, and 1000 units, which are wound on the wooden block _C_; these terminals may be connected to each other by means of stoppers, and therefore one or more of the resistances may be brought into circuit as desired, and to the ends of these terminals the wires of the artificial resistances are connected as shown on diagrams Pl. xxiv., Figs. 1, 2, 3_a_ and 3_b_; _f_ is the graduated slate disc, round which the platinum wire is stretched in a slight groove at the edge of the disc, and is inserted in such manner that about half its diameter protrudes beyond the slate. The ends of the platinum wire are soldered to two brass terminals _l_ and _l_^{1}, which are placed at the angles formed by the sides of the gap in the slate disc, and which form the junctures, as in the ordinary resistance bridge, between _A_, _n_, and the galvanometer on one side, and _B_, _X_, and the galvanometer on the other side, of the parallelogram. The terminal _l_ is permanently connected by a thick copper wire or metal strip to terminal _h__{1}, and the other terminal _l_^{1} is connected in a similar manner to terminal III. Slate is adopted for the material of which to make the disc _f_, because it is found by experience to be the material which is the least sensitive to variations in the weather or temperature.