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. h1, h2, h3, h4, 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], [3a] and [3b]; 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 l1, 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 l1 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.
The slate disc is graduated on its upper edge through an arc of 300 degrees, zero being in the centre, and the graduations figured up to 150 on each side at the terminals l and l1 of the bridge wire.
In the centre of the circular plate E of polished wood, supported upon three levelling screws b, b, b, a metal boss is inserted, in which turns the vertical pin a which carries the instrument. This pin, being well fitted to the boss, supports the instrument firmly, but at the same time allows it to be turned freely round its vertical axis without losing its horizontal position when once obtained.
On the arm D D, which turns on the pin a, and somewhat behind the handle g, there is a small upright brass arm d turning between two screw points r, and carrying in a gap at its upper end a small platinum jockey pulley e turning on a vertical axis. This pulley forms the movable contact point along the bridge wire, against which it is kept firmly pressed by means of a spring acting on the arm d. The arm D D, which is insulated from the other parts of the apparatus, is permanently connected with the terminal I. On the top of d a pointer Z or a vernier is fixed, which laps over the upper edge of the slate disc and points to the graduations.
To the pin a is attached a circular disc of polished wood C, about one inch thick, and having a groove turned in its edge for the reception of the insulated wires composing the resistances. The disc C has a projection c, which carries the five insulated terminals marked I., II., III., IV., V., as shown on [Fig. 1] and [2], Pl. xxiii. Terminals III. and IV. can be connected by a plug, II. and V. by the contact key K. Terminal I. is in connection with the lever D D.
[Fig. 3] and [4], Pl. xxiii. show the shunt box supplied with the galvanometer if specially desired; the copper connecting arms a, a are screwed to the terminals II. and IV. By inserting a plug at c ([Fig. 4], Pl. xxiii.), the galvanometer is put out of circuit altogether, whilst by plugging either of the other holes shunts of the value of 1/9, 1/99, or 1/999, are introduced into the circuit, and the effect upon the galvanometer is reduced to 1/10, 1/100, 1/1000, respectively of what it would have been without the insertion of the shunt.
[Fig. 5] and [6], Pl. xxiii., show a battery commutator allowing to bring into the circuit four different amounts of battery power. It is placed in the battery circuit whenever consecutive tests with different batteries are desired to be made, it being only necessary to change the place of the stopper in the battery commutator, the terminal screw a of the battery commutator being connected to terminal V. of the galvanometer, and the screws b, b, b, b to various sections of the battery: see diagram of connections, [Fig. 4], Pl. xxiv.
The application of the universal galvanometer will be clear from the diagrams on Pl ii.; instructions, however, for its practical use are added further on, and also tables for use when measuring conducting resistances.