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 _l_^{ 1} 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 Figs. 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_.
Figs. 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.