On the disc being turned as shown in the figure, the current coming by the binding screw _P_ passes into the piece _o_, the plate _r_, and finally the binding screw _b_, which by means of a copper wire leads the current to the apparatus in connection with _b_; then returning to the binding screw _b'_, the current reaches the plate _r'_, the piece _i_, _e_, and so to the battery by the binding screw _N_. If the disc is turned so that the handle _m_ is half way between _c_ and _c'_, the pieces _o_ and _i_, _e_, being no longer in contact with the plates _r_ and _r'_, the current will not pass. If _m_ is turned as far as _c_, the plate _o_ will then touch _r'_, and the current pass to _b'_, and return by _b_, thus reversing its direction. "Peg" switches are also often used; they are arranged so that the removal or insertion of a brass peg or plug cuts out, or completes a circuit. _Rheostat._--A rheostat is an instrument used for the comparison of resistances. [Illustration: SHUNT, COMMUTATOR, RHEOSTAT. PLATE XXV] Wheatstone's rheostat, which is shown in elevation at Fig. 91, consists of two cylinders _A_ and _B_, one of brass and the other of non-conducting material, so arranged that a copper wire can be wound off the one on to the other by turning a handle _C_. The surface of the non-conducting cylinder _B_ has a screw thread cut in it for its whole length, in which the turns of the copper wire lie, so that its successive convolutions are well insulated from each other. Two binding screws _D_, _D'_ connected with the ends of the copper wire are provided, to which the circuit wires are connected. A scale is attached at _E_, by means of which the number of convolutions on _B_ can be read off; and parts of a revolution are indicated on a circle at one end. The handle _C_ can be shifted from one cylinder to the other. Supposing the rheostat introduced into a circuit, and the whole of the copper wire wrapped on the metal cylinder _A_, then, on account of the large section of this metal cylinder, its resistance may be entirely neglected, but for every convolution of the wire on the non-conducting cylinder =B=, a specific resistance is introduced into the circuit. The amount of resistance can thus be varied as gradually as desired by winding on and off the cylinder _B_. This instrument is often used in connection with the thermo galvanometer. _Resistance Box._--The general arrangement of a resistance box is shown in the diagram Fig. 92. Between two terminal binding screws _T_ and _T__{1} secured on a vulcanite slab are fixed a series of brass junction pieces _a_, _b_, _c_, _d_; each of these is connected by a resistance coil to its neighbour, as shown at 1, 2, 3, and 4. A number of brass conical plugs with insulating handles of vulcanite are provided, which can be inserted between any two successive junction pieces, as between _T_ and _a_, or _a_ and _b_.