"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.
SHUNT, COMMUTATOR, RHEOSTAT.
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 T1 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.
With all the plugs inserted, the electrical current will flow direct from T to T1, the large metallic junction pieces directly connected by the plugs would offer no sensible resistance; but if all the plugs were removed, then the current would flow through each of the coils 1, 2, 3, and 4, and the resistance in the circuit would be the sum of the resistances of those four coils. With the plugs arranged as in the figure, the current would flow through coil 4 only, and the resistance in the circuit would be equal to the resistance of that coil.