The current flowing in a circuit may be increased or decreased by changing the length of resistance wire inserted into it. This is done by means of a rheostat. Rheostats are obtainable in a variety of different patterns, but the type most com
monly used in laboratories is illustrated in Fig. 35.2. It consists of a coil of constantan wire wound on a slate or enamelled iron former and provided with a sliding contact on a brass rod. Current enters the coil through a terminal at one end and flows along the coil until it reaches the sliding contact. From here it flows along the brass rod, and thence through the circuit. The resistance thus introduced into the circuit will depend on the position of the slider along the coil. * Constantan (60% copper, 40% nickel), Manganin (84% copper, 12% manganese, 4% nickel), Nichrome (80% nickel. 20% chromium).