In a single circuit, consisting of a straight wire and a parallel return wire there is little or no self-induction. When a circuit containing a primary induction coil and a battery is closed there is no spark because at the instant of closing the circuit the current is at rest and on account of self-induction the current cannot at once rise to its full value.
Mutual Induction.—This is a particular case of electromagnetic induction in which the magnetic field producing an electromotive force in a circuit is due to the current in a neighboring circuit.
The effect of mutual induction may be explained with the aid of fig. 135. If, as illustrated, a circuit including a battery and a switch, be placed near another circuit, formed by connecting the two terminals of a galvanometer by a wire, it will be found that whenever the first circuit, 1, is closed by the switch, allowing a current to pass in a given direction, a momentary current will be induced in the second circuit, 2, as shown by the galvanometer. A similar result will follow on the opening of the battery circuit, the difference being that the momentary induced current occurring at closure moves in a direction opposite to that in the battery circuit, while the momentary current at opening moves in the same direction.
Currents, besides being induced in circuit 2 at make or break of circuit 1, are also induced when the current in 1 is fluctuating in intensity.
The most marked results are observed when the make or break is sudden, the action being strongest at the break of the current in 1.
The inductive effect of the current in the arrangement shown in fig. 135 is very weak.
Ques. What name is given to circuit 1?
Ans. The primary circuit.
Ques. What name is given to circuit 2?