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?= Ans. The _secondary circuit_. =Ques. What names are given respectively to the currents in circuits 1 and 2?= Ans. The _primary_ and _secondary_ or _induced currents_. =Primary Induction Coils.=--These represent the simplest form of coil, and are used chiefly in low tension ignition to intensify the spark when a battery forms the current source. A primary coil consists of a long iron core wound with a considerable length of low resistance insulated copper wire, the length of the core and the number of turns of the insulated wire winding determining the efficiency. The effect of the iron core is to increase the self-induction. [Illustration: FIG. 136.--Primary induction coil as used for low tension ignition. Coils of this type are made in a great variety of form and size. Ordinarily the winding consists of about six convolutions of No. 14 copper wire. The winding is usually covered and the ends capped with ebonite heads so that the core wires are not exposed.] The spark produced, as previously explained, is due to self-induction, and it should be remembered that in the operation of the coil, the _spark occurs at the instant of breaking the circuit_, _not at the instant of making_.