=Self-induction.=--This term signifies _the property of an electric current by virtue of which it tends to resist any change of value._ Self-induction is sometimes spoken of as _electromagnetic inertia_, and is analogous to the mechanical inertia of matter. It is on account of self-induction of the induced currents in the armature winding of a dynamo, that sparks appear at the brushes when the latter are not properly adjusted, hence the importance of clearly understanding the nature of this peculiar property of the current. Self-induction is fully explained in the chapter following. CHAPTER XI INDUCTION COILS The induction coil has always been a popular piece of apparatus with those interested in electrical science; the experiments which can be performed with its aid are very numerous. It is of considerable importance, especially in its application to such useful purposes as X ray work, wireless telegraphy and _ignition_ for gas engines. The latter has caused manufacturers to give much attention to the development of the induction coil, resulting in many refinements of design and construction. Induction coils may be divided into two general classes: 1. Primary coils; 2. Secondary coils. The subject of electromagnetic induction has been fully explained in chapter X, but it may be said, with special reference to induction coils, that the operation of the two classes just mentioned is respectively due to: 1. Self-induction; 2. Mutual induction. =Self-induction.=--This is the property of an electric current by virtue of which _it tends to resist any change in its rate of flow_. It is sometimes spoken of as _electromagnetic inertia_ and is analogous to the mechanical inertia of matter. Self-induction is due to the action of the current upon itself during variations in strength. It becomes especially marked in a coil of wire, in which the adjacent turns act inductively upon each other upon the principle of _mutual induction_ arising between two separate adjacent circuits. Self-induction manifests itself by giving “_momentum_” to the current so that _it cannot be instantly stopped when the circuit is broken_, the result being a bright spark at the moment of breaking the circuit. On account of this spark a primary induction coil is used in low tension or “make and break” ignition systems. [Illustration: FIG. 135.--Diagram illustrating the action of mutual induction between two circuits; the one including a source of electrical energy and a switch; the other including a galvanometer, but having no cell or other electrical source. During the increase or decrease in the strength of the current as on closing or opening the key a current is _induced_ in the secondary circuit in a direction opposite to that of the primary current as indicated by the arrows.]