devices upon gas and gasoline engines and in many devices and experiments in which high-tension electricity is employed. [Illustration: FIG. 304.--The transformer has a closed core; the induction coil, an open core.] [Illustration: FIG. 305.--The laminated iron core of a transformer.] [Illustration: FIG. 306.--Cross-section of the transformer shown in Fig. 305 showing the magnetic field around the primary and secondary coils.] =309. The Transformer.=--This is like the induction coil in that it uses a _primary_ and a _secondary_ coil, and an iron core to carry the magnetic field. (See Fig. 304.) They differ in that the transformer has a _closed_ core or one forming a continuous iron circuit, while the induction coil has an _open_ core, or one in which the magnetic field must travel in air from the north to the south poles of the core. The transformer must always be used with an _alternating_ current while the induction coil may use either a direct or an alternating current. Further, the _induction_ coil always produces a higher E.M.F. while the transformer may produce an E.M.F. in its secondary coil that is either higher or lower than the one in the primary. The former is called "_step-up_" while the latter is a "_step-down_" transformer. The alternating current in the primary coil of the transformer produces an _alternating magnetic flux_ in the iron core. This iron core is _laminated_ (see Fig. 305) to prevent the heating that would result if a solid core were used. The alternating magnetic flux induces in the secondary coil an E.M.F. in accordance with the following rule. The ratio of the _number_ of _turns_ in the _primary_ to the _number_ of the _turns_ in the _secondary_ coil equals the ratio of the electromotive forces in these respective coils. If the secondary coil has 8 turns while the primary has 4, the E.M.F. of the secondary will be just twice that of the primary. Or, if in the primary coil of the transformer Fig. 306 is an E.M.F. of 110 volts, in the secondary will be found an E.M.F. of 220 volts. [Illustration: FIG. 307.--A commercial transformer.]