Fig. 305.—The laminated iron core of a transformer.
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.
Fig. 307.—A commercial transformer.
310. Uses of Transformers.—In electric lighting systems, dynamos often produce alternating currents at 1000 to 12,000 volts pressure. It is very dangerous to admit currents at this pressure into dwellings and business houses, so that transformers are installed just outside of buildings to "step-down" the high voltage currents to 110 or 220 volts. The lighting current that enters a house does not come directly from a dynamo. It is an induced current produced by a transformer placed near the house. (See Fig. 307.) In a perfect transformer the efficiency would be 100 per cent. This signifies that the energy that is sent into the primary coil of the transformer exactly equals the energy in the secondary coil. The best transformers actually show efficiencies better than 97 per cent. The lost energy appears as heat in the transformer. "The transfer of great power in a large transformer from one circuit to another circuit entirely separate and distinct, without any motion or noise and almost without loss, is one of the most wonderful phenomena under the control of man."
311. The mercury arc rectifier is a device for changing an alternating current into a direct current. It is frequently used for charging storage batteries where only alternating current is supplied by the electric power company. It consists of an exhausted bulb containing two carbon or graphite electrodes marked G in Fig. 308 and a mercury electrode marked M. It is found that current will pass through such a bulb only from the graphite to the mercury but not in the reverse direction. In operating the device, the secondary terminals of an alternating current transformer T are connected to the graphite terminals of the rectifier. A wire connected to the center of the secondary of the transformer at C is attached to the negative terminal of the storage battery SB. The positive terminal of the battery is connected to the mercury electrode of the rectifier tube through a reactance or choke coil R. This coil serves to sustain the arc between the alternations. Sw is a starting switch, used only in striking the arc. It is opened immediately after the tube begins to glow.
Fig. 308.—Diagram of a mercury arc rectifier.
Important Topics
Transformer, induction coil, mercury arc rectifier, construction, action; uses of each.