Nikola Tesla Books
18 HIGH FREQUENCY APPARATUS Such a transformer is known as an "open core" instrument because the magnetic lines of force set up in the straight iron core must reach around through the air as shown in Fig. 1 to complete the magnetic circuit. A modification is shown in Fig. 2 which illustrates a method by means of which the windings of the transformer are partially surrounded by iron. This provides a ready path for the lines of force with a large increase in the efficiency of the instrument. Such a transformer is said to have a closed core. There are many modifications of the closed core transformer, all of which have merits peculiar to the uses to which they are put. For lighting and power work, it is desirable to have the primary and secondary windings as closely "coupled" as possible and, to this end, most power transformers have very compact cores which almost cover the windings. The effect of this close coupling is to improve the "regulation" of the transformer, i.e., to reduce the fluctuation in voltage from no load to full load to a minimum. This type is shown in Fig. 4. Transformers for Condenser Charging.-In the early days of radio telegraphy, when transformers were first used Fig. 2.-Showing principle of closed core transformer. Lines of force pass through the closed magnetic circuit instead of through the air as in Fig. 1
