English: In creating powerful electric waves for communication over long distances it is necessary to employ an alternating current transformer supplied with alternating currents from a low frequency alternator D driven by an engine to charge the condenser. The transformer T₁ has its secondary or high-pressure terminals connected to spark balls S₁, which are also connected by a circuit consisting of a large glass plate condenser C₁ and the primary circuit of an air-core transformer T₂, called an oscillation transformer. The secondary circuit of this last is either connected between an aerial A and the earth E, or it may be again in turn connected to a second pair of spark balls S₂, and these again to a second condenser C₂, oscillation transformer T₂, and the aerial A. In order to produce electric oscillations in the system, the first or alternating current transformer must charge the condenser connected to its secondary terminals, but must not produce a permanent electric arc between the balls. Various devices have been suggested for extinguishing the arc and yet allowing the condenser oscillatory discharge to take place. Tesla effected this purpose by placing the spark balls transversely in a powerful magnetic field. Elihu Thomson blows on the spark balls with a powerful jet of air. Marconi causes the spark balls to move rapidly past each other or causes a studded disk to move between the spark balls. J. A. Fleming devised a method which has practical advantages in both preventing the arc and permitting the oscillatory currents to be controlled so as to make electric wave signals. He inserts in the primary circuit of the alternating current transformers one or more choking or impedance coils R₁, R₂, called “chokers,” which are capable, one or all, of being short-circuited by keys K₁, K₂. The impedance of the primary or alternator circuit is so adjusted that when both the chokers are in circuit the current flowing is not sufficient to charge the condensers; but when one choker is short-circuited the impedance is reduced so that the condenser is charged, but the alternating arc is not formed. In addition it is necessary to adjust the frequency so that it has the value of the normal time period of the circuit formed of the condenser and transformer secondary circuit, and thus it is possible to obtain condenser oscillatory discharges free from any admixture with alternating current arc. In this manner the condenser discharge can be started or stopped at pleasure, and long and short discharges made in accordance with the signals of the Morse alphabet by manipulating the short-circuiting key of one of the choking coils (see British Patent Specs. Nos. 18865, 20576 and 22126 of 1900, and 3481 of 1901).