Radio Broadcast (Nov 1922-Apr 1923)

Simple Bulb Transmitters >5 fields travel from the antenna 186,000 miles a second in all directions. As these two fields cross the receiving antenna tuned to the same wave, the lines of force (the electromagnetic field) create therein, by simple induction, a current, similar, excepting in strength, to the transmitting one. This current is augmented by the electricity which the antenna also "picks up" from the electrostatic field. Hence any method of charging a transmitting aerial with a rapidly interrupted or alternating current (for the field moves only as the current fluctuates) will set up radio waves. HOW WE OBTAIN HIGH-FREQUENCY CURRENTS THERE are several systems of generating high-frequency currents: the spark, alternator, arc, and bulb. The spark, until the advent of the audion, was by far the most popular means of generating radio waves, and was demonstrated by Hertz many years ago. When a condenser is charged to a high potential by a transformer or static machine, and the 100 OF A SECOND \N AMPLITUDES \ FIG. I terminals then approached to within sparking distance, the condenser will discharge across the gap in a spark. This spark, is not a single discharge as it appears to the eye, but a series of isolated discharges, first in one direction and then in the other, each one weaker than the last, until the condenser is discharged. The spark is, of course, accompanied by similar reversals in the current which, in electrical language, "alternates." But for radio-telegraphic purposes the condenser is immediately recharged, at which point the action is repeated, and the resulting radio current assumes the form of a group of oscillating discharges which can be indicated diagrammatically, as in Fig. 1. The alternator is a modification of the alternating current generator or dynamo, designed to produce oscillations of a very high frequency, an effect that is also achieved by the arc and bulb. The radio impulses set up by these last named systems are of a continuous nature and are shown in Fig. 2. In contrast with the preceding illustration, it will be noted that the successive oscillations in Fig. 2 are all of equal amplitude or power (that is there is no A A ^AAAAAAAAAAAAAAAAAA uVVn VVvVVv UVnl/vVVV FIG. 2 "damping") and the wave train is not broken up into groups of audio frequency. In Fig. 3, the coil or inductance L2 and condenser C comprise an oscillating circuit. Li is a similar winding connected through switch S to a battery. As the switch is closed a current flows through Li and sets up a magnetic field which, expanding, cuts L2. As L2 is cut by these lines of force, a current is induced in it, flowing, we shall assume, in the direction indicated by the outer arrows. However, when the switch is opened, the flux collapses and L2 is now cut by the lines of force in the opposite direction, and, by the same laws of induction, the induced current momentarily follows the direction indicated by the inside arrows. Hence, by rapidly opening and closing the switch S, an alternating current is induced in the second winding. This switch action is exactly what is effected by the three-element tube, except that the interrupting of the circuit is accomplished far more rapidly than would be possible by hand. Fig. 4 shows a somewhat revised circuit with the audion bulb replacing the switch. For this purpose almost any of your detector or amplifier tubes will do. When the filament is lighted, the potential on the grid, as determined by the condenser and leak, having first been adjusted to a slight positive charge, the tube fig. 3 will operate for a fraction of a second as indicated in A, Fig. 5. This small plus charge tends to draw the electrons (electrons are negative, and unlike charges attract one