Radio Broadcast (May-Oct 1922)

212 Radio Broadcast vibrating system) the less will be the force tending to produce the electric oscillations, and consequently the slower their frequency will be. The greater the inductance of the coil (just as the larger the mass of the vibrating weight in a spring pendulum) the greater will be the electric inertia of the circuit, and consequently the lower will be the natural alternating current frequency. REGARDING THE CONTINUITY OF OSCILLATIONS HOW long will such a circuit continue to oscillate, once the electric vibration has been started? Will the condenser continue to discharge and recharge indefinitely, or will the electric energy originally stored all be used up after a certain number of oscillations have taken place? The fact is that each successive re-charge is a little less than the one preceding it, because some of the electric energy is lost in heating the wire of the coil and circuit during each oscillation. The amount of energy thus lost is proportional to the electrical resistance of the circuit, which is simply a measure of the opposition which exists to current flow in any conductor. Clearly, the greater the resistance of the oscillating circuit, the more energy will be lost at each swing, and consequently the fewer electric vibrations that can take place before the current dies away to an immeasurably small value. Fig. I shows a simple oscillating circuit in which the resistance is small, being merely that of the wires and coil. In such a circuit the oscillations will continue for a comparatively large number of swings, and hence it is called a persistent oscillator. Fig. 2 represents a gradually reducing or persistent train of oscillations such as would exist in a persistently oscillating (or, as it is often called, feebly damped) circuit. On the other hand, we may increase the resistance of our circuit by inserting a resistor as in Fig. 3. This will make the circuit less persistent or more highly damped, and, since each condenser recharge will be considerably smaller than that which preceded it, the number of cycles of oscillation before the current dies away to a useless value will be much reduced. Fig. 4 indicates such a highly damped train of oscillations, the reduced number of vibrations resulting from an increase of the circuit resistance. If the resistance is made too large, the circuit will not develop any free electric oscillations whatever, for so much of the condenser energy will be used up in the first discharge that no inertia or recharging effect will appear. Now let us consider what this matter of cir Condenser I Inductance Coll Fig. I : A persistently oscillating simple resonant circuit cuit persistence, or the varying number of free oscillations, has to do with sharpness of tuning. We have seen that as the natural frequency' of a receiving antenna is varied, from a value Current 100% Time lOOfa Fig. 2: A persistent train of oscillations such as would occur in the circuit of Fig. 1. below the wave frequency of an arriving radio signal, upward to and then beyond that frequency, the current flowing in the antenna circuit increases to a maximum and then deResistor ■WLflRr Condenser > Inductance Coll Fig. 3: A simple resonant circuit including a resistance unit which decreases its persistence creases. The largest current value occurs at resonance, or when the frequencies are in agreement. The rapidity with which the current increases as the resonant point is approached is what determines the sharpness of tuning of the circuit. The antenna-to-ground circuit of Fig. 5 behaves almost exactly as does the closed res