Radio Broadcast (May-Oct 1922)

How to Begin to Enjoy Radio these two factors are present in a system, it is seen from the experiment that the system, when once set in motion, will vibrate or oscillate of its own accord. • It is evident then that if we wish an electric current to flow of its own accord to and fro (oscillate) in a circuit, there must be introduced into the circuit, each of these two factors. One of these factors is called inductance; the other factor is called capacity (sometimes called capacitance). The fundamental idea that 1 desire you to get now about inductance is this: When inductance is present in a circuit it tends to prevent any change in an electric current. Thus when a current tries to die away the inductance of a circuit will try to prevent it from dying away. Inductance acts like a weight in this respect. The fundamental idea I desire you to get about capacity is this: When capacity is present in a circuit it will store up electricity, but as soon as it begins to store some it tries to get rid of it. The more electricity that it stores, the harder it tries to get rid of it. Thus it is seen that capacity of a circuit corresponds to the springiness of the hack saw blade. Let us represent, as in Fig. i, a circuit having inductance and capacity and study it. Capacities A and B are equal to each other. B o ns Q. O Inductance Fig. I o a. O By some means let us put a charge upon the capacity represented by A in the figure. This charge is shown by the minus signs, which represent the small particles of electricity called electrons. As has been noted, the capacity TRIES TO GET RID of its charge and is able to do so, as there is a ciicuit leading to the other capacity. The electrons pass along the wire to the capacity B thus making a current through the inductance. If it were not for the inductance, when the capacity B had received half of the electrons that were on capacity A, the electrons would stop moving, for capacities A and B would each be trying to get rid of their electrons with equal force. But the electrons are moving from A to B and hence there is a current flowing. The inductance now comes into play and prevents the cur rent FROM DYING AWAY, hence causing all the electrons on A to move over to B. The conditions are exactly the same as they were at first and exactly the same events take place with the exception that it is now capacity B which is charged and which gets rid of its electrons. This to and fro movement will continue until the energy dies away by wastage. The to and fro movement of the electrons is an oscillating current. Thus it happens that if a circuit contains inductance and capacity, an electric current will oscillate in the circuit when energy has been supplied. This is the exact counterpart of a mechanical system represented by the hack saw blade and weight which has elasticity (springiness) and mass (weight). The reader should compare step by step the action that takes place in the mechanical and the electrical systems. He will find that the action of one is similar to that of the other. natural frequency and resonance NOW let us go back to the hack saw blade and weight experiment and notice another fact about it. Start it vibrating and notice the number of vibrations per second. Change the amount of weight on it and cause it to vibrate again. The number of vibrations per second is different from what it was. Change the stiffness of the blade (make it longer or shorter). Again the number of vibrations per second have been changed. What we have learned is this: that this mechanical system has a natural frequency of vibration which may be changed by changing either the stiffness of the blade or the weight attached or both. Electrical oscillations are exactly the same. In Fig. I the number of to and fro motions of the electrons (oscillations) depends upon the value of the inductance and capacity in the circuit. Changing either one or both of these will change the number of oscillations per second. This may be summed up by saying that an electrical circuit containing an inductance AND CAPACITY HAS A NATURAL PERIOD OF OSCILLATION, DEPENDING ON THE VALUES OF THE INDUCTANCE AND CAPACITY. Every receiving set has inductance and capacity. What you do when you turn the knobs on the receiving set is to change either the amount of inductance or capacity in the circuit. (On some sets there are also knobs used to control the current supplied the vacuum tube or tubes.)