Radio Broadcast (May-Oct 1922)

RADIO BROADCAST 109 This frequency is called "natural," because it is that of the system vibrating naturally or freely, without any external influence. If, now, one grasped the weight and used sufficient strength, it would of course be possible to force it to move up and down at a different rate, say once, or even ten times, per second. But this frequency would be forced frequency of movement, not a natural one. We may now consider the phrase "most-easily-attained" frequency used above. If one begins forcing the weight of this particular spring pendulum up and down at one cycle per second, and then gradually increases the rate of motion, he will find that as he approaches closer and closer to the natural frequency of four per second he will need to exert less and less effort to keep the weight swinging. At the exact natural frequency a mere touch for each vibration will maintain the oscillation; the spring and weight will seem to work together to keep on going at this particular rate. On the other hand, as it is attempted to move the pendulum faster and faster, at frequencies increasingly higher than four per second, the work required will be harder and harder. Thus, the natural frequency of four per second is the most easily attained frequency of vibration. An entirely similar set of conditions holds for an electrical circuit containing elements such as coils and condensers, which possess the electrical properties of inductance and capacitance. We may set up such a circuit, as in Fig. 3, and charge the condenser (which need consist merely of two plates of metal hung face to face and close together in the air), by connecting to it a high potential battery. On removing the battery, the condenserwill discharge through the coil, producing an alternating current which will swing back and forth at the natural frequency of the circuit. The frequency of this electrical oscillation can be changed at will by increasing the size of the condenser (i. e., its electrical capacitance) or the size of the coil (i. e., its electrical inductance). Varying these constants corresponds exactly to changing the flimsiness of the spring or the mass of the pendulum bob, in the analogous mechanical system previously described. So we have a way to control the natural or most easily attained electrical frequency of a condenser-coil circuit. The condenser of such an electrical oscillatory circuit need not be of the ordinary plateto-plate type. An aerial wire system acts. opposite to the ground below it, like an electrical condenser. The elevated wires constitute one " plate " of such a condenser and the earth forms the other; the two possess electrical capacitance with respect to each other. With the above in mind, and returning to the electrical system of Fig. i, let us imagine that a stream of radio waves having 833,000 Condenser Meter Coil Fig 3 cycles frequency strikes the aerial and induces corresponding voltages therein. Let us assume that the tuning coil has been adjusted so that its inductance, taken together with capacitance of the aerial system, gives to the circuit a natural frequency of the same value, 833,000 cycles per second. Reasoning that the induced voltages correspond to the hand driving the spring pendulum and that the resulting currents correspond to the motion of the pendulum weight, one would expect this agreement between arriving wave frequency and mosteasily-attained circuit frequency to result in the largest possible radio current flowing in the aerial-to-ground system. This is the fact, as we may determine by a relatively simple test. Consider that the frequency of the arriving waves falls slightly below 833,000 cycles per second, say to 831,000 cycles. The voltages in the aerial, and the currents produced thereby, will have this same lowered frequency of 83 1 ,000 cycles. But if the receiver is unchanged, this will no longer be the natural frequency, to which the system most easily vibrates; consequently not so much current can flow between aerial and ground. The same reduction in current would occur if the arriving wave frequency were slightly increased, say to 835,000 cycles per second. The greater the departure from the resonant condition attained at 833,000 cycles, when the arriving and natural fre