The great importance and value of properly "tuning" the circuit of radiotelegraphic apparatus cannot be overestimated and for that reason the subject can hardly be passed without some further explanation. Its effects are two-fold. In the first place it is always desirable and highly important that wireless messages should be, so far as is possible, selective, inasmuch as there are often several stations in the same immediate neighborhood operating at the same time. This result is reached by tuning and it is possible for them all to transmit different messages at the same time without confusion by the proper arrangement of the wave length. The second effect is the transmission of messages over long distances with the comparative consumption of small amount of power by adjusting the "period" or electrical length of the circuits until the oscillations "flow in harmony" with each other and resonance is secured.
Perhaps the only way that these results may be made clearly intelligible is by resort to a graphical example. Suppose that a very heavy weight were suspended from a chain as shown in the illustration and that it is struck at regular intervals, once every second, with a hammer.
Every time that the hammer strikes the ball it will give it an impulse and cause it to swing slightly. If the chain is short, the ball will swing faster, while if it is long it will swing more slowly. We will suppose that the ball is struck from such a direction that it starts to swing over toward A. The ball is so heavy and the hammer so light in comparison however that the ball does not swing very far and soon commences a return journey. If it should return to the point B just as the hammer delivers another blow the force of the blow will be expended in stopping the ball rather than adding to its motion because they are both traveling in opposite directions. However if the chain is lengthened so that it has a period of swing lasting one second, the succeeding blow will strike the ball after it has reached the point C and is on its return journey, thus imparting fresh energy because both the ball and hammer come together at the right time when they are both swinging together. Proper adjustment of the length of the chain will make it possible for the hammer to always descend at the right moment to add its energy and motion to that previously given the ball. The result will be considerable increase in the amplitude of the swing.
FIG. 86.—Chain and ball arranged to illustrate effect of tuning.
From this we may easily perceive how it is possible by shortening or lengthening the period of an electrical circuit to so adjust it that resonance is secured and each succeeding oscillation will take place at the proper time to assist the previous one, not dying away after one or two surges and becoming what is known in technical language as rapidly "damped."
FIG. 87.—Loose coupled helix.
The instruments for accomplishing these things consist as previously explained, in the case of a transmitter, of the helix and in the receiving station of various tuning coils and condensers.
