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CHAPTER V.

TUNING AND COUPLING, DIRECTIVE WAVE TELEGRAPHY.

Tuning has been mentioned in several places but not explained in any greater measure than was necessary to render a conception which would enable the reader to follow the text intelligently in order not to depart from the subjects under discussion there and consequently defeat the purpose of clearness.

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

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B

FIG. 86.-Chain and ball arranged to illustrate effect of tuning.

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.

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

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previous one, not dying away after one or two surges and becoming what is known in technical language as rapidly "damped."

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.

Helix and tuning coils are divided into the "inductive" or "loose" and the "direct" or close coupled types. Inductive tuning coils are known as "loose-couplers" and "receiving transformers." Inductive helixes consist simply of two helixes, separated from one another as shown in the accompanying illustration. The upper helix, called the

secondary, can be raised or lowered upon a central support. Varying the distance between the primary and secondary is varying the "coupling." There are several advantages

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derived by using loose coupled sending helixes, the chief of which lie in the fact that it is possible to radiate larger amounts of energy and also decrease the "damping."

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FIG. 89.-The principle of the hot-wire ammeter.

In order to tune a transmitter, the "hot-wire" ammeter is necessary. This instrument makes use of the property which electrical conductors possess to become heated and expand when a current is passed through them.

The accompanying diagram serves to illustrate the principle of the "hot-wire" meter. A piece of platinum wire is stretched tightly between two rigidly fixed posts. A thread leads from the center of the "hot wire" to a small spindle around which it passes once or twice. The spindle is also connected to a spring which exerts a continual tendency to turn the spindle but is prevented from so doing by the

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FIG. 90.-Diagram showing loose coupled helix in circuit.

thread attaching to the hot wire. Any tendency on the part of the string to slacken a little, however, will immediately permit the spring to turn the spindle. When a high frequency current is passed through the platinum wire it becomes heated and expands. The expansion of the wire allows the thread to slacken slightly with the immediate result that the spindle turns. The spindle carries a pointer at the upper end which shows the amount of turning. It is therefore easy to tell the comparative strength of current flowing accordingly as the deflection is great or small.

The meter is placed in series with the aerial and when the high frequency currents pass through it they heat and

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