Page:Popular Science Monthly Volume 88.djvu/963

 Experimental Electricity

��Practical Hints for the Amateur

���Wireless Communication

��Sharpness of Tuning in Radio

��Bv John Vincent

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��THE effect of increased resistance in a freely oscillating circuit was described in the May article of this series. It was pointed out that the more rapid loss of energy, brought about by the presence of this added resistance, reduced the num- ber of current os- cillations in the circuit. It was al- so indicated that when the persist- Fig. 1. A simple circuit ence of the circuit was thus reduced (as its damping or decrement in- creased), the system became less sharp- ly tuned.

Just what is meant by the "sharpness of tuning?" Before this can be answered, it is necessary to look more closely at the effects of tuning itself. This phe- nomenon of resonance is, perhaps, made of more use than any other in the science of radio telegraphy; and yet it is often grossly misunderstood, even by skilled operators and experimenters.

Mechanical illustrations of tuning, drawn from the art of music, have been described in book after book; yet there seems to exist some difficulty in carrying over, into the purely electrical cases, the physical facts which these analogies should teach. Suppose that one dis- regards, for the moment, the sympathetic tuning forks and the tuned strings (both of which vibrate, though only one is

��plucked), and that one considers a simple electrical circuit having in series an inductance, a capacity, a resistance, a current indicator and a source of high- frequency sustained voltage. Such a circuit is that shown in Fig. i. In the February article the effect of altering the circuit impedance by changing its inductance and capacity was described; when the values of the coil L and con- denser C just neutralized, for the frequency generated by the alternator E, resonance was secured and the current indicated by / became a maximum.

The same circuit may now be studied with the alternator at rest. If a charge of electricity is placed upon the con- denser and allowed to discharge freely through the circuit, there will be set up a feebly-damped alternating current of the character indicated by Fig. 2; this is on the assumption that the resistance R has a small value, as is usual in practice. The frequency of this free

���Fig. 2. Feebly-damped alternating current

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