Experimental Electricity 



Practical Hints 

 for the Amateur 



Wireless 

 Communication 



Sharpness of Tuning in Radio 



Bv John Vincent 



■nnMiJir 



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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