Experimental Electricity 



Practical Hints 

 for the Amateur 



Wireless 

 Communication 



Damping in Radio Circuits 



By John Vincent 



Q 



Fig. 1. Pendulum 

 with circular scale 



THE subject of damping and "loga- 

 rithmic decrement" of current and 

 voltage in radio telegraph senders 

 and receivers is often looked upon, by the 

 wireless experimenter, with a certain 

 degree of awe. This is usually because 

 many of the text- 

 books and articles 

 treat the matter as 

 though it were very 

 complicated and 

 hard to understand ; 

 the fact is indeed the 

 contrary, and the 

 matter of damping 

 is not at all difficult 

 to grasp. There is 

 no need of making 

 use of long mathe- 

 matical expressions 

 to figure out how much damping exists 

 in any circuit, and what damping itself 

 means. 



In the first place, it must be under- 

 stood that in speaking of the damping 

 of an alternating current one refers 

 merely to the rate at which the current 

 oscillations die away. If the oscillations 

 die away fast the damping is said to be 

 high, or if, on the contrary, the oscilla- 

 tions persist a long time before fading 

 out, the damping is feeble. A pendulum 

 having a freely pivoted joint at the top, 

 and swinging through the air, will 

 vibrate back and forth many times 

 before coming to rest; its oscillations, 

 which are, of course, mechanical, are then 

 feebly damped. But if the same pen- 



dulum is, immersed in water it will stop 

 swinging much sooner, because the 

 friction of the water offers resistance to 

 its motion ; in this condition the damping 

 is higher. If the pendulum is lowered 

 into a tank of heavy oil or molasses the 

 friction will be greater still, and the 

 oscillations will die out very quickly; 

 thus the mechanical system becomes 

 highly damped. 



If we arrange the pendulum with a 

 circular scale and pointer, as shown in 

 Fig. I, it becomes a simple matter to 

 measure its period and damping. To 

 find its period it is only necessary to 

 draw the bob to one end of the scale and 

 let it go, counting the number of 

 complete swings it makes in one minute. 

 The length of time taken for one 

 complete swing from left to right and 

 back, measured in seconds, is equal to 

 sixty divided by the number of swings 

 in one minute; this division gives the 

 time period of the pendulum. For 

 instance, if the bob is swung out to the 

 left and let go at the beginning of the 

 minute of timing, and if it swings back 

 to the left side thirty-six times and is 

 at the right-hand end when the minute 

 is up, the period will be 60 divided by 

 36.5, or 1.64 seconds. By lengthening 

 the cord or rod a little, the period 

 could be made exactly 2 seconds, or by 

 shortening it, i second. For the illustra- 

 tion of damping measurement given 

 below it is useful to make the pendulum 

 about 39 ins, long, which will make the 

 period about 2 seconds. The cord may 



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