Experimental Electricity
��Practical Hints for the Amateur
���Wireless Communication
��Damping in Radio Circuits
By John Vincent
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��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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