Page:Popular Science Monthly Volume 88.djvu/645

 Arc Light Interference

��IN the November and December issues of Popular Science Monthly there were published several queries and answers on the matter of arc light inter- ference with received signals. Our read- ers were asked to contribute suggestions which they found helpful in overcoming or reducing this sort of disturbance. A large number of replies have been received, and the proposed methods are here described. It appears that at least two kinds of

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��inductive disturbances are encountered. The first of these is the ordinary "induc- tion hum," heard in nearly all wireless stations that have alternating current power lines run- ning into or very near them. The second type is that which is caused by the flickering and fluttering of carbon arc lights in opera- tion, and which is usually transferred to the receiving wireless aerial by induction from ad- jacent power lines. The induction hum is the most

common and the easiest to eliminate. It is troublesome when direct-coupled re- ceiving tuners are used, but may often be cut out by changing over to an induc- tively-coupled receiver. One experi- menter states that by running his aerial lead through a fixed condenser i)cfore connecting to his tuning coil, he reduced the interference greatly. Another writes that he secured good results by placing a 7-volt tungsten lamp in series with the antenna lead, keeping the lamp lighted to a certain brilliance (determined by ex- periment) by means of a battery and rheostat. The real reason for any im- provement gained from either of these last two methods is not apparent; the use of inductively-coupled apparatus,

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���however, has not only been found effec- tive by practical test, but also is theo- retically correct.

Another way of cutting out the hum is efi^ected by merely opening the main switch which brings power into the house containing the receiving instru- ments. This method prevents internal induction from the leads, but, of course, cannot be used when it is desired to utilize the electric power for transmitting signals or for lights. In some stations the trouble has been stopped by connect- ing one side of the incoming i lo-volt line to ground, through a fuse which will blow on 2 amperes or so, or by grounding through a condenser or small iio-volt lamp. Which of the two power wires is to be grounded through the lamp, con- denser or fuse in this way must be de- termined by trial. In grounding through a condens- er the fuse should also be used, for protection in case the condenser punctures.

Some of the above methods are effective not only for the alternating current induction, but also for the ragged, harsh noises from arc lights. Especially helpful is the plan of grounding the power lines, for in many cases the arc induction is picked-up by the regular lighting lines and brought to the wireless station over them. Both kinds of disturb-

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��ance have also been reduced in wireless stations by con- necting the dia- phragms of the re- ceiving telephones to ground, either directly or through a condenser. Often

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��it helps merely to touch the aerial or ground lead with the finger, or to rest the hand upon a metal plate connected to the blocking condens-

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