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Page:Popular Science Monthly Volume 89.djvu/465

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Popular Science Monthly


��but the larger it is tiic farther you will be able to signal. For transmitting from room to room within the house, it will be sufficient to string some twenty feet of wire around the [)icture moulding near the ceiling.

If you have set up the apparatus properly the buzzer will hum strongly as long as you hold down the sending key and thus close the battery circuit. By pressing the key for short and long intervals you can produce short and long buzzes which correspond to dots and dashes in the Morse telegraph code; in this way messages can be spelled out letter by letter.

A Microphone Receiver

For the receiving sta- tion you will need to make another ground con- nection by fastening a wire to the steam or water pipes, and then the next thing is to build a second antenna or aerial wire system exactly like that at the sender. The second tuning coil, an old dry cell (preferably one which has become very weak), a tele- phone receiver and the microphone detector are to be connected together as shown in Fig. 4. Any telephone receiver will do; you can buy a 70-ohm watchcase instrument from an electrical store for about 75 cents, but if you intend to continue with wireless experimenting it will pay you to invest several dollars in a pair of telephones of high sensitiv'eness. These will not only make it pos- sible to receive messages from longer distances, but because of the headband with which they are fitted you will be relieved of the nuisance of holding the receiver to your ear and will have both hands free for manipulation of your apparatus.

The microphone detector is to be made as shown in Fig. 3, which indicates how two large double binding posts are to be mounted upon a hard rubber or wooden base. Two sharp sewing needles

���Fig. 3. How the Micro- phone Detector is to be Made

���Fig. 4. How the Tele- phone Receiver and Microphone are to be Connected Together

��are inserted into the upper holes of the binding posts, and between their points is lightly supported a short length of graphite from a soft pencil. The piece of graphite should be about one-half inch long, and should have its ends partially hollowed out so that it will hang easily upon the needle points. It is not to be clamped firmly, but allowed to rest so loosely that it may be revolved freely and even slid a very short dis- tance back and forth.

Operation of the Apparatus

After you have set up both stations according to the diagrams, have someone work the transmitter key, making regular test sig- nals such as "V" or "D", and go to the receiver. Listen carefully to the telephone receiver, and move the graphite piece of the microphone around slightly. You will notice that you can hear every touch; when the micro- phone is adjusted to its most sensitive condition there will be a continuous slight hiss in the telephone receiver, and even the slightest taps on the table or instrument base will be clearly heard. When the apparatus is ad- justed in this way you should hear the buzzes of the transmitter reproduced in your telephone, and so should be able to copy the signals sent out from the transmitting station.

If you have any difficulty in getting good results, try again with the receiver near- er to the sending station. When you have once trans- mitted good signals, move the stations farther apart. Remember that it is neces- sary' to have good ground that the two tuning coils that the


must be exactly alike, and sending and receiving antennas must be identical. If you are able to erect fairly large aerials for the two stations, such as, for instance, sixty foot lengths of wire supported by trees or f>oles, you should be able to transmit signals a

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