Popular Science Monthly 



451 



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 he 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 



connections 



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 



