458 



A United States patent issued to C. V. 

 Logwood, in 1915, No. 1,161,142, de- 

 scribes what has come to be known as 

 the "slipping-contact" detector for radio 

 telegraphy. This is shown in Fig. 4, and 

 consists of a grooved conducting cyhn- 

 der 12, which is rotated by a smaller mo- 

 tor 13 and has bearing upon its surface a 

 deUcately fine contacting wire, 16. This 

 apparatus forms a resistance- 

 varying device, which is connect- 



Popular Science Monthly 



Fig. 5. A resistance- varying device connected 

 into the receiving circuit 



ed into the receiving circuit as shown in 

 Fig. 5. Rapid irregular changes of re- 

 sistance, or in some cases actual break- 

 ing of the circuit, result in permitting 

 the large condenser 8 to draw an irreg- 

 ular charge from the condenser 6 in the 

 oscillating circuit. The condenser 8 dis- 

 charges through the telephone 7, and 

 gives the hissing response to sustained 

 or feebly damped waves that is charac- 

 teristic of this form of receiver. The 

 device has been found to be very sensi- 

 tive as compared with a rectifier and in- 

 terrupter for receiving sustained waves, 

 and in addition has the advantage of 

 drawing energy from the receiving sec- 

 ondary circuit at so small a rate that 

 very sharp tuning mav be obtained. 



Patent No. 1,144,969, issued to G. W. 

 Pickard, shows an interesting receiver 

 for radio telegraphy and telephony. The 

 circuit arrangement is shown in Fig. 6, 

 where the antenna A is connected 

 through an inductance Li to ground G. 

 Coupled to this primary coil, which is 

 tuned to the frequency of the incoming 

 waves, is a secondary Lj, shunted by 

 tuning condenser C2 and having associ- 

 ated with it the detector D, condenser 

 Cs, and telephone T. These elements 

 form the usual receiver, which is tuned 

 to the waves it is desired to receive ; the 

 present invention adds to this a closed 

 oscillating circuit formed of coil L2 and 



condenser Ci. This third inductance 

 coil L2 has a variable coupling to the 

 primary Li, and is used to create elec- 

 trical beats in the receiving circuits by 

 the peculiar coupling reactions which oc- 

 cur when the mutual inductance of the 

 system is given the correct value. The 

 inventor states, in effect, that when sus- 

 tained waves are received, the primary 

 and the closed circuits may be so related 

 that the inducing and induced currents 

 will react upon each other in such a way 

 as to produce electrical beats or ampli- 

 tude variations and at such frequency 

 that they may be picked up by the coil 

 Lj. The receiver is of nearly equal 

 value if the received waves are not com- 

 pletely sustained, but are only feebly 

 damped ; for highly damped, incoming 

 energy, however, the device is practi- 

 cally inoperative. From the patent spec- 

 ification, it appears that this is a new 

 type of receiver which will give variable 

 musical responses to signals transmitted 

 by spark or sustained-wave alternator- 

 senders. The tighter the coupling be- 

 tween L2 and Li, the higher the fre- 

 quency of the beat-tones produced. The 

 coil Lj should not be very tightly cou- 

 pled to the primary Li. 



A modified form of quenched-gap 

 sender is shown in Fig. 7, from U. S. 

 patent 1,162,830, issued to G. Von Arco 

 and A. Meissner. The invention 1 1 

 is intended to permit heterodyne ^j_J 







=;r5 



^yy 



Fig. 6. An interesting receiver for radio tele- 

 phony as well as telegraphy 



or beats reception from spark-senders, 

 without destroying the musical charac- 

 ter of the signal note. As is well known, 

 when a heterodyne receiver is used for 

 producing sustained-wave signals, the 

 tones produced are clear and perfectly 

 musical ; the same receiver, when trans- 

 lating signals from spark-senders, almost 

 invariably gives a hissing sound instead 



