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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
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��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
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