EXTENSION OE THE TELEPHONE SYSTEM TO SUITS 10! 



There is both an outgoing and an incoming radio channel. The 

 automatic repetition from the wire to the outgoing radio channel is 

 made possible thru ability to control the transmitter wave power 

 by the voice currents set up at the distant end of the telephone line. 

 It will be recalled that in the early radio telephone art, before the 

 vacuum tube, modulation was effected by the microphone transmitter 

 which required that the talker be present at the radio station. It is, 

 therefore, the electric-control type of modulator such as the vacuum 

 tube, as distinguished from the air-wave control modulator, which 

 permits of the talker being at the far end of a wire circuit. Con- 

 versely in the receiving channel, it is the fact that the detecting action 

 yields telephone currents directly, ready for propagation over a wire 

 circuit, that enables the radio channel to be extended to a distant 

 listener. 



Thus it is the thermionic tube modulator and detector which have 

 made possible the radio-wire transfer. It is the thermionic tube as a 

 reliable high-quality amplifier, however, that makes the transfer 

 practical; for it is the amplifier which enables the weak voice currents 

 received at the radio station from a land line subscriber to be boosted 

 to power proportions and thus control the considerable radio frequency 

 power required for transmission; and, again, it is the amplifier which 

 enables the extremely weak currents received from the radio link to be 

 so augmented that upon being placed upon a wire circuit, and perhaps 

 being further amplified en route, they may be heard in the regular 

 telephone at the other end. 



The other important feature of the radio-wire inter-connection is 

 the junction of the four-wire and the two-wire circuits by means of 

 the hybrid coil and balancing network as shown in Fig. 11. The 

 windings of such a coil are so designed as to establish a sort of Wheat- 

 stone bridge circuit. This bridge circuit accomplishes the joining 

 of the regular two-wire telephone circuit with the sending radio channel 

 on the one hand and the receiving radio channel on the other, while 

 .still maintaining an electrical separation between the two radio 

 channels. It is really, therefore, the connecting link between the 

 two-wire type of circuit of the telephone plant and the four-wire 

 circuit of the radio link. The hybrid coil type of circuit is taken 

 from the telephone repeater and carrier current art.- The radio 

 receiving circuit corresponds to the generator branch ol the Wheatstone 

 bridge, and radio transmitting circuit to the detector branch. I he 



-' " relephonic kepeaieis,'' B. Gheiaidi and F. B. Jewett, Journal oj American 

 Institute of Electrical Engineers, pages 1255-1395, November, 1919. 



" Carrier Current Telephony and Telegraphy," E. 1 1. Colpitis and ( ). B. Black'vell, 



Journal of American Institute of Electrical Engineers, pages 205-300, February, 1921. 



