Long Lines, March, 1922, pages 7-10:
Cross-Country diagram

THE completion of the carrier current telegraph system between Chicago and San Francisco makes it possible to set up New York-San Francisco telegraph circuits using carrier telegraph facilities for approximately the entire distance from coast to coast. For the comparatively short distance between New York and Harrisburg, about 200 miles out of the total of 3,370, cable circuits with the accompanying metallic Morse equipment have made carrier facilities unnecessary; but with this exception, New York to San Francisco by carrier is an accomplished fact and we may therefore speak of a transcontinental carrier telegraph system.
    The transcontinental telephone circuits between Chicago and San Francisco were unloaded early in 1920 for a two-fold purpose. First, to obtain an improvement in telephone transmission, since transmission of better quality can be obtained over non-loaded open wire circuits than over loaded open wire circuits; and second, to make the two pairs in the group of No. 8 gauge wires, constituting the transcontinental circuits, suitable for carrier current operation, since carrier current systems cannot be operated on loaded open wire circuits.
    Coincident with the unloading work a carrier telegraph system was planned between Chicago and San Francisco. The equipment as planned and as subsequently installed consists in effect of five separate systems, capable of being connected together with the utmost flexibility and providing the following telegraph facilities:
Chicago-Omaha20 Morse channels
Omaha-Denver10 Morse channels
Denver-Salt Lake  8 Morse channels
Salt Lake-San Francisco   6 Morse channels

a Morse channel being the equivalent of a physical telegraph circuit.
    The systems necessary to obtain these facilities are as follows:
    Two 10-channel systems between Chicago and Omaha, a distance of 495 miles, with repeaters at Davenport, a total of 9,900 channel miles.
    One 10-channel system between Omaha and Denver, a distance of 586 miles, with a repeater at North Platte, a total of 5,860 channel miles.
    One 8-channel system between Denver and Salt Lake, a distance of 580 miles, with a repeater at Rawlins, a total of 4,640 channel miles.
    One 6-channel system between Salt Lake and San Francisco (Oakland), a distance of 764 miles, with repeaters at Winnemucca and Sacramento, a total of 4,584 channel miles.
    A grand total of nearly 25,000 channel miles is thus provided using only approximately 3,000 miles of physical circuits.
equipment at Omaha

    These individual systems can be operated independently between their terminal points or connected together to provide through wires with or without drops at any or all terminals, i. e., Chicago, Omaha, Denver, Salt Lake and San Francisco. At Chicago connections can be made to Eastern systems, as previously indicated, making it possible to provide carrier telegraph facilities between Harrisburg and San Francisco. These carrier systems are superimposed upon the 8-gauge circuits of the transcontinental line without the sacrifice of any facilities previously derived from these wires. No reduction in the use of these wires for telephone purposes results and the wires are still available for direct current Morse circuits.
    While this apparatus represents the latest development in carrier telegraph equipment and contains a number of refinements and improvements not present in the earlier types, fundamentally it is substantially the same as previous systems, the first experimental type of which was tried out between Maumee and Chicago in 1917.
    A carrier telegraph system provides a means whereby a number of telegraph circuits can be obtained from a single pair of wires. The fundamental principles underlying these systems and descriptions of commercial types of apparatus are given in a paper entitled "Carrier Current Telephony and Telegraphy," by E. H. Colpitts and O. B. Blackwell, which paper is available and has been widely distributed among those interested in carrier current equipment. In general, in a carrier current telegraph system a number of separate telegraph messages are superimposed simultaneously on a single physical circuit by using separate currents called "carrier currents" for each message. These carrier currents are made to vary in accordance with the variations of current representing the telegraph message and therefore may be said to carry the telegraph message.
    Alternating currents of frequencies higher than those required for telephone transmission are employed, and these are separated from the telephone frequency currents at terminal and repeater points by means of filter equipment in the same manner that direct current Morse circuits are separated from the telephone circuits in the Morse composite set. Two currents of different frequencies per channel are used, one for transmitting in one direction and the other for transmitting in the reverse direction, and the different frequencies which are employed must differ sufficiently between themselves so that they may be separated from each other at the terminals by the use of proper electrical circuits. Thus for a 10-channel system a total of 20 separate currents of different frequencies are employed, each current differing in frequency from its nearest neighbor by approximately 250 cycles. At the terminal points each current is directed into the proper channel by means of a tuned circuit which admits currents of that frequency but no other, while at repeater points the whole range of frequencies is amplified in a manner similar to that in which the voice frequency repeater amplifies all of the frequencies which go to make up the voice.
    Each channel consists of sending and receiving equipment at each terminal. The sending equipment is made up first of an oscillator which is the source of the alternating current for that particular channel, adjusted to the frequency of the particular channel. The output of this oscillator is controlled by relay equipment which is in turn controlled by the sender's telegraph key. An oscillator amplifier is next provided by means of which the output of the oscillator can be amplified and the sending current adjusted to the proper value. From the oscillator the high frequency alternating current passes through a tuned circuit to prevent interference between channels, and thence to the line.
    The receiving equipment consists of a tuned circuit adjusted to select the receiving frequency of the channel, an amplifier circuit to increase the received current to the proper amount and a detector circuit by means of which the high frequency alternating current is rectified and made suitable to operate the receiving relay equipment. Thus are provided one-way circuits in each direction which properly combined make two-way telegraph circuits or channels.
    These channels are suitable for all kinds of Morse service and may be connected to other telegraph facilities with perfect flexibility. Open wire, metallic Morse and carrier channels may be connected together when necessary and operated full or half duplex, as service may require. They are also suitable for the operation of printers. During the testing of this system New York-San Francisco telegraph circuits were built up using metallic Morse equipment between New York and Harrisburg, carrier channels of the Harrisburg-Chicago telegraph system No. 1 (which, by the way, was the first commercial carrier telegraph system and was installed during 1919) between Harrisburg and Chicago, and channels of the Chicago-San Francisco carrier telegraph systems between Chicago and San Francisco. Moreover, two of these channels were looped back at San Francisco so that the telegraph circuit actually extended from New York to San Francisco and back, a total of 6,738 miles of telegraph circuit of which 6,340 were carrier. A perfectly satisfactory telegraph circuit was obtained.
equipment at Davenport and Omaha

    As previously stated the Chicago-San Francisco system is the latest type of carrier telegraph equipment in which a number of refinements and improvements have been incorporated for the first time. As such it constituted a trial system and was under the supervision of the Department of Development and Research during its installation. The installation work was done in part by the Long Lines Department, Division Equipment Installation Forces, in part by the Western Electric Company, and west of Denver where the equipment is the property of the Associated Companies the installation work was arranged for by the Mountain States and Pacific Telephone and Telegraph Companies. The testing work was handled by the Department of Development and Research in which work representatives of the Long Lines and the Associated Companies participated.
    Some sections of the entire system were completed somewhat in advance of others, and some channels in these sections were used for service in emergency, so that service was furnished over the system between Chicago and Omaha during November, 1921, and between Chicago and Denver early in December, 1921. The first Chicago-San Francisco carrier telegraph circuit was set up for test on December 8, 1921, and the first New York-San Francisco carrier telegraph circuit was set up for test on December 21, 1921. At a demonstration of the transcontinental telegraph system on January 6,1922, printer equipment was operated over the carrier between New York and San Francisco. Commercial service over the entire system was established January 23, 1922.