The Telegraph Office

A Relay Key from a Federal Telegraph Arc Transmitter, c. 1919

by Neal McEwen, K5RW

Copyright © 1997, Neal McEwen

To Telegraph Office Main Page

The image to the right is a relay key from a 2 KW Federal Telegraph arc transmitter, circa 1919. These relays were used to 'back shunt' key model K and model Q arc transmitters. What is an arc transmitter, Relay Key by Federal Telegaph what is 'back shunt' keying and why is a relay needed. To answer those questions, we need to take a look at early wireless technology.

The arc converter, sometimes called Poulsen arc, or arc, as it was commonly called, is a device used to convert DC energy into RF energy. The converter produces undamped or continuous waves (CW) and was used in radiotelegraphy.

An electric arc between two carbon electrodes energized by a source of direct current has a resistance defined by R = V / I. V is the potential difference between the arc electrodes. I is the current in the arc. If a curve is plotted giving values of current correspond to various values of potentials, we have plotted the resistance curve of the arc. The slope of the curve is negative. That is, for increasing current in the arc, the voltage across the arc goes down. Note that this is contrary to the resistance curve for normal conductors. This negative resistance characteristic when used with inductance and capacitance as shown in the simplified schematic diagram below is what causes the arc to oscillate at radio frequencies.

The detailed physics of the arc, though very, very interesting, is beyond the scope of this article. Lauer and Brown, shown in the bibliography below, has an excellent discussion of the principles of arc operation.

Very early development of the oscillating arc was by Danish scientist Valdemar Poulsen. Australian Arc Transmitter Schematic born and Stanford educated Cyril Elwell saw the promise of the arc and visited Poulsen. In 1909, Elwell and his financial backers, founded the forerunner of Federal Telegraph in San Francisco to exploit arc technology commercially. Soon Elwell further developed the technology and demonstrated its practicality to the U.S. Navy in 1913 with a 30 KW transmitter at Arlington, VA. The arc transmitter was heard significantly farther than spark transmitters of equal power.

The Navy was instrumental in recognizing that the future of radio lay in undamped wave transmission rather than damped wave (spark) transmission. Between 1913 and the end of W.W.I, the Navy developed a world wide communications network with arc transmitters supplied by Federal. One of the transmitters installed in Bordeaux, France at the end of W.W.I was capable of 1,000 KW. By 1921, 80% of all commercial and military transmitters were of the arc variety.

Federal Telegraph besides making transmitters, entered into the Arc Transmitter Relay Diagram telecommunications business by establishing a network of arc stations in the western United Sates and the Pacific. They competed directly with the landline and cable services of Western Union and Postal Telegraph.

Recall that for spark transmitters, the telegraph key is put in the primary of the spark high voltage transformer. But because the arc must run continuously, a telegraph key cannot be placed in series with the DC supply to the arc. Another means must be found to key the transmitter. Early arc transmitters simply placed a telegraph key across a few turns of the antenna coil. When the key was up, the transmitter was on the 'back wave' frequency. When the key was down, the transmitter was on the desired frequency. This was called the 'compensation method' of keying. The difference in frequency between the desired frequency, the 'working wave,' and the 'back wave' was typically one to five percent. Larger differences changed the operation of the arc. The compensation method of transmitting was a waste of the radio spectrum and required selective receivers to separate the two signals.

The 'uniwave method' of keying was developed such that only the desired frequency was transmitted. With the 'uniwave' method, when the key is up, the arc is shunted to a dummy load. The 'back wave' is shunted to the dummy load, hence 'back shunt' keying. A single pole double throw relay was used. A telegraph key was in series with the keying relay's solenoids. The relay shown above has a built in series resistor to limit the current on the 120 VDC keyed line. Referring to the schematic, note that the arc, denoted by 'X', is switched by relay 'K' between the antenna circuit and the dummy load. The dummy load is tuned to the same frequency Federal Morse Keyas the antenna with the L and C components.

The 2 KW Federal arc transmitters had both 'compensation' and 'uniwave' methods of keying. The uniwave method key is shown as a "Morse Key" in the transmitter schematics. Likewise the 'compensation method' key is shown as an 'Auxiliary Hand Key.' When the auxiliary hand key was used in series with a chopper, the arc transmitter was in ICW mode. This was used so crystal receivers could hear the arc signal. The key shown is also by Federal Telegraph. The label on this key reads "Morse Key CT-1576." The numbers after "CT" place it similar in vintage to the relay, perhaps a few months earlier. It is very likely that this key and the relay were used together in arc rigs.

Key collectors take note!!! "What does 'Auxiliary Hand Key' really mean?" This a frequent question among even advanced collectors. From Nilson & Hornung: "AUXILIARY HAND KEY -- The auxiliary hand key is provided for the use with the chopper as an alternative for the usual means of signaling by the back-shunt method. The auxiliary hand key may also be used for signaling on long waves in case of trouble with the regular means."

Notice the detail of the relay contacts. The position Federal Telegaph Relay, points detail of the stationary contacts can be adjusted. The contacts are also spring loaded. These mechanisms are necessary so that the relay will 'make' before it 'breaks' for either direction of armature travel. This prevents the arc from ever being with out an oscillating circuit, thus maintaining continuous operation.

This particular relay is a real brute. With a cast iron frame and dimensions of 8" x 4 1/2" x 5", it weighs in at nine pounds and the contacts are 3/4" in diameter. I have not measured the tension on the armature return spring but it is substantial when I move the armature with my fingers.

Needless to say, I'm quite pleased to have this relay key in my collection. It is a very good companion piece for the Federal "Morse Key." Furthermore, I was curious about the relay and how it was used, which prompted me to investigate arc transmitting technology and the Federal Telegraph Co. 

.BibliographyFederal Telegraph Relay, label

Aitken, Hugh G. J. The Continuous Wave: Technology and American Radio, 1900 - 1932. Princeton, New Jersey: Princeton University Press, 1985

Bucher, Elmer. Practical Wireless Telegraphy. New York: Wireless Press, 1921

Lauer, Henry & Brown, Harry L. Radio Engineering Principles. New York: McGraw-Hill, 1920

Mayes, Thorn L., W6AX. The Federal Telegraph Company 1909 -1920. Antique Wireless Association Monograph No. 3

Nilson, Arthur R. & Hornung, J. L. Practical Radio Telegraphy. New York: McGraw-Hill, 1928

Robison, S.S. Robison's Manual of Radio Telegraphy and Telephony for use of Naval Radiomen. Annapolis: United States Naval Institute, 1928

Signal Corps, U. S. Army. The Princinples Underlying Radio Communication. Washington: Government Print Office, 1921

For more information, visit the Telegraph Office home page

Neal McEwen,