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Ducretet & Roger, Paris, France

An Unusual Spark Key c. 1915 - 1919

by Neal McEwen, K5RW

Copyright © 1999, Neal McEwen

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An elementary spark gap transmitter schematic diagram is shown at the right. Note that the transmitting key is in series with the alternating current power source and with the primary of the spark transformer. Diagram of Spark TransmitterSeveral hundred volts AC at ten, twenty or even fifty amps was typical of the input to the the transformer.

Keying this type of voltage and current required a key much different than the typical key used in American Morse landline service. Wireless keys, of even small transmitters, used larger contacts to dissipate the heat generated by the high current. Arcing between the contacts was also a problem due to the high voltage. Arcing caused pitting, and after so long in service the contacts were rendered useless. Many spark keys were made with replaceable contacts.

Ducretet & Roger made the unique looking key shown at the right. (Click on the image for a full size view.) The keying contacts are immersed in a cup shaped vessel of oil. Such keys are called "oil break" keys to denote that the contacts "make and break" in oil. Typically oil break keys take advantage of the fact that oil is a good thermal conductor and a good electrical insulator. Therefore it dissipates heat away from the contacts,... and no less important, minimizing arcing between and prolonging the life span of the contacts.

The Ducretet and Roger key is not large by spark key standards. The base is only 5 1/8" by 3 1/8". The large protective skirt under the keying knob, the oil immersed DuCretet & Roger oil break spark keycontacts, and the ventilated lever suggest that this key carried a large amount of current in relation to keys of similar size. However, the carrying of high current was not a consideration in the design of this Ducrete and Roger key.

The large diameter 'see through' holes in the lever are a clue to its application. The holes were used to lighten the key for use in in aircraft. This technique was used in other parts of the aircraft's airframe. Due to size and weight limitations, aircraft spark transmitters were small, not requiring a high current key. Why an "oil break" design then? This key was expressly designed to operate in an explosive atmosphere. With the contacts immersed in oil, the key is a "flameproof" key.

The oil cup is brass and is countersunk into the wooden base. It measures an inch and a quarter in diameter and one inch deep. This allows approximately 15 cubic centimeters of oil in the vessel. The lower contact is integrated into the bottom of the cup. The upper contact is on the rod attached to the lever. Open or closed, the contacts are immersed in the oil. The cup assembly is has a cork cover which fits against the lip of the cup.. The cover has been raised in the photograph to show details of the cup.

This key, minus the oil cup, is typical of European designs. The lever return spring is behind the fulcrum and in tension rather than compression. The lever is straight, rather than curved like its American cousins. Ducretet and Roger built a similar key without the oil cup.

Eugene Ducretet (1844 -1915) was a scientific instrument maker, opening his first shop in 1864. He made galvanometers, Whimshurst machines, Tesla apparatus, Crookes tubes, etc. Ducretet also made telegraph instruments including keys and Morse registers. The Ducretet name is associated with the early development of wireless in France; he was an early experimenter and maker of wireless apparatus. Descriptions of his experimental Ducretet and Roger oil break spark keytransmitters and receivers are shown in Electrical World and Engineer in 1899. Ducretet wrote a wireless telegraphy guide in 1901, declaring himself a "constructeur" or builder. Ernst Roger was a collaborator in the Ducretet experiments as early as 1898.

Ducretet died in 1915, leaving the company in the hands of his son Fernand and partner Ernst Roger. During World War One, the Ducretet and Roger shops were devoted entirely to the manufacture of military communications apparatus and special equipment for the Navy such as periscopes and microphones. In 1931 the company was sold to Thomson-Houston.

The Ducretet and Roger mark is pressed into the top of the wood base and is shown at the right of the bibliography section below. Unfortunately, a mounting hole obscures some of the mark.

The era of "oil break" keys was short lived. Wireless engineers learned to used large keying relays or "relay keys" with smaller hand keys. The task of handling high current and high voltage was transferred to the relay. The operator's key carried low voltage DC and keyed a large transmitting relay. However, the requirement for "flameproof keys" continued through W.W.II and beyond. Later "flameproof" designs were hermetically sealed and not "oil break" designs, however.

Should the Ducretet and Roger key be called an "oil break" key or a "flameproof" key. Though most collectors will continue to call it an "oil break" key, given its application in explosive atmosphere environments, it should probably be called a "flameproof" key. Both are right; it is a "flameproof" key of "oil break" design.

The Ducretet & Roger key represents an interesting period in wireless history.

BibliographyDuCrete & Roger makers mark

Brenni, Paolo. 19th Century French Scientific Instrument Makers, VIII: Eugene Ducrete (1844 - 1915), Bulletin of the Scientific Instrument Society, No. 46. 1995

Ducretet, E. Guide Pratique Telegraphie Hertzienne Sans Fil, 1901

Kreuzer, James. Conversation paraphrasing French wireless texts, with Neal McEwen, 1 August 1998

Moreau, L.R. The Story of the Key, 1987

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

Renaud, Paul. Annual Exhibition of the French Society of Physics, Electrical World and Engineer, 1899

Vanden Berghen, Fons. Letter describing Ducretet telegraph instruments, to Neal McEwen, 17 August 1998.

B. Neal McEwen, K5RW

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