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A French "load compensating" Spark Key

An Unusual Spark Key

with two sets of high current contacts c. 1908 - 1916

by Neal McEwen, K5RW

Copyright © 2001, Neal McEwen

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The photo of the key to the right has occupied my curiosity for some time. (Click on any of the images for a full size view.)  Notice that the key has two sets of high current contacts.  A 'normal' spark key has one set of contacts.  When the 'normal' key is down, the the alternating current supply is connected to the transformer.  When the 'normal' key is up, the circuit is broken.

The contacts on the key to the right are shrouded by cylindrical cups that hold oil.  Immersing the contacts in oil eliminates arcing of the contacts, thereby preventing pitting.  In some cases, "oil break" keys are used where a key is employed in a potentially explosive atmosphere, such as on aircraft or on board ship.  Also note that the lever and hence the knob and skirt are abnormally high above the base for a telegraph key.

There are no makers marks on this key.  There is not a clue to its origin or application.  Is it a wireless key or some sort of laboratory curiosity?  Why does it have two sets of contacts functioning like a Single Pole Double Throw (SPDT) switch?

The first clue to the key's identify appeared in the "Antique Wireless Association Review, " Vol. 14, 2001, in the article Spark Keys: The Interplay of Wireless History and Technology.  A key, similar to the one shown in the AWA Review is shown to the right.  The article sates, "One of the largest of the oil break keys, probably designed and made by an unknown French manufacture, was used on the S.S. Santa Marta in 1915."  However, no explanation is made concerning why there are equally large contacts front and rear.  No explanation is given regarding the high lever.

The image shown at the right is a partially assembled view of the AWA Review key. 

So why does the key above have two sets of contacts?  The answer to this took me down many dead ends and to many unsupported conjectures -- both my own and others.  The diagram to the right and below shows how this key works and how the two sets of contacts are used.  Let's walk through the diagram.

The circuit diagram above does not tell us why the lever is so high.  A close examination of the pictorial shows that the key does not have the gap adjusting screw found on traditional telegraph and wireless  keys.  "Nouveau matériel de télégraphie sans fil de la Marine," 1908 describes the fulcrum and the lower contacts.   "Fulcrum -- at the bottom of the fulcrum there is a cylindrical rod secured by a clamping ring. The height of the fulcrum and therefore the lever can be adjusted by setting this rod.,...  The gap adjustment consists in finding a good compromise between the height of the lever and the height of each [oil] cup." 

The key in the photo at the top of the page appears to be some sort of hybrid.  It has the high lever, yet the contact spacing is adjusted the traditional manner.

Why has this information been so hard to find?  It has been locked away and nearly lost in French military texts. A scientific instrument collector in France explained to me that such books are extremely difficult to find.  At the start of World War One, the French did not want their military installations, wireless technology or the locations of their wireless stations known to the enemy.  So they destroyed such books.  The sixth entry in the bibliography below, from which the two pictorials above are taken, references a book of which only three are known to exist.  One has pages cut from it to 'sanitize' it.

These keys appear in French military texts from 1908 to 1916 and are shown a 'key for medium current.'   I wonder what a key for high current looks like!  An earlier version of the "load compensating" key is shown to the right.

Back to the key!  Is the mystery solved?  Now we know why the key had two contacts and a high lever.  But why did the French feel that load compensation was needed?  As far as I have been able to determine, no other nation used this configuration and the French did not do it for long.  Otherwise we would see more of these "load compensating" keys.  Very, very few are known to exist.

Let's look a little deeper.  I am guessing that the alternator was on the same shaft as a rotary gap.  Commercial spark transmitters of the period  use a motor-generator.  The motor is feed from the DC mains if on board ship.  The motor is mechanically connected, via a drive shaft, to an alternator which supplies AC, through the key, to the spark transmitter.  A rotary spark gap is also on the same drive shaft.

When a motor generator is connected to a spark transmitter, it is intermittently loaded with the make and break of the key.  For simple motor generator sets under load, the alternator frequency slows and the voltage drops.  This will cause a change in 'tone' of the copied signal and possibly loss of the spark.  Advanced motor-generator sets were self regulating through the use of shunt wound motors and / or compound wound alternators.

Perhaps the French found that using a load compensating resistor was easier than using advanced motor-generator sets.  Key up or key down the load is the same, hence the frequency does not change nor does the voltage drop.  The rotational momentum of the motor - generator would keep the frequency and voltage constant during the brief intervals between key up and key down.

The earliest technical treatise on motor-generator sets for spark use that I can find is 1915 and they show shunt wound motors.  Perhaps these keys were designed before shunt wound motors.  Or perhaps the alternator was driven by a gasoline powered motor.  I sure would like to see the schematic diagram of the transmitter and motor - generator that this key went with!!!

Special thanks to Alain Tambourini and Yann Conan for helping with French sources and translations.


Bucher, E.L., Practical Wireless Telegraphy, Wireless Press, New York, 1921.  [Description of motor - generator sets]

Hawkhead, J.C. and Dowsett, H.M. Handbook of Technical Instruction for Wireless Telegraphists, Wireless Press, London, 1915. [Description of motor - generator sets]

Conan, Yann, Email to Neal McEwen,  30 October 2001. [Translation -- application of the two contact key]

Moller, Jan. Email to Neal McEwen, 13 November 2001. [Gasoline powered alternators]

Nouveau matériel de télégraphie sans fil de la Marine, Paris, Imprimerie Nationale, Avril 1908. ("New wireless
equipment for the Navy").

Notice provisoire sur le matèriel de télégraphie sans fil,  Ministère de la guerre,  Genie Imprimerie Nationale, Paris, 1909.  (Temporary note on the wireless telegraphy material.  War Ministry - the Engineer Corps")

Tambourini, A. Email to Neal McEwen, 31 October 2001. [Explanation of rarity of French military wireless texts]

B. Neal McEwen, K5RW

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