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A Swiss PTT "Silent Key" c. 1920

made by G. Hasler, Bern, Switzerland

by Neal McEwen, K5RW

Copyright © 2001, Neal McEwen

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It is not everyday that you find a key with such interesting supporting documentation.  Usually it is a struggle to find any relevant
information on a key.  This time luck prevailed and I also discovered why some European keys have the leaf spring contacts. (Click on any of the images for a full size view.)

This key, a Swiss PTT design, was made by Gustav Hasler of Bern, Switzerland.  Hasler was started by Dr. Gustav A. Hasler (1850 - 1900) and Heinrich Albert Escher and traces its origin to 1865 from the Eidgenössischen Telegraph works.  Although a Swiss PTT standard and manufactured for at least fifty years, this particular key dates approximately 1920.  The design was used not only by the Swiss Post Office, but also by the Swiss Army and Swiss railroads.

Notice the lower contacts.   They rest of a leaf spring made of steel.  This contact design appears on other keys manufactured in Europe, including keys from Germany, and Denmark.  The leaf spring contacts are explained in "Un Siecle De Telecommunications En Suisse 1852-1952 Volume 1, Telegraphy," published by the Swiss PTT.

In the image from the PTT book, the key on the right has similarly engineered leaf spring contacts.  The caption says "avec ressorts de contact pour la transmission silencieuse."  this translates to "with spring contacts for silent transmission."

When and where silent contacts were first used is not known.  However we do know that the design was used by Siemens and Halske of Germany for their keys starting in 1871.  In Faszination Morsetasten: German telegraph keys Collector's guide, the keys are referred to as "geräuschlose" or "lautlose" keys, meaning "noiseless or "quite" keys.    There are many World War Two era German military keys employing the leaf spring design.The leaf spring contact design was used through the 1950s and beyond in several countries including East and West German, Switzerland and China..

Why were silent contacts needed?  Since these keys are quite large, made in large numbers and used by the Swiss PTT, we can assume that they were not made for clandestine or secret operations.  The German Telegraph Administration use the "lautlose" keys in larger offices. It is then probable this design was favored where there were many operators in a room.  The silent contacts would reduce the noise in a telegraph office and hence enable an operator to better copy his own sending.  In most European landline circuits, the sounder, if present, was in the circuit only on receive; on transmit, the operator listened to the clicking of his own key and / or watched the galvanometer swing in order to monitor his own sending.  (Many European landline circuits did not use telegraph sounders; they used Morse register to record the incoming signals with ink on paper tape, later to be transcribed by an operator.)

A Hasler catalog dated 1898 shows a line drawing of the silent key's brother.  Also shown in the catalog are galvanometers and Morse registers, both essential for a telegrapher's operating position in Europe at the turn of the century.

Besides the silent contacts, this is a typical European landline key.  Notice that there are a set of contacts at both the front of the lever and the back of the lever.  Most European landlines were of the 'open circuit' design; keys employed in 'open circuits' require a normally open set of contacts and a normally closed set of contacts and electrically the keys resemble a Single Pole, Double Throw, (SPDT) switch.

By contrast, North American land lines were 'open circuits' requiring only one set of contacts in the front and a circuit closing lever on the side, electrically resembling two Single Pole, Single Throw (SPST) switches in parallel.  The lever return spring is behind the fulcrum and in tension, typically European.  American keys have the lever return spring in front of the lever and in compression. 

What makes the Swiss PTT key design especially interesting is that it was used as a model for a postage stamp to commemorate the 50th anniversary of the Union Schweizerischer Kurzwellen-Amateure, (USKA) the Swiss Shortwaves Amateurs Union.  The USKA is the Swiss equivalent of the American Radio Relay League.

Hasler made this key of brass and mahogany.  The leaf springs are made from steel. The knob, at first glance appears to be polished wood; however, on closer inspection, it proves to be animal horn, polished to a high luster.  The key is well made and stands as a tribute to Swiss engineering and manufacturing.  Watches are not the only thing the Swiss make well!


Ascom AG home page A Brief History of Hasler

de Henseler, M. Email to Neal McEwen, 21 Oct. 2001 [Swiss PTT keys, Stamp, USKA]

Hasler, G. Preis-Verzeichnis, Telegraphen - Werkstatt, Bern, Switzerland, 1898

Kelter, W. Schweizer Pioniere der Wirtschaft und Technik, Vol. 14. Zurich, Switzerland

Ulsamer, G. Faszination Morsetasten: German telegraph keys Collector's guide, Emden, Germany, 2001

Ulsamer, G. Email to N. McEwen, 12 Dec. 2001 [silent key production post WWII]

Un Siecle De Telecommunications En Suisse 1852-1952 Volume 1, Telegraphy, Swiss PTT

B. Neal McEwen, K5RW

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