Cable telegraph keys have been in use since the first Transatlantic Cable was completed in 1858. The first thing that stands out when looking at a cable telegraph key is the fact that it has 2 levers, one for dots and one for dashes. One lever produces a positive current and the other a negative current. At the receiving end of the cable, this difference between positive and negative currents is used to decode the Morse signal.
One early method of decoding this signal involved the use of an Optical Galvanometer. A mirror was attached to a galvanometer coil suspended between 2 magnets. A positive current through the coil would cause the mirror to rotate one direction, and a negative current could cause the mirror to rotate in the opposite direction. A light was shined on the mirror which would reflect against a long horizontal ruler. Thus, the rotation of the mirror in either direction would deflect the light beam to the right or left depending on whether a dot or a dash was received. The operator would watch the movement of the light beam against the ruler to read the Morse signal. (Click here for a drawing of a mirror galvanometer, and here for an actual example of an instrument)
An improved method of reading cable signals came from using a Siphon Recorder. The Siphon Recorder used a small glass tube connected to an ink well to draw a line down the length of a paper tape. The siphon was attached to a coil much like with the Mirror Galvanometer. Changes in direction of current would cause the ink line created by the siphon to move towards one edge of the tape or the other depending on whether a dot or dash was received, thus giving a permanent record of the incoming signal. It took some practice to be able to read the decoded signal as it was not a sharp transition of the line in either direction. Capacitance on the cable caused a "smearing" of the incoming signal. You can see an example of this later. (Click here to see a drawing of a Muirhead Siphon Recorder, and here to see a picture of an actual instrument)
Using a hand cable key to send messages was rather slow, so to speed up the transmissions of cable messages, a paper tape was pre-punched with holes to represent a Morse signal and then fed through a Wheatstone Transmitter to automatically send the coded message. For an example of a Morse message from a Siphon Recorder as well as a pre-punched transmitting tape, click here (Above photos courtesy of atlantic-cable.com)
For everything you could possibly want to know about cable telegraph, please visit Bill Burns excellent website dedicated to the Atlantic Telegraph.
(Click on the pictures below to see larger versions of the photo)
Classic style cable key by Elliott Brothers, London.
|A similar style cable key by Muirhead & Company Ltd.|
Cable Key Marked Singpore Workshop
Elliott Brothers key with knife-edge pivots (w1tp.com).
|Prussian cable key with a pair of camelback keys.|
|Muirhead & Co key with locking mechanisms to hold down levers in transmit position.|
|Silvertown cable key with sheet metal strap levers. Because of the stiffness of these levers, keys with this type of lever were generally used for cable testing rather than actual cable telegraphy.|
|Muirhead & Co key with similar strap levers.|
|Early Silvertown cable key with strap levers.|
|A more ornate British cable key. Maker unknown.|
|A pair of Double Paddle keys, likely made for double current telegraph use, but keys like this were also used for cable telegraphy. The key on the left was made by Silvertown, and the one on the right is a Walters Model 6K.|
|A British double current style key with 2 levers for use with cable telegraph. (From the French Cable Station Museum in Orleans, Massachusetts)|
|Saunders Signalling Key by Muirhead & Company, Westminster|
|An Italian cable key. (IK6BAK collection)|
|Cable key with "Victor" style knife edge pivots. Possibly made by ES Greeley.|
|Western Electric cable key comprised of a pair of Steiner Keys.|
|Cable key by Bunnell Telegraph & Electric Co, New York.|
|Unknown maker cable key.|