Spark Keys

Wireless Specialty Apparatus Key

Just before Horace Martin invented the bug or semiautomatic key, Guglielmo Marconi, continuing the work of Hertz, Maxwell, and others invented wireless radio. Early wireless telegraph, from about 1895 to 1924, was called "Spark." The spark key was inserted in the high voltage part of the circuit and sparks literally flew from the key when code was sent. Spark was initially conceived as a way the Navy could keep in touch with ships at sea. Spark also helped keep civilian ships in touch with the shore. The first newspaper ever published aboard ship containing wireless news from shore was published aboard the St. Paul en route for England on November 15th, 1899. Landline was then intended to provide the link on the continent. Spark keys required huge contacts to handle the immense current and voltage across the contacts. The key shown here is a Wireless Specialty Apparatus Company spark key with cooling fins on the contacts.

Early wireless radio depended on a spark between brass balls to generate a transmitting signal. Very high and dangerous voltages were required. What is the spark voltage between brass balls 2 centimeters in diameter? It depends on the distance between the balls, the spark length. Pick a spark length to change the distance between the balls and click on the "Discharge" button to see the voltage needed to generate a spark. Adapted from Robison's Manual of Radio Telegraphy and Telephony, 1918, Appendix A, Table 1.

WWI Marconi Mach.Div Boston Clapp-Eastham Boston key

Manufacturers such as the Marconi Company, Machinery Division of the Boston Navy Yard, Clapp-Eastham, Murdock and many others made spark keys. These keys tended to be larger and bulkier than the landline telegraph keys of the time. The key on the left was made by the Marconi Wireless Telegraph Co., Ltd, of London, England and is a typical World War I instrument. The center key was made by the Boston Navy Yard and the rating stamped into the label reads 300 volts at 50 amperes! The key on the right is a marble-based Clapp-Eastham key made ca. 1915. It is known as the "Boston Key" and was also available on a bakelite base and with at least 3 different size contacts.
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Murdock key

The spark key on the left was made by the Wm. J. Murdock Co. of Chelsea, Mass. There were also offices in Chicago and San Francisco. Murdock also sold other radio gear in addition to spark keys.


The spark key on the right is another Boston Navy Yard model, with the contacts encased in solid metal. This presumably protected it from its surroundings.

 and Roger Oil Break Key

As early as 1901, the U.S. Navy was considering buying equipment from Marconi, Ducretet, Rochefort, Slaby-Arco, and Braun-Siemens. Ducretet and Roger, of Paris, France, sold the U.S. Navy two complete wireless sets in 1902 for $2,614.37. This Ducretet and Roger aircraft oil break key dates to about 1915. It represents the merging of two technologies in their infancy: airplanes and wireless. The contacts are submerged in oil to prevent the spark from igniting the gasoline fumes in the early airplanes.

The semiautomatic key invented by Horace Martin could not handle the large currents associated with spark telegraphy. Building on the Fleming Valve, or diode, of the 19th century, Lee De Forest invented the Audion tube, or triode. The Audion was capable of undamped oscillation and allowed the development of continuous wave wireless (CW.) The semiautomatic key or bug now could evolve into the most popular telegraph instrument of the mid 20th century.

Electro-Magnetic Induction

As a magnet enters a coil of wire, an electromotive force is induced in the coil of wire. If there is a complete circuit, a current of electricity will flow. This effect is known as Electro-magnetic Induction. As the magnet is moved into the coil, the needle of the galvanometer is deflected indicating that a current of electricity has been generated in the coil. If we withdraw the magnet from the coil, the needle of the galvanometer will again be deflected, this time in the opposite direction. Adapted from "The Elementary Principles of Wireless Telegraphy", by R.D. Bangay.

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