In the United States, Lee De Forest, who patented the first triode vacuum tube (audion) in 1907 and 1908, was experimenting with a high-frequency spark-gap generator for use in surgery and filed a U.S. patent application describing his work on 10 February 1907, which was issued on 17 December 1907. This device was the first electrosurgical unit, and there are no better words than his own in his patent for describing it; he stated: Be it known that I, Lee De Forest, a citizen of the United States, and a resident of New York, in the county of New York and State of New York, have invented a new and useful improvement in Cauteries, of which the following is a specification. My invention relates to devices for producing burning effects by means of high-frequency electrical currents, and more particularly to cauteries employed in surgery. (A cautery is a heated rod used to cut and coagulate living tissue.) I have discovered that when a conductor is attached to any part of a high-frequency oscillating circuit, the high-frequency electrical current passing into a body, for example, a human body, will produce a heating or burning effect at the point of contact of the electrode conveying such currents to said body, and that the intensity of said heating or burning effect is inversely proportional to the terminal area of said electrode. If therefore the electrode ends in a fine point or cutting edge, or relatively small area of other shape, an extremely intense burning effect may be obtained.