English: A miniature voltage-tunable O-type backward wave oscillator tube from 1956, manufactured by Varian Industries. The backward wave oscillator, invented in 1951 by Rudolf Klompfner, is a specialized linear-beam tube which generates microwaves. The accompanying ad copy says this tube can operate over an 8.2 - 12.4 GHz range with supply voltage of 300-600 V. The body is approximately 4 inches long. The magnet required for operation weighs less than 5 lbs.
This image is from an advertisement for Varian Co. without a copyright notice published in a 1955 magazine. In the United States, advertisements published in collective works (magazines and newspapers) are not covered by the copyright notice for the entire collective work. (See U.S. Copyright Office Circular 3, "Copyright Notice", page 3, "Contributions to Collective Works".) Since the advertisement was published before 1978 without a copyright notice, it falls into the public domain.
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This work is in the public domain in the United States because it was published in the United States between 1929 and 1977, inclusive, without a copyright notice. For further explanation, see Commons:Hirtle chart as well as a detailed definition of "publication" for public art. Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.