English: The first commercial klystron in 1940, manufactured by Westinghouse. Part of the tube is cut away to show the internal construction. The klystron, invented in 1937 by Russel and Sigurd Varian at Stanford University, is a linear beam vacuum tube which generates microwaves. It could generate 200 W of power at 40 centimeters (750 MHz) with 50% efficiency. On the left is the cathode and accelerating anode which creates the electron beam. In the center between the wooden supports is the drift tube, surrounded by the two donut-shaped cavity resonators, the "buncher" to which the input signal is applied, and the "catcher" from which the output signal is taken. The output terminal is visible at top. On the right is the cone shaped collector anode which absorbs the electrons.
This 1940 issue of Electronics magazine would have the copyright renewed in 1968. Online page scans of the Catalog of Copyright Entries, published by the US Copyright Office can be found here. Search of the Renewals for Periodicals for 1967, 1968, and 1969 show no renewal entries for Electronics. Therefore the magazine's copyright was not renewed and it is in the public domain.
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 (70 years p.m.a.), Mainland China (50 years p.m.a., not Hong Kong or Macao), Germany (70 years p.m.a.), Mexico (100 years p.m.a.), Switzerland (70 years p.m.a.), and other countries with individual treaties.