original and remarkably hard to find c. 1940's original illuminated test-o-matic radio tube tester can sign

c. 1940's original t.v. and/or radio repair shop or store illuminated can sign that would have hung in the display window and/or near a test-o-matic device used to test the integrity of your vacum radio tubes. the light source within the can is a single incandescent standard watt light bulb. the white panel with black print slides out with ease to quickly access and replace the light source. measures 18 x 5 x 4 inches. the 19th century saw increasing research with evacuated tubes, such as the geissler and crookes tubes. scientists who experimented with such tubes included eugen goldstein, nikola tesla, johann wilhelm hittorf, thomas edison, and many others. these tubes were mostly for specialized scientific applications, or were novelties, with the exception of the light bulb. the groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology. though the thermionic emission effect was originally reported in 1873 by frederick guthrie, it is thomas edison's 1884 investigation of the edison effect that is more often mentioned. edison patented what he found, but he did not understand the underlying physics, or the potential value of the discovery. vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of radio broadcasting, television, radar, sound reproduction, large telephone networks, analog and digital computers, and industrial process control. some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.some small cracking on one side of sign, price adjusted accordingly.