Marked: "DAYRAD Tube Checker / Mfd. by the Radio Products Co., Dayton, Ohio. / Licensed Under Weston Patent 1704566 Type B". Includes leather carrying strap. Tube tester designed for A.C. plug in ...

A vacuum tube is just that: a glass tube surrounding a vacuum (an area from which all gases have been removed). What makes it interesting is that when electrical contacts are put on the ends, you can ...