Marconi Radiotrons Substitution List
This pamphlet was published by Marconi, the company responsible for patenting the radio in 1904. This specific pamphlet was a substitution list for replacement 'radio tubes'. Radio tubes generate and amplify radio-frequencies, but have a temporary shelf life and need to be replaced periodically. Before the introduction of television, and especially during wartime, having functional radios was crucial for any family.