There were two versions of this film made; this is the short version. Watch for footage of ridiculously huge vacuum tubes that were used for radio transmissions.
From the original 1940 documentation:
"With Western Electric Vacuum Tubes in the starring roles, this film tells the fascinating story of tube development from the first crude bulbs of Edison and De Forest to the powerful and efficient tubes in use today, and shows the prominent part they play in radio, long-distance telephony, public address systems, sound motion pictures and the phonograph.
Animated sequences depicting a traffic cop halting an on-rush of electrons inside a tube and a troupe of monkeys tossing pebbles at a grid explain the three-element tube so clearly that a non-technical audience can understand how it operates. (emphasis ours - ed.)
Scenes in the tube shop show the precision workmanship that goes into the making of broadcasting and telephone repeater tubes. The camera moves from one intricate operation to another while skilled craftsmen transform spools of wire and rods of glass into these magic lamps of today."
Footage courtesy of AT&T Archives and History Center, Warren, NJ