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Western Union Desk-Fax, 1950s.
The Desk-Fax was a facsimile transceiver introduced by Western Union around 1952 to allow business subscribers to send outgoing telegram forms to WU offices for transmission, and to receive images of printed telegrams which arrived at the WU office, thus eliminating messenger trips. The Desk-Fax did not use regular dial phone lines like modern fax machines; rather, it was linked to the local WU office by dedicated wires.
To send, the user wrapped a WU telegram blank (about 4” x 6”) around a metal drum and pushed a button. The drum rotated and slowly moved lengthwise, allowing a focused light beam to scan the entire document. The reflected light was modulated by passing through a rotating, notched “chopper” wheel, and finally was focused on a phototube which converted it to an AC signal (a 2500 Hz carrier on-off amplitude modulated by the scanned image).
Messages were received on a special paper called “Teledeltos”, which had an electrically-conductive black carbon-bearing layer. Writing was done by a fixed, fine wire stylus to which a high-voltage was applied, burning through the coating where a black mark was required. The stylus pressed against the paper, which was wrapped around the machine’s drum.
All of this was accomplished with five vacuum tubes (plus the phototube), six relays, four electric motors, and assorted other electronic and electromechanical components. Transmission time took about 3 minutes.