Etymology of 10-4

From the 1940 ten-codes devised by the Association of Public-Safety Communications Officials-International (APCO), based on an unnamed standard invented by Charles "Charlie" Hopper for the Illinois District 10 State Police in 1937 that subsequently spread to the rest of Illinois, Iowa, Missouri, and Minnesota and documented by APCO in 1939. "Ten" was originally chosen to help novice operators fill an appropriate amount of time so that the dynamotor used to power 1930s radio transmitters could spin up to full power before the second number was transmitted. The second numbers were assigned with the aim that more useful common messages would be given smaller numbers, and similar messages would be roughly grouped into "brackets". The message "acknowledged" was expected to be extremely common and useful, and so was mapped to "four", the first available number after the three codes used to report and manage connection quality, which the committee gave highest priority.

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