Alan Goldberg on the TCI listserv informs me that the Bell System measured call traffic in 100 second units, called CCS, with 36ccs equivalent to full usage in one hour, or "Erlang" on any given line. The Erlang is named after Agner Krarup Erlang, a Danish mathematician who did statistical analysis of telephone traffic at the Copenhagen exchange while working for the Bell System in the early 1900s. Erlang first published his work in 1908, so it appears that this type of watch was in use prior to 1908.