On Sat 2020-06-13T11:44:19-0700 Paul Eggert hath writ:
“Art. 2. - Pendant la période s’étendant de la nuit du 10 mars au 11 mars (minuit), à la nuit du 30 juin au 1er juillet 1911 (minuit), l’indication de l’heure transmise aux navires en mer, par les stations côtiéres radiotélègraphiques ouveries au service public, sera suivie de la mention : « Heure de l’Europe occidentale ».”
This means that the clocks used at the coastal radio transmitters were reset on the night of March 10/11, and that the signals were broadcast using the new time starting that night, and that the Morse code transmissions were made more complicated than before by keying an alphabetic string after the time pulses. Likely the transmitted string was just the abbreviated initials. At this point radio transmitters were spark gaps which emitted broadband noise. If more than one was in use then ionospheric propagation could switch reception between one and another. Compare with the sinking of the Titanic in 1912 when there were so many ship and land transmitters in operation that the ionosphere mixed strings of characters from them to produce news reports that the Virginian was towing Titanic. See the size of the US Navy gear of that era at http://www.navy-radio.com/commsta/arlington.htm which was used in 1914 to determine the longitude difference between Paris and Washington.
We have more detail to help us resolve this ambiguity, in the next page of the cited source, which says “Jusqu’a la nuit du 30 juin 1911 exclusivement, aucune modification ne sera faite à l’instant de l’envoi des signaux horaires par l’Observatoire de Paris. [Until the night of June 30, 1911 exclusively, no modification will be made at the time of the sending of time signals by the Paris Observatory.]” That is, through June radio time signals were still sent at the same time intervals as before, with the labels altered to be new time. In particular, signals were sent daily “La nuit à minuit 0 m. 0 s., minuit 2 m. 0 s., minuit 4 m. 0 s. de temps moyen de Paris [At 00:00:00, 00:02:00, 00:04:00 Paris Mean Time]” with labels “11 h. 50 m. 39 s., 11 h. 52 m. 39 s., 11 h. 54 m. 39 s. [23:50:59, 23:52:39, 23:54:39]”.
This means that the paper clock at Observatoire de Paris (which was constructed by choosing one from all the physical clocks in the basement and the astronomical observations of stars) was not reset until July 1. So during the interval from March 11 through June 30 the observatory continued to send Paris Mean Time and it was incumbent on the staff at the radio transmitters to do math to verify that their local clock was offset by 9:21 from the observatory clock. Until the WW2 German occupation in 1940 May the Observatoire de Paris clock was conveyed to the transmitter at Tour Eiffel by a dedicated telephone line. So this likely means that the time signals from Tour Eiffel stayed on Paris Mean Time through June 30, whereas the coastal transmitters changed on March 11.
We want to know what happened during the change around 1911-03-11 00:00. For example, were the radio signals sent at 1911-03-11 00:04 old time labeled “00:04:00 Paris Mean Time” or were they labeled “23:54:39 Western European Time”? The 1911-03-11 change was supposed to occur from “de la nuit du 10 mars au 11 mars (minuit)”, which suggests that it was intended to affect the broadcasts of 00:02 and 00:04 old time as they occurred after midnight old time. And it would have been odd for the broadcast of 00:00 old time to have been treated differently from the other two. So this suggests that for the radiotelegraph, the transition occurred at 00:00 old time.
Not really any of the above. In that era the time broadcasts were not continuous. Each station broadcast time signals during a few brief intervals each day. Other intervals during the day either had no broadcasts (powering a 5 kW transmitter was not always feasible) or they were broadcasting other information such as commercial messages. If everyone had transmitted at once with spark gaps they would routinely be jamming each other, and more important messages, when the ionosphere shifted. Bulletin Horaire has a fun note saying that the Saigon transmitter stopped sending time signals on 1941 December 8 because of more exigent radio traffic. The intermittent nature of time signals persisted into the 1960s where German DCF77 broadcast old UTC (based on UT2) during some hours, SAT (stepped atomic time, based on PTB cesium which was a contributor to what later became TAI) during other hours, and nothing at all during other hours. In any issue of Bulletin Horaire it can be seen which stations were broadcasting at which hours. As the earliest example from 1921 http://adsbit.harvard.edu/full/1922BuBIH...1....3. page 8 shows Tour Eiffel demi-automatic signals at 10:45 and 22:45. page 11 shows rhythmic signals from Tour Eiffel a 10:00 and from Lyon at 08:00 The "automatic" signal format was decided by the original International Time Conference in 1912 which chartered the existence of BIH. The "rhythmic" signal format was a different presentation also known as "vernier" in common use from the 1920s until 1962. Evolution of rhythmic is depicted on pages 320/321 of http://adsbit.harvard.edu/full/1929BuBIH...3..303S So the answer is that because no station was broadcasting continuously, there was no ambiguity about whether they were broadcasting before or after the 1911 March 10/11 boundary. Anyone listening to Tour Eiffel was getting old Paris mean time. In any case, any of these radio broadcasts were irrelevant except to a few trained radio operators. The behavior of the railway clocks was the most obvious official indicator, and a lot of the population would probably have relied on whatever their local church bells did. -- Steve Allen <sla@ucolick.org> WGS-84 (GPS) UCO/Lick Observatory--ISB 260 Natural Sciences II, Room 165 Lat +36.99855 1156 High Street Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 https://www.ucolick.org/~sla/ Hgt +250 m