By Jim Duff
Radio Room Clock
RADIO ROOM CLOCKS
The sinking of the Titanic resulted in the Radio Act of 1912 that required 24-hour radio watches. The disaster also led to clocks in the newer radio rooms featuring three-minute periods marked in red. That three minutes provided a silent period when only emergency radio messages could be transmitted.
First: where does it come from?
It all started quite some time ago, the early days of radio communication, and it has to do with maritime radiotelephone communication in distress situations on the typical marine MF bands: the 2182 and 500 KHz international bands for emergency and distress. In fact the sinking of the “RMS TITANIC” triggered a lot of safety rules and this is believed to be one of them.
Why a radio silence period?
This allowed any station with distress, urgent or safety traffic the best chance of being heard at that time, even if they were at some distance from other stations, operating on reduced battery power or perhaps reduced antenna efficiency, as for example from a dismasted vessel.
All stations using 2182 KHz were required to maintain a strictly enforced three-minute silence and listening period twice each hour, starting at h+00, h+30.
As a visual aide-memoire, a typical clock in a ship's radio room (see picture) would have these silence periods marked by shading the sectors from h+00 to h+03 and from h+30 to h+33 in green.
Similar sectors were marked in red for what used to be the corresponding silence and listening period on 500 KHz between h+15 and h+18 and from h+45 to h+48.
This frequency was used for Morse Code signalling – and is not generally used today.
It is marked in red on the dial, clearly and forcibly calling attention to the radio operator thereto, are the two 3 minute silent periods which must be observed by all radio stations at 15 and 45 minutes past each hour."
"The dial has accurate 4 second marks in red around the outside edge, over which the sweep seconds hand passes, enabling the radio operator to accurately transmit the 4 second alarm signal provided by the International Telecommunication Convention and the International Conference on Safety of Life at Sea." Mayday Mayday Mayday …---…
Modern ships use the Digital Selective calling system (G,M,D,S,S,) – which do not require listening periods as messages are delivered electronically to a consul in the Radio Room.