Postwar version of the Ships Radio Room Clock. The red sectors show silence periods on the 500 KHz distress frequency, green on 2182 KHz. 

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.

I've always been fascinated by these clocks used in shipboard radio shacks. See the diagram in the photos from the Chelsea Clock Company (USA) Special Radio Room Clock.



In the US Government specifications for the Chelsea clock, it notes "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." And later, it indicates that, "also 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." Thus, the red marks on the dial edge are for the seconds alarm, and the red sector triangles for 3 minute radio silent periods on the 500 kcs. distress frequency. Later models featured additional green sectors at the top of each hour and 30 minutes past each hour for the monitoring of the 2182 kcs. distress frequency.

Features:

Corrosion-resistant ABS housing

Scratch-resistant acrylic lens

Easy access hinged front bezel

Dial diameter: 4"

Weight: 10 oz.

Mounting flange diameter: 6"

Mounting hardware included.

One "AA" battery required (High quality Alkaline battery recommended for best performance).

One year manufacturers warranty.