The "Fitzroy" clock benefits from a super durable, tarnish-free coating and QuickFix fitting system. Overall case diameter 15cm, depth 4cm. The instrument uses a good quality quartz movement and a dial with black Arabic numerals upon a face which features indicators for radio silence periods. Requires 1xAA battery. The instrument is supplied with a base plate which is screwed to the wall or bulkhead (instructions and drill hole template included). The instrument then simply slots over the base plate and with a gentle twist, is locked on. A reverse twist removes it again. This makes changing batteries easy, removal for storage easy, and avoids the tricky job of not scratching your polished case when screwing the instrument to the wall! TARNISH-FREE - Tarnish-free instruments have been coated with a newly developed protective coating using advanced surface treatment technology. Far superior to lacquers and varnishes, it can withstand more than 720 hours (the longest test period for this process) under NSS (Neutral Sea Salt) laboratory testing. NSS testing is a certificated and standardised test method used to check corrosion resistance of coatings on metals such as brass, zinc and steel. This means your instruments will stay looking good for longer even when exposed to the harsh marine environment for extended periods. RADIO SILENCE ZONES: Tragically, the radio-signalling equipment carried by RMS Titanic was of little use since the distress signals sent after hitting an iceberg in 1912 went unheard for several hours. The disaster spurred an international review of maritime safety which made several recommendations, one of which led to the introduction of radio silence periods. These are three-minute periods immediately following each quarter-hour reserved exclusively for making, and listening for, distress calls. The green sectors represent silence periods on the SSB (Single Side Band)) distress frequency of 2182kHz, as carried by commercial and some pleasure craft. The red sectors are for Morse code distress signals on the 500kHz band, as used in radiotelegraphy. Radio room clocks have traditionally been marked with the sectors as guidance for the operators.