an atomic clock which uses the 1.4 GHz transition in a hydrogen atom to measure time to within 0.45 nanoseconds over a span of 12 hours

plural of hydrogen maser atomic clock

plural of face that would stop a clock

A normally unreliable person or instrument can occasionally provide correct information, even if only by accident.

Alternative form of a stopped clock is right twice a day

plural of clocker

A person who clocks (illegally winds back the milometer of) a motor car / (slang) A low-level drug dealer who operates on the streets. / (Scotland, dated) A clucking hen.