Outer box, Outer box open, Inner box, Inner box open, Dial, Movement, Movement marks, Angle view, Side view, Escape wheel, Balance closeup, Fuzee, Instructions, Tub bottom

Hamilton Marine Chronometer Model 21. The Hamilton Marine Chronometer was the winner in the WWII competition for the Navy/Military standard chronometer. As a result they were produced in great quantity. This example has both the inner box and the outer box, but does not have the externally operated balance arresting tool. The instruction page in the top of the box refers to the use of this tool, so it is likely that either the instructions were added during a refurbishment, or that the box has been switched with another chronometer.

The Model 21 was a distinct departure from standard chronometer practice, but worked extremely well. The more conservative Elgin design with a Guillaume Balance did not meet the military specifications, while the new "Ovalizing Balance" from Hamilton with an uncut stainless steel rim and Invar cross arm proved both accurate and durable.