Tachometer, Strobomeca, Test Instrument 1942

This is an interesting artifact, a "Strobomeca", which is a hand-held instrument,  used to determine the RPM's of a rotating object such as a propeller. It is essentially a mechanical stroboscope.

The user first selects and installs the slotted disc corresponding with the estimated speed of the object to be measured (e.g., a propeller). Then the small chrome wheel on the back is wound, which begins to rotate the disc. The user then observes the rotating object through the small viewing lens, where an image appears to be rotating clockwise or counter-clockwise. The outer housing of the instrument is then rotated until the image appear to be still, at which time the correct speed can be read on the index ring. 

The kit came standard with two discs, but others could be ordered for different speed measurements. This kit has the two-slot and the eight-slot discs included, as well as instructions and a purple-felt lined case.  The instructions are dated 1942. 

The case measures 4 inches by 3.5 inches by 3.5 inches, and the instrument is ~ 3 inches diameter. It appears to be in exceptional condition given its age. The winding mechanism functions and the loaded disc rotates.  A very cool little artifact.