Filar Micrometer

filar micrometer [′fī·lər mī′kräm·əd·ər]

(design engineering)

An instrument used to measure small distances in the field of an eyepiece by using two parallel wires, one of which is fixed while the other is moved at right angles to its length by means of an accurately cut screw. Also known as bifilar micrometer.

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Filar Micrometer 

an auxiliary device used with microscopes and with geodetic, astronomical, and other instruments in order to precisely measure small distances or angles. A filar micrometer consists of a housing containing a grid of fixed wires that are mounted in the focal plane of the instrument and are viewed through the eyepiece together with the image of the object being observed. A movable wire, which can be displaced with the aid of a finely threaded screw, is superimposed on the image. The screw has a graduated drum with a scale. The scale divisions are used to read the magnitude of the displacement of the movable wire with respect to the fixed wires. In astronomical instruments that are used to take measurements in various directions of the celestial sphere, the filar micrometer is mounted such that it can be rotated around the optical axis of the tube of the instrument.