What is "filar micrometer"

Bifilar \Bi*fi"lar\, a. [Pref. bi- + filar.] Two-threaded; involving the use of two threads; as, bifilar suspension; a bifilar balance.

Bifilar micrometer (often called a bifilar), an instrument form measuring minute distances or angles by means of two very minute threads (usually spider lines), one of which, at least, is movable; -- more commonly called a filar micrometer.

A filar micrometer is a specialized eyepiece used in astronomical telescopes for astrometry measurements, in microscopes for specimen measurements, and in alignment and surveying telescopes for measuring angles and distances on nearby objects. The word filar derives . It refers to the fine threads or wires used in the device.