Micrometers comprising a measuring head, extension rods with or without spacing collars, handle (in smaller sizes). Three sizes of measuring head, in inch and metric units. Stick micrometers for measurement of greater internal lengths (6 in or 12 in micrometer unit, fitted with micrometer of 1 in range which, with series of extension rods, permits measurement up to maximum length required). Material, thimble and barrel, micrometer screw, extension rods, spacing collars, adjustment, accuracy, rigidity, handle, gap setting gauges, marking, case and packaging, test methods.