A silica standard of length

Abstract

The paramount qualities which must characterise a material suitable for a primary standard of length are constancy and durability. In addition to the absence of secular variation it should exhibit no linear thermal hysteresis, i. e . no lagging of length behind temperature. The ideal material would furthermore, from the operator’s point of view, possess a zero coefficient of expansion—a convenience which, until recent years, has not been even approximately satisfied—should take a high polish, permit the ruling of suitable lines, and should answer all requirements as to hardness, elasticity, density, resistance to oxidation, etc. These qualifications lead us to look for a pure substance—chemically simple—which will combine in itself as many as possible of the important essentials. In this connection it will be interesting to take notice of some of the materials which have been used at different times for primary line standards of length.