In slip gauge interferometry, every deviation d of a slip gauge of length L produces a unique set of fractional differences for the wavelength of radiations used and this deviation (d_(fd))_L (deviation associated with the set of fractional differences for the slip gauge length L) is obtained by use of a slide rule. In practice, deviation d is given by (d_(fd))_L plus wavelength and temperature corrections (involving #alpha#) over the slip gauge length. This equation is utilized to find #alpha#. Conventionally, one slip gauge length and two temperature values (measurement at two different temperatures) are used to find #alpha#. Since temperature is a major error causing factor, it is preferable that temperature measurements take place only once, since its measurement twice implied the possibility of (1) doubling the error in the measurement of the parameter itself and also (2) increasing the error in measurement of other environment related parameter, like barometric pressure, dry bulb temperature, wet bulb temperature, etc. involved in wavelength correction, because two different temperature, amounts to two different environmental conditions. If measurement at one temperature is undertaken using two slip gauge lengths (of identical deviation) then not only these major errors are taken care of but also another error involving difference between actual and nominal length of slip gauges is also eliminated.
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