Length, Volume, and Density Measurement

For higher precision, the scale is magnified with an optical microscope. Accuracies of 0.2 pm are possible in this way. Precision techniques for subdivision of scales and special instruments for comparisons have been developed. The highest precision can be reached by observing differences in interference fringes, set up by monochromatic light between the ends of the objects to be compared. For the maintenance of the standard meter a precision of 1 in 10 is possible as mentioned in Appendix 8. 

Measurement of Length by a Linearly Variable Differential Transformer, LVDT 

LVDT due to temperature changes, a differential setup may be used, as indicated in the bottom sketch. A reference sample is connected to the coil, so that only the differential expansion of the sample is registered. Quartz is used frequently for the construction of the connecting rod and also as reference material. It has a rather small expansivity when compared to other solids as can be seen from Table 4.1. Another material of interest is Invar, an alloy, developed to have a low expansivity. 

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