Electrical Measurement of Physical Values The Determination by Electrical and Magnetic Means of Quantities not in Themselves of an Electrical Nature

This paper was prepared under instructions from the Committee on Instruments and Measurements, with a view to forming a basis for the Committee's work in connection with electrical measurements as applied outside of the strictly electrical field. The system of classification used is in accordance with the nature of the quantity under measurement, rather than with the principle involved; and the subject is treated under nine headings. Measurement of Temperature: There are recognized two principal methods of electrically measuring temperature; the thermoelectric circuit, and the effect of temperature upon the resistance of many conducting materials. Outlines are given of the elementary applications of these, and a number of practical instrutments embodying one or the other of the principles are described. Reference is made to several types of calorimeters embodying electrical methods of temperature measurement, and to hygrometers or humidity meters, in which the principle of the thermocouple has been applied. A brief description is also given of the optical pyrometer. Measurement of Stress, Strain or Small Changes in Physical Dimensions: General reference is made to the changes which take place in both the electrical and magnetic characteristics of materials when exposed to mechanical stresses, with particular attention given to resistance effects. Several applications of the carbon pile in strain determination are mentioned. Recent developments in the use of the piezo-electric effect of certain crystals are described; and the use of the thermionic tube and its associated circuits for measuring very small dimensional changes is briefly treated.