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In order to relate the physical and chemical properties of carbon to the microphonic properties of this material, it has been found necessary to make measurements on physical quantities which are influenced by primary causes beyond the control of the observer. In many problems of the physical and engineering sciences it is possible for the observer to control within narrow limits, the causes of variation of a quantity while it is being subjected to measurement. Certain problems arise, however, in these sciences as in the field of economics and biology wherein it is impossible to do this. In general let Xi and X2 represent two quantities related to others U1, U2, ... Ua; Vi, V2, ... Vb; W3, W2 ... Wb in the following way:

proceedings of the national academy of sciences of the united states of america

On the Measurement of a Physical Quantity Whose Magnitude is Influenced by Primary Causes beyond the Control of the Observer and on the Method of Determining the Relation between Two Such Quantities