The Measurement System

Abstract

The measurement of any variable in a physical system inevitably disturbs the system, so that we are never able to become aware of the state of the system before the measurement was taken. This statement may appear rather sweeping but on close examination it will be seen that even the most sophisticated measurement technique involves an interchange of energy either from, or to the measured system. The standard determination of length by means of optical interferometric methods compares the length of the standard sample to the wavelength of light emitted from a particular atomic transition in the Krypton atom. Even at this high degree of sophistication the length of the standard sample has to be observed for comparison and the mere act of illuminating the standard causes a disturbance to it by radiation pressure. In this particular standard measurement, the technique has been so designed that this disturbance error is negligible, but the fact that the measurement procedure does affect the measured variable should be paramount in the reader’s mind since quite significant errors may be incurred inadvertently. This phenomenon will henceforth be referred to as parameter loading.