Typically, a measurement system consists of several active elements. A factor is needed to discriminate an object and perceive its amplitude or frequency. This information is then transmitted throughout the system through physiological signals. If the object itself is active, such as a stream of water, it can power the signal. If passive, it must stimulate the signal either by an energetic probe, such as a light source or X-ray tube, or by interacting with the carrier signal. Finally the physical signal is compared with a reference signal of known quantity which has been subdivided or multiplied by the desired measurement range. The reference signal is derived from objects of known quantity by a process called calibration. Comparison can be an analog process in which signals of constant amplitude are brought to parity. An alternative comparison process is quantization by counting, that is, dividing the signal into parts of equal and known size and increasing the number of parts.
Other measurement system functions simplify the basic process described above. Amplification ensures that the physical signal is strong enough to complete the measurement. To reduce measurement degradation through the system, the signal can be coded or converted to digital form. Magnification, the amplification of a measurement signal without increasing its power, is often necessary to match the output of one element of a system to the input of another element, such as a readout meter with the perception of the human eye. Mixing sizes.
An important type of measurement is the analysis of resonance, or the frequency of variation within a physical system. This is determined by harmonic analysis, which is usually reflected in the sorting of signals by a radio receiver. Calculation is another important measurement process, in which measurement signals are mathematically manipulated, usually by some form of analog or digital computer. Computers can also provide a control function in monitoring system performance.
Measurement systems may also include devices for transmitting signals over large distances (see telemetry). All measurement systems, even the most automated ones, include some way of displaying the signal to the observer. A visual display system may include a calibrated chart and a pointer, an integrated display on a cathode ray tube, or a digital readout. Measurement systems often include recording elements. A common type uses a writing stylus that records measurements on a moving chart. Electrical recorders may include feedback reading devices for greater accuracy.
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The actual performance of measuring instruments is affected by a number of external and internal factors. External factors include noise and interference, both of which mask or distort the measurement signal. Intrinsic factors include linearity, resolution, accuracy, and precision, all of which are characteristic of a given instrument or system, and dynamic response, drift, and hysteresis, which are effects of the measurement process itself. The general question of error in measurement raises the topic of measurement theory.
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