Scientific instruments are devices used for measurement and hence for scientific investigation and control. They extend the observing faculties of the human senses, providing accuracy and a greater range. They can also detect and measure phenomena, such as X-rays, which humans cannot sense.
Early instruments, used mainly in the fields of astronomy, navigation, and surveying, measured the basic quantities of mass, length, time, and direction (see astrolabe; balance; clocks and watches; compass). With the rise of modern science came several new instruments including the micrometer, microscope, telescope, and thermometer. During the Industrial Revolution and after were invented instruments to numerous to mention; and today in science, industry, and even the home there are a host of instruments to measure every conceivable quantity.
A few simple instruments, such as the ruler or balance, work by direct comparison, but most are transducers, representing the quantity measured by another sensible quantity (usually the position of a pointer on a scale). All instruments require initial calibration against a known or calculable standard. In general, an instrument interacts with the measured phenomenon, and the resultant change in its state is amplified, if necessary, displayed by means of a pointer, pen, light beam, oscilloscope, etc., and recorded usually on chart paper, by photography, or on some digital storage medium.
Although precision instruments are designed for high accuracy, inevitably errors are introduced: amplification produces noise, the slowness of the instrument's response results in lag and damping, and the intrinsic nature of the response may be defective owing to hysteresis (see hysteresis loop) or drift; moreover, the observer may misread the scale because of parallax or interpolation (see interpolate) errors. Most fundamental of all, the act of measuring a system may significantly alter the state of the system (see also uncertainty principle).