Every sensor nudges the very thing it is trying to measure
Drop a room-temperature thermometer into hot coffee and the reading is already slightly wrong, because the glass has cooled the drink while the drink warms the glass. Sensors always interact with what they measure. Good design keeps that meddling small, and shrinking the device is one of the most reliable ways to do it.
A sensor turns some quantity or condition into an electrical signal and passes it on, often to a processor. They hide everywhere: touch-sensitive lift buttons, lamps that brighten when you tap the base, motion detectors, and countless others nobody notices. Cheap microcontrollers and tiny machined parts have pushed them far beyond the old trio of temperature, pressure and flow, while humble analog devices such as potentiometers and force-sensing resistors remain common in cars, aircraft, robots and medicine.
The key figure is sensitivity, how much output changes per unit of input. If mercury climbs a centimetre for each degree, that is the slope. Real devices stray from the ideal in many ways: the slope itself can be off, a constant offset can creep in, the response can bend away from a straight line, and readings can drift over months as the hardware ages. Some remember their history, giving different answers depending on whether the input was rising or falling, an effect called hysteresis. Digital sampling adds its own traps, including aliasing. Calibration can correct systematic faults, while filtering tames random noise at some cost in speed. Resolution, the smallest change detectable, is not the same as accuracy, which may be far worse.
Miniaturisation changed the field. Microsensors built in bulk with MEMS techniques usually react faster and more sensitively than bigger versions, and cheap disposable sensors now handle one-off checks without recalibration or contamination. Chemical sensors pair a recognition step, where target molecules latch onto receptors, with a transduction step that produces a signal; if the receptor is biological, it is a biosensor.
Transistor technology spawned a family of its own. Piet Bergveld's ion-sensitive transistor of 1970 was among the first. Willard Boyle and George E. Smith devised the charge-coupled device in 1969, ancestor of camera sensors, and the first optical mouse, built at Xerox in 1980, used a similar chip.
Source: Sensor
