A sensor is a device that emits an output signal (voltage,
current, displacement, etc.) that is dependent upon a physical quantity
(temperature, pressure, light, flow, etc.). The output of the sensor is
functionally related to the input. This relationship is defined as the
sensitivity and is the ratio of the change in output to the change in input.
For example, if mercury in a thermometer raises one centimeter when the
temperature increases by one degree Celsius, the sensitivity of the thermometer
would be 1 cm/°C. The most common
sensors output a voltage in response to temperature, pressure, or flow.