devices act as both sensor and transducer. A thermocouple has two junctions
of wires of different metals; these generate a small electric voltage that
depends on the temperature difference between the two junctions. A thermistor
is a special resistor, the resistance of which varies with temperature. A
variable resistor can convert mechanical movement into an electrical signal.
Specially designed capacitors are used to measure distance, and photocells
are used to detect light (see Photoelectric Cell).
image-orthicon tube and the vidicon tube were invented in the 1940s and were
a vast improvement on the iconoscope. They needed only about as much light to
record a scene as human eyes need to see. Instead of camera tubes, most
modern cameras now use light-sensitive integrated circuits (tiny, electronic
devices) called charge-coupled devices (CCDs).
used for recording visual information is made of an array of photodiodes
(devices that conduct electricity when light strikes them) on top of a
semiconductor (a material that conducts electricity better than electrical
insulators but not as well as electrical conductors). When light strikes a
photodiode, an electric current proportional to the amount of light is sent
to a capacitor, which stores the charge. The semiconductor processes the
signal from the capacitor and sends it to a computer or other device that can
analyze the data about the light that hit the CCD.