Electrical systems are only able to respond to voltage and current signals in the electrical domain: amplitude, frequency, phase and time constant. An input transducer is necessary to convert a signal from its domain of origin (non-electrical) into the electrical domain.
A transducer may generate an electrical signal by varying one of the
following: voltage, current, resistance, capacitance, self-inductance,
mutual-inductance, 
, 
, 
or 
The most fundamental transducers respond to temperature,
electromagnetic radiation intensity, force, displacement or chemical
concentration.
If coupled to the time domain these devices can be used to measure any
physical or chemical quantity.
Examples of input transducers are: a radio antenna, a photo-diode,
a phototube, a piezoelectric crystal, a thermocouple, a Hall effect
device, a mechanical switch, a strain gauge, an ionization chamber,
etc..
The output transducer transfers signals out of the electrical domain and into the domain that can be perceived by one of the five human senses. A substantial amount of power is usually required to transfer information out of the electrical domain. Examples of output transducers are the motor, cathode-ray tube, loudspeaker, light-emitting diode, radio-frequency transmitter, etc.