Because such pulses of power can be regulated directly by the controller, a computer or microprocessor can keep direct track of a step motor without the need for a sensor tracking position. Resistance actuators generate either light or heat. There is no mechanical activity when they are used, and no switch is involved in turning them on as they respond directly to the current. A word on temperature Thermistors are not actuators. They change resistance on the basis of the temperature and in that role are a sensor and resistor.
Thermostats are units with variable sensitivity designed to adjust to the environmental parameters.
They are a subsystem within a control situation and may fulfil the role of a controller. Thermocouples are bi-metallic strips, the components of which expand at different temperature rates causing the strip to bend. It then acts as the contacts in a switch. It undertake the role of both sensor and actuator in one unit. Magnetic field actuators cause a movement directly within a fluctuating magnetic field.