Control Systems
The device necessarily used for automatic temperature control in a furnace is

 Any of above.
 Thermostat.
 Thermocouple.
 Auto-transformer.

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Control Systems
In the integral control of the single area system frequency error is reduced to zero. Then

 integrator o/p decreases and speed changer position comes down.
  integrator output and speed changer position attain a constant value.
 integrator o/p decreases but speed changer position moves up.
 integrator o/p increases but speed changer position comes down.

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