Which of the following devices is necessarily required for automatic temperature control in a furnace ?

A. Variation of resistance
B. Variation of voltage
C. Periodical switching on and off of the supply
D. All of the above methods

A. Coreless transformers
B. Current transformers
C. Motor-generator set
D. Multi-phase transformer

A. To achieve better heating
B. To increase the life of roof refractory
C. To have better stirring action
D. To reduce problem of oxidation
E. All of the above

A. Current
B. Voltage
C. Power factor
D. All of the above

A. Water
B. Steam
C. Sold ice
D. Melting ice

A. By disconnecting some of the heating elements
B. By varying the operating voltage
C. By varying the current through heating elements
D. By any of the above method