Rectifiers

31. Mercury, in arc rectifiers, is chosen for cathode because

1. its ionization potential is relatively low
2. its atomic weight is quite high
3. its boiling point and specific heat are low
4. it remains in liquid state at ordi¬nary temperature
5. all of the above

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32. Normally, which of the following is used, when a large-scale conversion from AC. to D.C. power is required?

1. Motor-generator set
2. Motor converter
3. Rotary converter
4. Mercury arc rectifier

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33. Selenium rectifier can be operated at temperatures as high as

1. 25°C
2. 40°C
3. 60°C
4. 75°C

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34. The advantage of mercury arc rectifier is that

1. it is light in weight and occupies small floor space
2. it has high efficiency
3. it has high overload capacity
4. it is comparatively noiseless
5. all of the above

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35. The applications of selenium rectifiers are usually limited to potential of

1. 10 V
2. 30 V
3. 60 V
4. 100 V
5. 200 V

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36. The cathdde voltage drop, in a mercury arc rectifier, is due to

1. expenditure of energy in ionization
2. surface resistance
3. expenditure of energy in overcoming the electrostatic field
4. expenditure of energy in liberating electrons from the mercury

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37. The efficiency of an electrolytic rectifier is nearly

1. 80%
2. 70%
3. 60%
4. 40%

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38. The efficiency of the copper oxide rectifier seldom exceeds

1. 90 to 95%
2. 85 to 90%
3. 80 to 85%
4. 65 to 75%

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39. The ignited or auxiliary anode in mercury arc rectifier is made of

1. graphite
2. boron carbide
3. aluminium
4. copper

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40. The internal efficiency of a mercury arc rectifier depends on

1. voltage only
2. current only
3. voltage and current
4. r.m.s. value of current

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