A row of 25 decorative lights, connected in series, is connected to a main transformer. When the supply is switched on, the lights do not work. The owner uses a voltmeter to test the circuit. When the voltmeter is connected across the third bulb in the row, a reading of zero is obtained. Which of the following cannot be the only fault in the circuit?
Correct answer: B. The filament of the third bulb has broken.
- A. The filament of one of the other bulbs has broken
- B. The filament of the third bulb has broken.
- C. The fuse in the mains transformer has blown.
- D. There is a break in the wire from the supply to the transformer.
Explanation
When a filament of a bulb in a series circuit is broken, it creates an open circuit causing the voltage to be maximum across that bulb. In this case, the third bulb's broken filament would result in a high voltage reading, not zero. The correct answer is that the filament of the third bulb has broken. This creates an open circuit, leading to the absence of current flow and hence zero voltage. The other options are incorrect because they do not explain the observed zero voltage across the third bulb.
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