The correct option is (c).
Explanation
The operation of a standard electric tester involving a filament bulb is based on the heating effect of electric current (Joule heating). When current passes through the filament, electrical energy is converted into heat and light energy. For weak currents, alternative indicators like Light Emitting Diodes (LEDs) or magnetic compass needles are often used.
Statement-wise Analysis:
- Statement 1 is Incorrect. The glowing of a bulb in a standard tester is due to the heating effect of electric current, not the magnetic effect. The current heats the filament to a high temperature, causing it to emit visible light. The magnetic effect of current is utilized in testers that employ a magnetic compass needle, which deflects even when the current is very weak.
- Statement 2 is Correct. For a filament to glow, it must reach a specific high temperature. If the current flowing through the circuit is too weak, the heat generated ($H \propto I^2$) is insufficient to raise the filament's temperature to the point of incandescence (glowing).
- Statement 3 is Correct. While some liquids conduct electricity (electrolytes), their electrical resistance is often significantly higher than that of metal wires. Consequently, the current flowing through the liquid may be too weak to heat the filament of a standard bulb sufficiently to make it glow. In such cases, an LED or a magnetic compass is required to detect the current.
Key Takeaway:
Standard filament bulbs rely on the heating effect of current and require a minimum current strength to glow. For detecting weak currents, such as those in conductive liquids, testers based on the magnetic effect (compass needle) or LEDs are used.