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What is a fluorescent tube ?

When the fluorescent tube is switched on almost full supply voltage is applied to the starter. The potential across bimetallic electrodes of the starter causes a small glow discharge at a small current not enough to heat up the tube filaments. This discharge is enough to heat the bimetallic strips of the starter itself causing them to bend and make contact. As a result, the large current flows through the electrodes their temperature being raised to incandescence and the gas in the immediate neighborhood inside the tube gets ionized. At the same time when the contacts of the starter are closed potential difference across it is reduced to zero. After two or three seconds glow discharge inside the starter stops and the bimetallic strips cool down which breaks the contacts apart. This sudden break induces a very high voltage in the coke connected in series due to its inertia effect and it is sufficient to initiate the discharge in the main tube light. When the fluorescent tube is alight, current passes through the tube which offers a very low resistance. The voltage across the tube is about 110 volts and rest of the supply voltage of about 110 volts is dropped in the choke. Due to the low voltage, the starter ceases to glow. Starters are so designed that this potential difference across the lamp in running condition is insufficient to restart the glow discharge in the starter. So the contacts remain open, thus the tube is put in operation.


Why is a condenser connected in parallel with the starter of a fluorescent tube ?

To suppress the radio interference a condenser of about 0.05 μF capacity is connected in parallel with the starter.

Why is a resistance connected in series with the radio suppressor condenser?

A resistance of about 100 ohms is connected in series with the radio suppressor condenser to check the condenser surges so as to prevent the starter contacts welding together.

Why is a condenser connected across the supply line in fluorescent tube circuit?

To improve the power factor a condenser is connected across the supply line. The size of the condenser should be 3.25μF for 40 watt and 7.5 μF for 80 watt lamps.

Is the choke necessary for tube light working in d.c. circuit ?

Yes, the choke is necessary to create the voltage impulse at the time of starting.

Why is an extra variable resistance connected in series with the chock on d.c. supply ?

As there is no effect of inductance on d.c. supply an extra variable resistance is connected in series with the choke to increase the effective resistance to absorb the difference between the mains voltage and the correct lamp voltage during running.

Why is a reversing switch used for fluorescent tube light in d.c. circuit ?

A reversing switch is used to reverse the current at intervals to prevent the blackening of the tube at positive end due to migration of the mercury from the positive end to the negative end of the tube.

Which lamp will give more of incandescent lamp and fluorescent tube light for the same wattage?

Fluorescent tube light.

Why are fluorescent lamps most commonly used?

Fluorescent lamps are most commonly used due to its following advantages over other light sources :-
(a)Increased efficiency, (b) Longer life of lamp, (c) Gives illumination in a variety of colours, (d) Gives glare or glitter free light, (e) High lighting intensities are achieved without excessive temperature rise, (f) Gives diffused and shadow light, (g) Gives more comfortable illumination and (h) Through initial cost is more it is economical.

Why is light efficiency high in fluorescent tube lights ?

The light efficiency in fluorescent tube light is high because the invisible radiation is converted into visible radiation with the action of phosphor coating inside the tube which reduces the loss light energy.

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