40. Model of an incandescent lamp
Students noted that the coil of wire in an electromagnet became hot if kept
connected to the battery. (Experiment No. 7) If a wire is made white hot by
electricity without melting it, it would give off light. This is the principle of
the incandescent lamp. A basic model of an incandescent lamp can be made
easily using a strand of picture hanging (iron) wire.
Material:
A piece of picture hanging wire -- Two pieces of bell wire each about 15 cm
long. -- A large clear pill bottle.
Method:
1) Using a small nail or awl, make two small holes in the pill bottle lid about 2 to 3
cm apart and pass the bell wires through them as lead wires.
2) Strip the insulation from both ends of the wires for about a centimetre.
3) Separate a single strand of wire from a 10 cm long picture wire.
4) Cut a 3 or 4 cm long piece of from it and twist the ends over the ends of the lead
wires and replace the lid in the bottle.
5) Connect a flash light D cell to the ends of the lead wires outside the bottle and
watch the iron wire inside. Does it glow ?
You may need a supply of pieces of iron wire to
replace as they may be burnt or melted as they glow.
Try a battery of two 1.5 Volt D cells connected in
series (positive pole connected to the negative,) and
taping the cells together. ( = 3 Volts)
Experiment with iron wires of different lengths and
find out when they last longer. When you are using a
longer wire, make it into a small coil by winding it
around a piece of coat hanger wire.
Comments:
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Longer wires offer more resistance to electricity and less current is
allowed to pass.
Thicker wires offer less resistance than thinner wires.
The heat produced by electricity depends on the resistance offered,
current that flows and the voltage. Some metals offer more resistance
than others.
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Silver offers less resistance than copper.
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Copper is used as lead wires as it is much less expensive than silver.
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Iron offers more resistance than copper.
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Iron rusts easily and it also melts due to more heat produced by more
resistance.
If you can get a piece of nichrome wire (28 or 30 gauge) about 3 or 4 cm
long, try it instead of iron wire.
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Nichrome does not melt easily and it lasts much longer.
NOTE: The hot glowing wire should not be too near the
walls of the plastic bottle as the heat will melt the plastic.
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In a real incandescent lamp tungsten or osmium wire is used as it does
not melt easily and offers much resistance to electricity to glow white
hot.
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In addition, air, containing oxygen is also removed from the bulb and
then sealed to prevent the wire burning in oxygen.
Bulbs filled with nitrogen, or argon or a halogen give even brighter light.
Try if presence of carbon dioxide changes the life of the iron wire or
filament.
Carbon dioxide can be produced inside the pill bottle with a teaspoon of
vinegar and a pinch of baking soda ( sodium bicarbonate.)