MadSci Network: Engineering |
Hi Johnathan!
To measure the light output of a light bulb accurately, you should
probably use a real light meter like the kind photographers use. You will also need to always keep the light meter exactly same distance away from the bulb, so it doesn't measure different brightness as you move the
bulb and the meter around. Also,
you should not change any of the objects surrounding the bulb, since
changes in the nearby objects can fool the light meter into
thinking that the brightness of the light bulb has changed. Perhaps
you could tape the light bulb and the light meter inside a box,
that way you could put the whole thing in a freezer, yet the light
meter would not be fooled by any white walls held near the bulb.
A silicon solar cell will work, but it will register the light bulb's invisible
(infra-red) light much more strongly than a human eye
would. The photocell will think that the light is bright, even if it looks
fairly dim to your eyes. It might work for you anyway, since you
are looking for changes in the light, and not trying to measure the
actual brightness which human eyes would see. If you wish to use a solar cell, use a current
meter (measures milliamperes) to measure the output current of the
solarcell while the cell it is short-circuited
(a current meter acts like a short circuit, so simply
connect a current meter to the solar cell's wires.)
To measure the resistance of a light bulb, you should measure the
current going through the bulb, and at the same time measure the
voltage across the bulb. You could use both a current meter and
a voltmeter at the same time. For less accurate measurements, you
can use a single
"digital voltmeter" instrument. First connect in in series with the
bulb in order to measure the current. Later you would connect it
in parallel with the bulb and measure the voltage.) Once you know the
amperes and the volts, you can calculate the resistance of the light
bulb by dividing the voltage by the current. Volts divided by current
gives "ohms" of resistance.
Be careful! If you are using a line-powered light bulb, it will
have 120 Volts on any exposed metal parts, and this is very
dangerous, it can shock you or even kill you. If you wish to
measure the voltage and current of a 120-volt bulb, you should
work with an adult who has experience with high voltage home
wiring, and who knows all the safety procedures for working
on "live" circuits. Never attempt to remove
the insulation of 120-volt wires yourself. Never try to measure the
volts or amperes of a 120-volt light bulb without help from
a trained adult.
It is far safter to use batteries to power your bulb. Perhaps use
a 6-volt or a 12-volt battery.
If you use batteries, you should keep measuring the
battery voltage, since any changes in the battery voltage can
cause the brightness of your bulb to change. It might be wise to
use a "regulated" DC power supply such as the type used in
high school physics classes. That way you can keep the voltage
at the same level all the time. With batteries, the voltage will decrease a
little bit all by itself as the batteries get used up, and it might fool you into
thinking that the heat or cold was causing the light to change.
Try the links in the MadSci Library for more information on Engineering.