MadSci Network: Engineering
Query:

Re: effect of cold on incandescnet light bulbs

Date: Mon Jan 18 03:10:13 1999
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Engineering
ID: 914460885.Eg
Message:

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.


Current Queue | Current Queue for Engineering | Engineering archives

Try the links in the MadSci Library for more information on Engineering.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-1998. All rights reserved.