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Still, you have too many variables. How much current will you put through it. Are you going to turn it on and off, or just burn it? Will it turn on slowly or instantaneously? But anyway, what you need to do is pick a bulb and find manufacturer's data for it. This link is just one I found quickly.
What I am triing to do is test different 9V batteries. I want just to connect a 9V battery to the lightbuld and leave it in a dark box. On one end of the box will be a photoresistor. Every couple of minutes I will measure the resistance of the photoresistor. What I want to see is which battery will make the lightbult the bright for the longest.
I worded my question wrong by mistake, sorry. I need to know how long a 9V BATTERY (any type) will last when it is attached only to a lightbulb.
As an example, look at this CML catalog page -- check out the data for the PR20 (8.63V) lamp -- a typical lamp used in a 6-cell flashlight.
The catalog shows this lamp to have a design current draw of 0.5A (500mA). A 655mAh 9V battery like the Eveready X22 will provide power to this lamp for an absolute maximum of 1.31 hours, which would completely deplete the battery. In reality, the battery would cease to light the lamp long before that time period has elapsed, as the battery output would fall off at some point to a level insufficient to cause the fillament to incandesce.
It would be a better experiment to use a lamp (or series string of lamps) which will operate at some reduced voltage and at a lower current. For example, look at the CM2156 on this page. Placing three of these lamps in series would still make for a simple enough circuit. Each of the three lamps in such a series string would drop 3V, and the overall circuit current would be a more useful 30mA. Now it will much simpler to match your experimental findings to the battery maker's datasheets (see links in thread cited in my previous post); the current draw will be in line with the battery maker's test standard.
I have read in a book that to calculate how long a battery will last is by dividing the mAh of a battery by the current usage of the cuircuit (mAh/mA). That should give me the amount of time the battery will last in hours. That means the circui you suggested would take about 20-21 hours to discharge the battery (9V battery, right?) on a enengizer.
Sorry to complicate things again, but the mA-Hr rating of a battery is based on a specific discharge rate (constant current) until the battery's voltage falls to a specific terminal voltage. If you are just testing the batteries, I would abandon the light bulb idea and use either just a resistor or if you want to be fancy (First Prize!) use a constant current load (easy enough to do). Then measure either when the resistor's voltage gets to a certain level (like 6V or whatever) or when the battery can no longer source the specified current (like 10mA) into the constant current load.
It sounds like the data gathered in the experiment as described will actually be quite good if those who might use the data are interested in how long a flashlight will work. If I were a consumer I'd appreciate data in a form that means something to me - a test that represents what I might do. On the other hand, data resulting from constant current draw or other refinements might be more useful.
I do not know if the light output of the bulb will change significantly as it ages. That might be something to watch for, to the extent that you can. If I had six different batteries (label them A,B,C,D,E&F) to test I'd purchase three of each then test one each of A thru F, then another of A thru F then the final group of A thru F. Keeping that order might help to reveal aging of the bulb.
Worth noting is that the resistance of the bulb will change with the voltage applied. Note also that the light spectrum changes with voltage yet your photocell may have different sensitivities at different wavelengths. These things are good to know but may not affect your experiment to a great degree.