Powering a light bulb with a battery

Powering a light bulb with a battery
Just connect the positive terminal of the battery with one electrical contact of your light bulb and the negative terminal with the other electrical contact of the bulb. Many bulbs have one electrical contact with screw threads on it, with the other contact as a round dot on the end of the base. Other bulbs will have metal prongs sticking out. It’s notoriously hard to get good electrical contact on batteries and bulbs by soldering wires on. The spring contacts in flashlights work much better (but even they are troublesome from time to time).




It’s important to select a bulb which matches what your battery can put out. If the battery has too low a voltage, the current flowing through the bulb will be small and the bulb’s filament won’t get hot enough to visibly glow. If the battery has too high a voltage, so much current will flow that the filament will get too hot and vaporize.



Standard bulbs are designed to work with a voltage of around 120 V, which is an unusual range for batteries. Ordinary flashlight bulbs are designed to work with about 3V, easy to obtain with two batteries in series. Bulbs from cars are usually designed to work with about 12V, the output of a car battery or of eight standard battery cells in series.



You might think that using a lower voltage would only slightly dim the light, but actually the effect is much more severe. First, the heating power in the bulb goes as the square of the voltage, at least until the voltage gets big enough for the bulb to heat up and increase its resistance. Second, the amount of visible light produced in the bulb is virtually zero until the filament temperature gets close to the standard operating temperature. Thus using one fourth of the power will give much less than one fourth of the light output. If you use a bit too low a voltage, the bulb will glow orangeish, because it can still put out some colors of light but not the blue part of the spectrum.



You yourself generate about 60 W of heating power, the same as a 60 W bulb, but I’ll bet you don’t visibly glow very much. That’s because your temperature is too low to give off visible light. The light you do give off is infrared, which can be detected but not directly by our eyes.