A small battery and fine steel wool can be used to start a
fire. Simply bridge the battery terminals with the steel wool and a spark
will instantly form in the steel wool.
How does it work?
The basic principle involved is that you need enough "electrical current"
flowing through the steel wool to get it hot enough to "light." If the steel
wool is too "thick" (strands are too large in diameter), then there isn't
enough "resistance" in the strands. In that case - you are just discharging
the battery - and not ending up getting fire.
Another factor is the total voltage that is applied to the strands. The
higher the voltage, the more current that will flow, and the hotter the
wires will get. (The relationship is linear - twice the voltage, twice the
current.) So, the more cells that you stack in "series" the hotter the wires
will get. If you stack two cells, then you have just doubled your chances of
success. If you put three cells in series, then you are three times closer
A nine volt battery is great - since it have a reasonable high voltage -
and the terminals are close together. You can ignite steel wool with a
2-cell flashlight. Just take the "end" off of the flashlight and brush the
steel wool against the contacts.
The best route is to get an assortment of steel wool, and an assortment
of batteries - and then to play for a while. Soon it should become pretty
obvious what the optimal combinations might be.