In the last section, we saw that mercury atoms in a fluorescent lamp's glass tube are excited by electrons flowing in an electrical current. This electrical current is something like the current in an ordinary wire, but it passes through gas instead of through a solid. Gas conductors differ from solid conductors in a number of ways.
In a solid conductor, electrical charge is carried by free electrons jumping from atom to atom, from a negatively-charged area to a positively-charged area. As we've seen, electrons always have a negative charge, which means they are always drawn toward positive charges. In a gas, electrical charge is carried by free electrons moving independently of atoms. Current is also carried by ions, atoms that have an electrical charge because they have lost or gained an electron. Like electrons, ions are drawn to oppositely charged areas.
To send a current through gas in a tube, then, a fluorescent light needs to have two things:
- Free electrons and ions
- A difference in charge between the two ends of the tube (a voltage)
Generally, there are few ions and free electrons in a gas, because all of the atoms naturally maintain a neutral charge. Consequently, it is difficult to conduct an electrical current through most gases. When you turn on a fluorescent lamp, the first thing it needs to do is introduce many new free electrons from both electrodes.
There are several different ways of doing this, as we'll see in the next couple of sections.