Down the Tubes The central element in a fluorescent lamp is a
sealed glass tube. The tube contains a small bit of mercury and
an inert gas, typically argon, kept under very low pressure. The
tube also contains a phosphor powder, coated along the inside of
the glass. The tube has two electrodes, one at each end, which
are wired to an electrical circuit. The electrical circuit,
which we'll examine later, is hooked up to an alternating
current (AC) supply. When you turn the lamp on, the current
flows through the electrical circuit to the electrodes. There is
a considerable voltage across the electrodes, so electrons will
migrate through the gas from one end of the tube to the other.
This energy changes some of the mercury in the tube from a
liquid to a gas. As electrons and charged atoms move through the
tube, some of them will collide with the gaseous mercury atoms.
These collisions excite the atoms, bumping electrons up to
higher energy levels. When the electrons return to their
original energy level, they rel