For the construction of a

sunlamp, a mercury vapour arc

discharge was the most

effective source of ultraviolet

radiation. In a basic

configuration, the sunlamp

consisted of two electrodes

that were located at the outer

ends of a sealed glass tube,

filled with mercury vapour.

When a DC-voltage was

applied over the electrodes,

electrons would be forced to

travel from one electrode to

the other, causing collisions

between the gas atoms and

the moving electrons. Due to

these collisions, photons

would be released with some

dedicated frequencies in the

ultraviolet part of the

spectrum.

In the diagram above, a simplified representation of the voltage-current characteristic

of a gas discharge tube is given. As can be read from the diagram the relation

between voltage and current is not linear and the impedance may even become

negative. For a better understanding the presentation of the various discharge

transitions is drawn disproportionally. Also the transitions at the breakdown points D

and G of the characteristic are idealised and drawn sharper than will be the case in

reality. The diagonal straight lines represent the voltage-current characteristic of the

serial resistor at different values of the supply voltage U. At I=0 the voltage over the

resistor UR will be zero. At UL=0 UR will be equal to the supply voltage and,

according to Ohm's law, the maximum current U/R will flow through the resistor. In any

stable situation, for instance in F, UR is equal to U minus UL and that is on the

crossing point of both voltage-current characteristics.

The physical processes in a gas discharge between two electrodes are very much

depending on the actual current the discharge is stabilised on. Basically there are

three discharge states with a relatively sharp transition from one state to another.

The discharge type between B and C is called a 'dark discharge', that between E and

F is called a 'glow discharge' and a discharge beyond G is called an 'arc discharge'.