Question

the funxtion of auxiliary electrod in a mercury vapour lamp is

Answer 1

An auxiliary starting electrode is placed next to one of the main electrodes to facilitate lamp ignition. When the lamp is first energised the full open circuit voltage is applied across the arc tube. The distance between the electrodes is so large that the resulting voltage gradient is not high enough to cause ionisation of the gas filling. However the same voltage is also applied between one electrode and the auxiliary via a small resistor. The gap between these electrodes is much smaller, and the voltage gradient is sufficiently high that ionisation will occur. A small discharge strikes, the series resistor of 10-30kW limiting the current flow to about one thousandth of the normal lamp current. Once free electrons, ions and photons have been produced in the arc tube it is then very easy to strike a discharge across the main electrodes. The discharge quickly makes this transition because it can then bypass the starting resistor, and there is no electrical resistance between the main electrodes. Some lamps, particularly high wattage types often include two auxiliary electrodes, one beside each main electrode and connected via a resistor to the opposite end of the arc tube. This is also standard practice in mercury lamps designed for use in colder atmospheres.