The extensive use of the mercury vapor lamp depends entirely upon the versatility of the mercury vapor as regards pressure, temperature, voltage and other characteristics, each change resulting in a lamp of different spectral quality and efficiency.
The mercury vapor lamp is similar in construction to the sodium vapor lamp. It consists of a discharge envelope enclosed in an outer bulb of ordinary mass. The discharge envelops may be of hard glass of quartz. The space between the bulb is partially or completely evacuated to prevent nest loss by convection from the inner bulb. The outer bulb absorbs harmful ultraviolet rays.
The inner bulb contains argon and a contain quantity of mercury. In addition to two main electrodes a starting electrode, connected through a high resistance.