Dynamical similarity laws of the high pressure mercury vapor discharge have been set up. These laws are based on principles which determine characteristics of a high pressure discharge in thermal equilibrium. Characteristics of discharges can be determined by measurements on models if the model contains the same mass of mercury gas per cm and is supplied with the same wattage per cm of length of tube as the main construction. The temperatures of the walls of discharge tube should be the same. In formulating the above requirements self‐absorption has been neglected. Experimental tests have shown the validity of the similarity theorems.