Consists essentially of a sealed glass tube provided with two electrodes and containing a gas. When an electrical potential difference is applied, a current passes and with a suitable gas, light will be emitted. In the case of mercury vapor, this light is of a bluish color and has proven effective in distinguishing dirt from coal. A special starting electrode close to one of the main electrodes initiates the discharge, and a choke coil in series with the lamp serves to limit the current passing, since the resistance tends to fall with increasing current.