PURPOSE: To raise the precision of analysis by controlling the volume of the alternating current sent to two electrodes in a lamp and by thereby equalizing the luminous strength of both electrodes.

CONSTITUTION: The cylindrical electrode 2 and the cuplike electrode 3 are provided on the central optical axis in the lamp 1, while on the outer periphery of the electrode 3 a magnetic field parallel to the direction of the optical axis is provided. The electrode 2 is connected to a power source 11, while the electrode 3 is also connected to the power source 11 through the intermediary of diodes 8 and 9. The positive current flowing from the power source 11 through the diode 8 makes the hollow part of the electrode 3 luminous. Next, when the negative current flows, the inside of the cylinder of the electrode 2 is made luminous, and the current flows to the power source 11 through a variable resistance 10 and the diode 9. Accordingly, the luminous volume of the electrode 2 can be controlled by the adjustment of the resistance 10.