Two movable photo-multiplier tubes are set in the position of the plate holder of a medium quartz spectrograph. The photo-current of each is amplified by an AC amplifier, and, after rectification, is used to charge a condenser. A thyratron is connected parallel to the condenser for the internal standard line (Al 2660?). At a present value of the voltage of this condenser, the thyratron operates so as to switch off the reading instrument which is connected with the other condenser for the analysis line (Cu 2247?). The final reading on the meter is proved to be proportional to the ratio of intensities of the two spectral lines.With the help of a set of standard samples, calibration curve for analysis of copper in Al-alloy is obtained. It is a straight lino in the concentration interval 2.3-6.7%. Beproducibility of the whole instrument including the light source is measured by repeating ten times the calibration process, each time determining the content of Cu in a given sample. The mean relative error of those ten determinations is 4%. The results of spectroscopic determination and chemical analysis have a deviation of 1%.