A small Nier type mass spectrometer for the use in high vacuum is presented. This mass spectrometer has the ion orbit radius of 15 cm, the deflection angle of 60° and the resolving power of about 120. The analyzer tube consists of entirely glass made envelope, the inner surface of which is silver-plated. In the electrical power sources new designs are applied, and some of them are as follows : first, in the electromagnet current regulator, (1) the magnet current is regulated by a circuit where transistors are used incombination with ordinary electron tubes, (2) the magnet current is swept in such a manner that mass numbers have the good linearity with time, and (3) a mass number can be returned back by a small quantity at any instant. Second, in the ionizing current regulator, the ionizing voltage is kept constant by using a transistor
Detailed descriptions of the theory on the linear approximation of the sweeping mass numbers are made. The magnet current is swept by a capacitor-resistor discharging circuit. Conditions for the most suitable approximation are given in a table.
