A new method and device will be described for measuring the density in a vapor stream of any material that will be deposited on a suitable substrate in vacuum. For this purpose a ``nude'' ionization gauge is used. The new ionization method differs from others in that it discriminates between the ions of the vapor and those resulting from the background pressure in the vacuum system. The ionization gauge is placed over a hole in the substrate holder; this hole is periodically opened and closed by a shutter vibrating at a low frequency, so that the ion gauge will receive a pulsed vapor stream and detect an alternating ion current superimposed on the direct ion current. A narrow‐band amplifier will amplify only the alternating ion current of the frequency of the shutter vibrator. A current integrator may control the total amount of evaporated material. The ionization gauge used in this method can easily measure the total pressure when the filtering system is bypassed. Calibration curves will be shown and a theory given. Sensitivities of the order of 1 A/sec and lower have been obtained.