Determining Planck’s constant - Separation of wavelengths with a straight-view prism on the optical bench

The experiment P6.1.4.2 uses an open arrangement on the optical bench. The wavelengths of the light are dispersed using a direct-vision prism. The light of precisely one spectral line at a time falls on the cathode. A capacitor is connected between the cathode and the anode of the photocell which is charged by the anode current, thus generating the opposing voltage U. As soon as the opposing voltage reaches the value –U₀, the anode current is zero and the charging of the capacitor is finished. U₀ is measured without applying a current by means of an electrometer amplifier.
Note: The opposing voltage U can alternatively be tapped from a DC voltage source. The I-measuring amplifier D is recommended for sensitive measurements of the anode current (see P6.1.4.4).