Indication of the balance position of the dial is effected in the following manner. Assume
that the dial is initially set off-balance so that the two light beams leaving slit S3 and slit S2 (or slit
S4) are of unequal intensity. The light beams then pass through a rotating sector wheel, driven by
a motor, which chops them and allows them to proceed alternately into a second
monochronometer and, finally, to fall on the photomultiplier located behind slit S5. (The purpose of
the second monochronometer is to eliminate scattered light, and to direct the slit images to the
photomultiplier tube such that the images fall on the same part of the cathode.) Since the two light
beams falling alternately on the photomultiplier are of unequal intensity, they give rise to a
pulsating electron current flowing out of the photomultiplier. This current is amplified by an
alternating current amplifier, rectified by a commutator, and causes a deflection on an indicating
direct current microammeter. The rectification causes a positive reading on the microammeter if
intensity is different in one sense (for example: IS2>IS3) and a negative reading if the intensity
difference is of the other sense. If now the dial is turned to the balance position, the two light
beams falling alternately on the photomultiplier become of equal intensity. They then give rise to a
steady, direct current which cannot be amplified by an alternating current amplifier. Since there is
no pulsating current to amplify and rectify, the microammeter reads zero. Thus, a null reading on
the microammeter is an indication of the balance position of the dial. The relative intensities of the
two wavelengths on which observations are made may then be obtained from the balance position
dial reading and calibration tables supplied with the instrument.