Summary The development, since 1935, of a photoelectric amplifier backed by an a.c. galvanometer is described. The accuracy attained is equal to that of an ideal d.c. amplifier. The a.c. amplifier, however, is not sensitive to external disturbances and can be built with ordinary radio accessories. Fig. 1 gives its principal features. An a.c. galvanometer, consisting of a coil suspended in an alternating magnetic field is fed by the same a.c. source as the synchronous motor with the disk, interrupting the light that falls on the photocell F. This device is sensitive to a.c. of 50 cycles/sec only and the right phase, in contrast to the rectifying circuits used elsewhere. It has a bandwith of 1 Hz with a time of indication τ0 = 1 sec. The error of the reading is exclusively determined by the essential fluctuations (comp. fig. 6c) caused by the Brownian motion, the shot effect of the gridcurrent ig and of the plate current ia of the first tube, of the thermal current ith and of the photo-electric current if. Their magnitudes are given by (1), (2) and (3), when T is the temperature of the first resistance R, Z is the input impedance and S is the mutual conductance of the first tube. These equations are deduced in § 5. The inaccuracy of the photo-electric current if is īf = 3,5 × 10−14/√τ0 A. Applying an RCA 57 tube, operated at low voltages and with a floating grid, we could reduce the noise so that if = 2,5 . 10−15/√τ0 A. The influence of the thermal current, prevailing by far for Cs-cells, was eliminated by cooling the cell with liquid air (fig. 5). This cooling reduced the fluctuations of the reading by a factor 200 (comp. fig. 6a, b, c; sensitivity in the first case 1/10). The corresponding mean error in the intensity of the light amounted to 0,85/√τ0 μ erg/sec (gas-amplification factor ƒ = 15 both for K- and for Cs-cells).