Similar to the thermoionic effect used in the valves, when the light strikes the surface of some materials it can emit electrons also kown as the photo- emission.
In 1839, Alexander Edmond Becquerel of France observed firstly the photelectric effect, when he could generate a voltage by illuminating the junction of an electrolyte.
During the twenties due to the continuous evolution of the valve technology it was employed in the manufacturing of the first devices using the photo-electric effect.
Basically in those early devices, the photo emissive layers were applied to a metal plate, the cathode. It was assembled inside a glass tube provided also with a second electrode or anode. In the first devices the emissive material was the Potassium Hidrate. When the light falls upon the coated plate, the electrons are emitted from its surface.
The cathode and the anode being connected respectively to the negative and positve side of a DC power source, the later will collect the electrons emitted by the cathode. In this way, the number of electrons is directly proportional to the amount of light falling on the surface of the coated metal plate.
Due to this assembling arrangement those first devices were called as Phototubes. They were orignally used in the sound heads of the early types of sound reproduction system from film recordings.
However, such kind of sound reproduction systems using phototubes, later known as photo cells, like sound track sensors devices, soon were replaced by a more reliable technology - themagnetic recording.
a) In the projector a light source is applied over the sound track. The modulations are collected by the photo cell whose signals are amplified and finally are reproduced by the loudspeaker.

How the talkies were made:

a)the microphone in the camera collects the sound waves converting the same in electrical signals.

Several types of photo-cells and photo-tubes made in the USA:
a) type 9A, with two anodes and sensitivity of 10 micro A/Lumen
b) type 919, usually used in photometers
c) type 918, gas photo-tube common used in projectors with a sensitivity of 10 micro A/Lumen
d) type CE-A26, gas foto-tube common used in projectors