Give the construction and working of photo emissive cell.

Explanation

Photo emissive cell :
Its working depends on the electron emission from a metal cathode due to irradiation of light or other radiations.
Construction :
It consists of an evacuated glass or quartz bulb in which two metallic electrodes - that is, a cathode and an anode are fixed.
The cathode C is semi-cylindrical in shape and is coated with a photo sensitive material. The anode A is a thin rod or wire kept along the axis of the semi-cylindrical cathode.
A pootential difference is applied between the anode and the cathode through a galvanometer G.
Working :
When cathode is illuminated, electrons are emitted from it. These electrons are attracted by anode and hence a current is produced which is measured by the galvanometer.
For a given cathode, the magnitude of the current depends on
(i) the intensity to incident radiation and
(ii) the potential difference between anode and cathode.