Hallwachs Experiment - photo electric effect

Photoelectric emission is the phenomena by which a good number of substances, chiefly metals, emit electrons under the influence of radiation such as γ rays, X-rays, ultraviolet and even visible light. This effect was discovered by Heinrich Hertz in 1887 while working with resonance electrical circuits. A year later, Hallwachs, Elster and Geitel investigated the phenomenon with a simple experimental arrangement.

Hallwachs Experiment

Hallwachs experimental set-up to study the photo electric effect is shown in Fig  It consists of an evacuated quartz bulb with two zinc plates cathode C and anode A. The plates are connected to a battery and a sensitive galvanometer. In the absence  of  any  radiation  incident  on  the plates, there is no flow of current and hence there is no deflection in the galvanometer.

But, when an electro magnetic radiation like ultraviolet radiation is allowed to fall on the plate C which is connected to the negative terminal of the battery, a current begins to flow, indicated by the deflection in the galvanometer (G). But, when ultraviolet radiation is made to fall on A, there is no deflection in the galvanometer. These observations reveal that the particles emitted by the plate C due to the photoelectric effect are negatively charged. These particles were found to be electrons. The observed current known as the photoelectric current is due to the flow of electrons.

After the study of photoelectric effect by Hallwachs, scientists J.J.Thomson, Lenard, Richardson, Compton did a series of experiments to study the relationship between photoelectric current, intensity of incident radiation, velocity and the kinetic energy of the photo electrons, and their dependence on the wave length of incident radiation used.

Effect of intensity of incident radiation on photo electric current

Keeping  the  frequency  of  the        Photo electric incident  radiation  and  the  potential          difference between the cathode and the anode at constant values, the intensity of incident radiation is varied. The corresponding  photoelectric current is measured in the microammeter.         

It is found that the photo electric      current  increases  linearly  with  the intensity      of incident radiation (Fig).

Since the photoelectric current is directly proportional to the number of photoelectrons emitted per second, it implies that the number of photoelectrons emitted per second is proportional to the intensity of incident radiation.