Millikans Experiment

Millikan's method was to set up a 'workshop in vacuo' by enclosing the photosensitive surface in a highly evacuated quartz tube. Light shining on the surface liberates electrons. By applying an electric field in the right direction the electrons can be continually swept away, forming an electric current. They are now 'out in the open' and their behaviour can easily be studied.

Figure 13.2 illustrates an apparatus such as was used by Millikan. The photoelectrons, which have been ejected from the metal, are now in a vacuum, and can be subjected to electric forces for analysis. Since they are negatively charged, the photosensitive plate, or emitter, is made the negative electrode. At the opposite end, the positive electrode, is the collector of the electrons.

The electric field accelerates the electrons away from the emitter creating a current through the tube and around the circuit.

Figure 13.2 Experimental set-up to draw the electron cloud across an evacuated tube.

Adverse field:

Accelerating field:

Emitter Collector

Emitter Collector

+ -

- +

We are now ready to study the properties of the electrons emitted when the photosensitive surface is illuminated by light of different frequencies.* For each frequency the resulting

* Even when light is behaving like a particle, we can describe the photon in terms of wave characteristics such as frequency and wavelength.

current current

Figure 13.3 Current for different frequencies of the incoming light plotted as a function of the voltage across the tube.

current is plotted as a function of the voltage across the tube and the results are presented in Figure 13.3.

As the accelerating voltage is increased the current increases untill it reaches the saturation current where all the electrons are swept across the tube as they are liberated. In a separate experiment, it can be shown that the magnitude of the saturation current depends on the intensity of the incoming light. (In this example the light of frequency f2 happens to be the most intense.)

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