DAVISSON AND GERMER’S EXPERIMENTS

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In the year 1927 Davisson and Germer working in the Bell telephone laboratories of USA ,accidentally stumbled upon an experiment in which they observed diffraction of electrons in a crystal of nickel the way X-ray diffraction occurs in a crystal. When they calculated the wavelength that should be attributed to the incident beam on the basis of Bragg’s law, the value matched exactly with the value of the wavelength evaluated by using de-broglie equation. In evaluating the wavelength from de broglie equation, the momentum value of the electron found from the actual experimental set up were made use of. Thus de broglie hypothesis was proved conclusively by the Davisson and Germer experiment.


C is an ionization chamber to which a sensitive galvanometer is connected . The electrons produce a small current upon entry into the ionization chamber which is detected by the galvanometer. The ionization chamber can be moved along a circular path S provided on a graduated circular scale whose Center coincides with the point O where the electrons are striking the targets . There is a provision for rotation of the crystal about an axis through O and perpendicular to the plane of the circular path S. The ionization chamber is made of two metallic walls which are insulated from each other. The electrons which enter into the ionization chamber through its inlet are decelerated due to retarding potential applied between the two walls . This is to ensure that only the fastest electron i.e those with velocities what they had just before hitting the target are able to enter the inner cylinder


When the expt. was repeated with accelerating potential of 44v,48v, 54v, 60v, and the polar graphs were plotted, curves as shown in fig. 2a,b,c,d,e,f are obtained. the surprising observation was that , a slight but distinct maxima was observed in the curve for observation at 44v. With increasing voltage the maxima appeared more up to 54v and thereafter it declined and faded away . the exact value of voltage and the scattering angle at which the spur becomes more pronounced i.e. the ionization current becomes maximum was v=54v and Φ=50°.