Science . ector used for picking up the scattered elec-trons can not be brought nearer the primarybeam in our present apparatus. The principalfeatures of the angular distribution are twomaxima of emission, one back along the pathof the bombarding electrons (^ = 0) and an-other lateral to the primary beam whose posi-tion depends upon the bombarding relative importance of these two maximaalso depends upon the speed of the 1 shows such a distribution curve fora bombarding potential of 150 volts. Theintensity is measured as the ratio of thecurrent entering the Faraday bo
Science . ector used for picking up the scattered elec-trons can not be brought nearer the primarybeam in our present apparatus. The principalfeatures of the angular distribution are twomaxima of emission, one back along the pathof the bombarding electrons (^ = 0) and an-other lateral to the primary beam whose posi-tion depends upon the bombarding relative importance of these two maximaalso depends upon the speed of the 1 shows such a distribution curve fora bombarding potential of 150 volts. Theintensity is measured as the ratio of thecurrent entering the Faraday box collectorto the total current reaching the target. Theopening in the Faraday box subtends of unit solid angle to the spot under bom-bardment. The retarding potential betweenbox and target for the curve. Fig. 1, is • 135volts, so that only electrons that have lostnot more than 10 per cent, of their initialenergy are caught. The effect of bringing theretarding voltage nearer the bombarding volt-. 524 SCIENCE [N. S. Vol. LIV. No. 1404. age is to reduce the number of electronscaught and to increase the sharpness of thepattern. On decreasing the bombarding poten-tial without altering the ratio of retard-ing to bombarding potential the lateral maxi-mum moves away from the primary beam to-ward the plane of the target, and the ratioof the intensity of this maximum to that ofthe other becomes greater. In attempting to interpret these results wehave been led to consider the scattering ofelectrons by a positive nucleus of limitedfield, one for which the central force on anelectron is Ee/r for values of r less thanp, and zero for all values of r greater than a field would exist for a concentratedpositive charge E surrounded by a sphericalshell of uniformly distributed charge — E andof radius p. The field of a system compris-ing a central positive nucleus of n electroniccharges surrounded by n electrons uniformlydistributed over the surface of a sphere ofradius p will
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