260 Sig. Quirino Majorana on the Contact Theory. 



With the apparatus used it may be assumed that it is 

 necessary to bring the zinc plate within 0*1 mm. distance 

 from the fibre before attraction occurs. We may take the 

 same value for this distance in the hypothetical case of 

 the two areas of silver and zinc ; so that # = 001 cm. and 

 « = !#=: 0*015 cm. Now substitute in the expression for A 

 the values of the quantities which occur in it ; these are, in 

 the apparatus used, P = - 032 dyne; Z=10 cm.; V = 0'9 volt 

 = 0*003 electrostatic unit. We thus obtain 



. 16. 7T. (0-015) 3 . 0-032 AOO 

 A= 27(0-003)MO - = °' 22 Sq - mm - 



The ideal case of two areas of silver and zinc might be 

 realized by leaving the conditions of sensitiveness of the quartz 

 fibre unaltered, and using a small disk of silver fixed at its 

 lower extremity, the disk having an area of about - 2 sq. mm. 

 Opposite this disk it would be necessary to place another one 

 of zinc of the same diameter, with a screw-motion attached. 

 The small magnitude of the areas of these disks shows the 

 probability of the hypothesis that the areas which mutually 

 attract in the experiment described are of the same order of 

 magnitude. 



Since experiment has shown that unlike metals attract each 

 other, the idea may present itself that like metals, which are 

 each charged to the same potential, will repel each other. 



Now if we replace the zinc plate in the preceding expe- 

 riment by one of silver, not only is no attraction observed, 

 but neither is there any sensible repulsion. The objection 

 might be raised that the walls of the box in which the expe- 

 riment is conducted are at the same potential as the fibre and 

 the plate. If, however, the walls are covered with tinfoil 

 connected to earth, there is still no observable repulsion. 

 This is explained when it is remembered that the portions of 

 the surfaces of the fibre and plate which are near each other 

 have a capacity only differing slightly from that which they 

 would have if separated. Whereas in the case of attraction 

 the quantities of electricity on the two nearest elements of the 

 two unlike metals are enormous compared with those which 

 they would possess if separated, in the case of two like metals 

 there is no increase in surface-density, but a tendency for 

 the densities on the two surfaces to annul each other ; and 

 hence there is no mutual action between them. 



I have also succeeded in exhibiting the attraction of unlike 

 metals by means of the torsion-balance. The first attempts 

 were unsuccessful, having been made in air with metallic 



