248 Sig. Quiriuo Majorana on the Contact Theory. 



discharged to earth, they acquire charges to which the state- 

 ments in (b) are applicable. 



The separation-charges are those obtained in the ordinary 

 Volta experiment, and they are exactty equal and of opposite 

 sign to the approach-charges previously studied, when the dis- 

 placements of the disks in the two experiments are equal but 

 in opposite directions. 



The proof of the above statements may be obtained from 

 some of Exner's experiments ; although this physicist con- 

 cluded from the phenomena observed by him that the contact- 

 theory is unsatisfactory, and quoted them in support of his 

 theory of surface-films of electrified oxides. 



I wish to proceed to the proof of the statements by means 

 of the following experiments. In them I have used a modified 

 Hankel electrometer *. The gold-leaf is replaced by a very fine 

 silvered quartz fibre. This has various advantages: — (a) the 

 electric capacity of the instrument is absolutely negligible ; 

 (b) the position of the quartz fibre is much more easily located 

 in the microscope than the gold-leaf would be ; (c) the zero- 

 point is much more constant, and the sensitiveness is also 

 greater. 



Two parallel insulated disks of about 15 cms. diameter, one 

 of gilt brass and the other of zinc, carefully flattened and 

 polished in the lathe, are placed at a distance of a few centi- 

 metres apart. By means of a screw-motion they can be 

 brought within 0*5 mm. of each other without making any 

 contact between them. Let the disk of gilt brass be put to 

 earth, and the disk of zinc to earth and to the silvered quartz 

 fibre of the electrometer, which is charged from 50 Daniell 

 cells. On breaking the communication between the zinc and 

 earth no deflexion is observed if all disturbing causes are 

 removed. Then, on causing the zinc plate to approach the 

 gilt disk, by means of the screw, a small deflexion occurs, 

 which continues to increase during the motion of the disks, 

 especially when the}' are very near together. When they 

 are 0*5 mm. apart the quartz fibre is displaced through 

 — 2*5 microscope scale-divisions, the sensitiveness of the in- 

 strument being about 3*5 divisions per volt. As long as the 

 disks remain in this position, the quartz fibre also remains 

 permanently deflected from the zero ; but if they are ;igain 

 separated, it returns exactly to zero. This is the case when 

 the distance between the disks is only 2 or 3 centimetres. 

 If, after the disks have been placed near each other, the zinc 



* M. E. Mallby, Meth. zur Bestimm. grosser elektrolytischer Wider- 

 stande ; Zeitschr. fiir Physik. Chemie, xviii. p. 133. 



