Fig. 1. 



256 Sig. Quirino Majorana on the Contact Theory. 



fig. 1 ; by its side is a square plate of brightly polished zinc 



of 1 centim. side, which can be made to approach the quartz 



thread by means of a fine-motion 



screw. The whole system is 



hermetically sealed in a box with 



glass sides, from the outside of 



which communication can be 



established either with the zinc 



plate or the quartz fibre, and the 



former can be displaced by means 



of the screw. 



A microscope, the optic axis 

 of which is normal to the quartz 



fibre and slightly inclined to the plane of the zinc plate, 

 enables the extremity Q of the fibre and its image in the 

 zinc plate to be easily observed. The quartz fibre should 

 be slightly inclined to the zinc, so that when the two come 

 into contact the appearance in the microscope is similar to (b), 

 fig. 1. The device of observing both the fibre and its image 

 has proved very useful in detecting small movements of the 

 fibre; such movements are in fact doubled. The experiments 

 to be described may be performed with a projection- micro- 

 scope, thus making them visible to an audience ; in this case 

 the illumination must not be too intense, otherwise the quartz 

 fibre will be displaced from its position of rest by the con- 

 vection-currents set up. 



Let the fibre and the plate be connected together by a 

 metal wire, and with the earth. The latter operation is 

 necessary in order to avoid accidental disturbances. On 

 observing with the microscope and working the screw gently 

 to bring the zinc nearer to the fibre, a sudden movement of 

 the fibre towards the zinc is noticed when the extremity Q 

 and its image are about 0'2 millim. apart ; the quartz fibre 

 is then attracted towards the zinc. 



That this attraction is due to the metals being different is 

 easily proved thus : — A silver plate does not attract the silvered 

 fibre ; there is an attraction, although slight, on substituting 

 aluminium for zinc ; it is easily observable in the case of 

 copper, and very feeble in the case of gold. On gilding the 

 silvered fibre feeble attraction occurs with a zinc plate, but 

 no attraction with a gold plate. Plates of other metals behave 

 similarly with the gilt fibre and with the silvered fibre. 



In order, however, to study the phenomenon better, let us 

 consider the arrangement shown in fig. 2. The plate L 

 and the fibre Q are connected with the points M and C 

 of a german-silver wire M N, traversed by the current from 



