Fm. 4. 



MepcuftY 



Sig. Quirino Majorana on the Contact Theory. 26 L 



disks, of which one was fixed and vertical, and the other 



parallel to the first and carried by a horizontal arm sus^ 



pended from a long fibre of metal or 



quartz. The difficulty encountered 



in such experiments arises from the 



resistance which opposes the approach 



of one disk towards the other, due 



to the expulsion of the layer of air 



between the disks. I was therefore 



led to conduct the experiments in a 



vacuum. 



Inside a glass box is suspended a ' 

 small arm carrying at its extremities 

 two vertical disks of gilt brass 

 2 centim. in diameter; one of these is merely for tbe purpose of 

 counterpoising the other one, which is used in the experiment. 

 A disk of zinc is carried by a screw which passes through an 

 ebonite plug cemented in a hole drilled through one of the 

 sides of the glass box, and a mercury joint (as shown in fig. 4) 

 ensures absence of leakage even with high vacua. By 

 turning the screw outside the box the disks of gold and zinc 

 can be brought into contact. 



The arm carrying the gilt disk is suspended by a long 

 vertical metal fibre enclosed in a vertical tube which is also 

 air-tight. A mirror attached to the movable system enables 

 its motion to be observed. From outside the box metallic 

 communication may be established either with the gold or the 

 zinc disk. 



With this apparatus, when the vacuum is very high 

 (500 mm.), the same experiments as were described in the 

 case of the quartz fibre can be repeated, but a much longer 

 time is necessary. The sudden attraction between the zinc 

 and gold disks is observed when their distance apart is only a 

 small fraction of a millimetre. This attraction may be 

 annulled or increased by connecting the two metals with the 

 poles of a suitable battery. 



Even at ordinary pressures, however, a torsion-balance 

 may be used. It is then necessary to use a vertical rigid 

 metal disk and a stiff wire of the other metal, the latter being- 

 suspended horizontally. When the end of the wire is at a 

 sufficiently small distance from the disk it is suddenly 

 attracted, and the troublesome resistance offered by the ex- 

 pelled air in the case of two disks does not occur. 



I have described the latter arrangements to show how the 

 experiments may be repeated in various forms ; but it should 

 be stated that the most satisfactory method is by means of the 



