Sig. Quirino Majorana on the Contact Theory. 249 



is momentarily put to earth or connected with an insulated 

 conductor of large capacity, the electrometer returns to zero, 

 and on separating the disks a large positive deflexion of the 

 quartz fibre is obtained amounting to 22 scale-divisions. The 

 two charges thus obtained — negative in the first, and positive 

 in the second case — are equal, because if the quartz fibre is 

 not brought back to zero by removing the small deflexion of 

 — 2*5 divisions no positive deflexion is obtained. The fact 

 that the charge on approach gives a smaller deflexion than 

 that on separation, although the charges are equal, is explained 

 by considering the different values of the capacity of the 

 system in the two cases. The charge on approach of the disks 

 is produced while the capacity is increasing, and the zinc is 

 brought near the copper, which is at a different potential. 

 The charge on separation is obtained in the inverse manner, 

 while the capacity is decreasing. The two charges, therefore, 

 although equal, are indicated on the electrometer by very 

 different deflexions, because, being distributed over capacities 

 of different magnitude, they are at different potentials. 



It is evident that the deflexions observed will be reversed 

 in sign but of the same amount it we repeat the experiment, 

 putting the zinc to earth instead of the gilt disk, and con- 

 necting the latter to the electrometer. 



In repeating the experiment of Volta we do not usually 

 observe the approach-charge, either because it produces only 

 a small deflexion of the electrometer-needle or because the 

 communication of one of the metals with the electrometer is 

 established after having put the metals in contact with each 

 other. The experiment of Exner, in which the disks of zinc 

 and copper are alternately discharged after being brought near 

 each other, is explained in the manner already indicated, and 

 the proof that it is contrary to the contact-theory is not valid, as 

 Julius, Pellat, Ayrton and Perry, and others have previously 

 shown. 



Finally, it may be mentioned that in carrying out the 

 experiment described I have ensured that no disturbing causes 

 existed which might mask the phenomena. If under the same 

 conditions as in the experiment two disks of the same metal are 

 used, no measurable deflexion is obtained, because, although 

 each is then charged to a definite potential, the two potentials 

 are equal and the variation of capacity of the system is small 

 (not more than 1 to 2). On repeating the experiment with 

 two disks of the same metal, raising one of them to a higher 

 potential than the other (by tapping-off at two points in a 

 battery-circuit), we obtain the same deflexion of the electro- 

 meter connected to one of the disks if the potential-difference 



