Sig. Quirino Majorana on the Contact Theory. 257 



an accumulator cell of large capacity, which has long been 

 charged. If the wire Mis is of sufficiently high resistance 

 the accumulator will not sen- „. „ 



siblv discharge during the ex- "' ' 



periment, and the points M 

 and N will be kept at a 

 potential-difference of about 2 

 volts; this can be occasionally 

 tested by means of an electro- 

 meter. The point of contact C 

 is movable along MN, so that 

 the potential-difference between £ 

 L and Q may be varied at will. 

 Suppose the commutator R so 

 arranged that the current flows from N to M. The silver 

 on the quartz fibre then receives a positive charge ; the zinc 

 plate receives no charge if the point M be connected to earth. 

 By adjusting the position of the slider C, a point may be found 

 at which the potential of the silvered fibre is such as to give 

 no attraction between the zinc and the fibre. If the zinc is 

 bright and. clean this corresponds to a potential -difference of 

 about 09 volt betw r een the points M and C. On reversing 

 the direction of the current by means of the commutator R, it 

 is easily seen that the attraction becomes much more marked 

 than when no current is flowing. The quartz fibre commences 

 to move visibly towards the zinc even when the distance be- 

 tween them is 0"5 rnillim. 



It seems evident, then, that the attraction is due to the 

 difference of electrical states of the metals used. On equalizing 

 the potentials of the silver and zinc by means of a battery of 

 electromotive force equal to the difference of potential existing 

 ;it their contact, the attraction practically disappears. Accord- 

 ing to the theory of Volta this might be obtained by using an 

 electrolytic combination of the silvered fibre and the plate, 

 but such an arrangement would be exactly equivalent to 

 using the battery as described above. Since moving the 

 screw of the plate L causes vibrations in the apparatus, how- 

 ever solidly it be constructed, we may proceed as follows, in 

 order to exhibit the attraction more clearly : — Let the 

 accumulator A send a current from ]S to M. The slider C 

 having been adjusted so that the potential-difference between 

 M. and C is 0*9 volt, bring the plate within about O'l millim. 

 from the fibre. There is no attraction, but on stopping the 

 current the fibre is observed to fall suddenly on to the zinc. 

 Leaving the circuit broken and separating the fibre from the 

 zinc until they are about 0*5 millim. apart, there is scarcely 

 Phil. Mag. S. 5. Vol. 48. No. 292. Sept. 1899. U 



