Sig. Quirino Majorana on the Contact llieory. 251 



S' negative. On rotating the drum we therefore produce a 

 continuous current, always in the same direction. 



It would be difficult to calculate a priori the intensity of 

 this current, principally owing to the fact that it would not 

 be easy to determine the capacity of the various parts of the 

 apparatus. If, however, the moving and fixed armatures are 

 very near together, that is to say, if the diameter of the 

 drum differs but little from that of the fixed armatures, a 

 good approximation may be made. 



Let us consider the apparatus in the position represented 

 in the figure. The electrical charges on the system have 

 then a small value, since the differences of potential between 

 the metallic plates facing each other are zero ; but if the 

 drum be rotated through a half-turn, and the capacity of one 

 of the two condensers in the apparatus be called 0, then 

 C(Zn/Cu) will be the quantity of electricity which has passed 

 from the copper to the zinc of the drum, assuming S and S' 

 to be joined by a wire. We assume that the portion of the 

 capacity of each armature which is independent of the 

 presence of the other one is small enough to be negligible. 

 On rotating through another half-turn this quantity of 

 electricity will flow back again, and so on. 



Xow since the armatures of each of the condensers are 

 assumed to be very close together, if we denote their distance 

 apart by e, and the area of each by S, both being expressed 

 in centimetre units, we have 



S 

 C = -. — electrostatic units. 



47T£ 



Jn farads this becomes 



S 1 



lire ' 9 x 10 u " 



On multiplying this capacity by the difference of potential 

 (Zn/Cu) we obtain the quantity of electricity flowing between 

 the brushes during each half-revolution, and if the drum 

 makes n revolutions per second, the current (I) will be 



A current of exactly the same intensity, but rapidly alter- 

 nating, will traverse the strip AB between the fixed armatures. 

 The existence of this connecting strip is essential; without it 

 the flow of electricity between the moving armatures would 

 take place to a much smaller extent. 



In the actual instrument the surface of each armature was 



