250 Sig. Quirino Majorana on the Contact Theory. 



between them is 0*8 to 09 volt. This value, therefore, 

 represents the electromotive force of contact of the zinc-gold 

 couple used. 



As an application of the principle of producing free charges 

 by the approach or separation of different metals, I propose to 

 describe two pieces of apparatus which, when set in rotation, 

 can furnish continuous electric currents. These currents are 

 very feeble, but they are due simply to the formation and 

 neutralization of the charges referred to. 



A drum T, of wood or ebonite, capable of rotation, is pro- 

 vided on its cylindrical surface with two insulated metallic 

 plates, one of zinc and the other of copper (fig. 1). Each of 



Kar. 1. 



these plates covers slightly less than 180° of the circum- 

 ference of the drum. The two bars of a commutator, mounted 

 on the axle of the drum, are connected each to one of the two 

 metals. Two brushes rest on these collectors as indicated in 

 the figure. The drum T is enclosed by two hemi-cylindrical 

 armatures concentric with itself and fixed on supports ; these 

 armatures are metallically connected by means of the strip 

 AB. 



Suppose the drum to be rotating in the direction of the 

 arrow, and consider what happens in the first half-revolution. 

 The zinc of the drum is brought near the fixed copper 

 armature, and may be said to become negatively charged ; on 

 the other hand, the copper is being brought near the fixed 

 zinc armature and becomes positively charged. Hence a wire 

 connecting the brushes S and S' will be traversed by a current 

 from S to S'. After the first half-turn the conditions are 

 reversed, since the zinc of the drum is being separated from 

 the fixed copper plate, and the copper on the drum is separated 

 from the fixed zinc plate. Since, however, the positions of 

 the collectors are changed, the brush S is always positive and 



