252 Sig. Quiriuo Majorana on the Contact Theory. 



of area 86 sq. cm.; the distance between the fixed and 

 movable armatures was about 0"1 cm. When the apparatus 

 is rotated at 20 revolutions per second, the contact difference 

 of potential between zinc and copper being 0*8 volt, we have 

 for the current 



T 86x20x0-8 _,_ 1A 9 



I = a ?Ti — n — TnTT = 2 ' 42 x 10 amperes, 



27rx0'lx9x 10 u L 



which is thus extremely small. 



A very sensitive astatic galvanometer of high resistance, 

 however, indicates a deflexion of 4 or 5 millimetres on a 

 scale 2 metres away. This deflexion changes sign on re- 

 versing the direction of rotation of the drum ; and on deter- 

 mining the sensitiveness of the instrument it is found that 

 the magnitude of the deflexion agrees with the calculated 

 value of the current. Further, the deflexion is naturally 

 proportional to the speed of rotation and disappears entirely 

 when the fixed armatures are removed. 



The apparatus described is somewhat analogous to the 

 duplicator of Belli or the replenisher of Lord Kelvin. It 

 differs from these, however, by the fact that it is designed to 

 furnish an electric current instead of statical charges ; a study 

 of the intensity of these currents gives a means of deter- 

 mining the electromotive force of contact of the metals 

 experimented upon. It further differs from the replenisher 

 in the construction of the rotating armature ; in the actual 

 instrument this is constructed with two different metals, and 

 the intensity of the current which would be obtained if it 

 were made of only one metal would be about half as great. 

 On covering the armature with tinfoil the galvanometer did, 

 in fact, deflect through 2 or 3 millimetres. 



It would be difficult, if not quite impossible, to observe 

 the current which flows through the strip AB in the figure, 

 between the two fixed armatures. For this purpose an 

 extraordinarily sensitive electrodynamometer would be re- 

 quired. 



In the determination of the intensity of the current obtain- 

 able from the apparatus described, no account has been 

 taken of the resistance of the galvanometer, except to state 

 that it is desirable to have the resistance as high as possible. 

 This secures that the number of ampere-turns shall be great, 

 and there is really no reason to fear that the resistance, even 

 if it amounts to a thousand ohms, will diminish the intensity 

 of the current obtained. 



In order to facilitate the observation of electric currents 



