Makch 24, 1916] 



SCIENCE 



411 



negative to copper. This being the ease, and 

 both being conductors, they should when 

 brought near together each induce a free 

 charge upon the other. 



This phenomenon was actually observed by 

 Exner, who describes an experiment for show- 

 ing it in Repertorium der Physile, XVII., 444 

 (1881). In this experiment a zinc plate was 

 placed in a horizontal position, and after being 

 discharged to earth was insulated. A similar 

 copper plate could be lowered parallel to the 

 zinc plate and very near it. This copper plate 

 was earthed, then insulated and brought very 

 near to the zinc plate and connected to an 

 electrometer. This caused an electrometer de- 

 flection of + 9 scale divisions, due to the free 

 charge induced upon the copper plate by the 

 zinc. The copper plate and electrometer while 

 still connected were earthed, and the elec- 

 trometer deflection returned to zero. They 

 were then again insulated, and while still con- 

 nected, the copper plate was raised from the 

 zinc plate. The electrometer then showed a 

 deflection of — 9 scale divisions, due to the 

 bound charge which had been induced upon 

 the copper plate. After the copper plate was 

 removed the zinc plate was tested and showed 

 no free charge, it having been insulated 

 throughout the experiment. 



This seems to show conclusively that a 

 zinc plate which has been discharged to earth 

 and insulated is capable of inducing a free 

 positive charge upon an insulated copper 

 plate which is brought near it. 



Exner also showed that when a platinum 

 plate and a zinc plate, after having been dis- 

 charged to earth and then insulated, are 

 brought very near together each induces a free 

 charge upon the other which may be shared 

 with an electrometer. If the electr6meter be 

 connected first with the platinum plate it will 

 show a positive charge. If the electrometer 

 and plate be discharged to earth and again 

 insulated and the electrometer connected to 

 the zinc plate, it will show a negative charge. 

 After this has been discharged to earth and 

 the plate and electrometer again insulated the 

 platinum will show another positive charge. 

 In this way Exner was able to take twenty 



successive charges, alternately positive and 

 negative, from his plates before their induced 

 free charges were entirely discharged. This 

 corresponds exactly to discharging the con- 

 ductors of an insulated Leyden jar alternately. 



The free charges induced by the approach 

 of different metals to each other are discussed 

 by Majorana in Phil. Mag., XLVIII., p. 241, 

 where they are called approach charges. 

 Majorana also showed the attraction of one 

 metal upon another at very small distances. 



It is difiicult to see how these induced 

 charges can be accounted for by any chemical 

 explanation. ISTeither can they be accounted 

 for on the assumption of a double electric 

 layer of any kind on the surface of the metal, 

 since the distance between the positive and 

 negative surfaces in such a layer would neces- 

 sarily be so small that their differential effect 

 would vanish at very small distances, and the 

 induced charges may easily be observed when 

 two plates of different metal are more than a 

 centimeter apart. They may even be shown 

 at much greater distances by using a hollow 

 conductor of one metal and introducing the 

 other metal into it. In this way an induced 

 charge may be taken from the outer surface 

 of the hollow conductor without bringing the 

 two metal surfaces near together. In this 

 case all talk of a double electrical layer is ex- 

 cluded, as is also any chemical action taking 

 place within the hollow conductor after the 

 inner metal is introduced. 



This induced charge upon the outer hollow 

 conductor may be shown even while the inner 

 metal is in contact with the earth or with the 

 inside of an earthed hollow conductor. A 

 simple method of doing this is as follows : 



A Dolazalek quadrant electrometer, E, in 

 the diagram, is enclosed in a cage of fine wire 

 mesh which is earthed through a wire soldered 

 to the water system of the laboratory. The 

 outer case of the electrometer and one pair of 

 quadrants are connected to this cage. The 

 other pair of quadrants is connected to a hol- 

 low metal cylinder, which may conveniently 

 be about 15 centimeters long and 2 cm. in in- 

 ternal diameter. This cylinder, C, in the dia- 

 gram, is supported horizontally upon hard 

 rubber blocks inside the cage. 



