sulating stand N. Let G be a galvanometer, and Bl and B2 

 its binding posts. Connect the plate M to the binding post 

 Bl of the galvanometer by a wire W. Connect B2 of the 

 galvanometer to the ground Gr. Rub a gutta-percha rod, R, 

 with cat's fur and bring it near M. In this case the galva- 

 nometer deflects to the black. Keep the rod in position, until 

 the galvanometer comes to rest. Upon removing the rod the 

 galvanometer deflects to the red. If a rubbed glass rod is 

 handled in the same way the results are the reverse of those 

 produced by the gutta-percha rod. 



The gutta-percha rod is negatively charged. Consequently 

 it has an excess of electrons on its surface. As it approaches 

 M it repels a stream of negative charges through the gal- 

 vanometer to the ground, deflecting it to the black. As the 

 rod is removed the electrons flow in from the ground to the 

 plate M in order to neutralize the positive charge on the plate, 

 due to the repelling of the electrons from it by R. When the 

 glass rod is brought near, it attracts electrons from the ground, 

 and they flow through the galvanometer to the plate deflect- 

 ing the galvanometer to the red. When the glass rod is re- 

 moved the electrons flow from the plate through the gal- 

 vanometer to the ground, deflecting it to the black. 



Now if the cell C be placed on the galvanometer with its 

 zinc Zn, connected to Bl, and its copper pole Cu connected 

 through the key K to B2, the galvanometer will deflect to the 

 black! when the key K is closed. Bl and B2 should be con- 

 nected by a heavy conductor, when the battery is connected 

 in order to prevent injury to the galvanometer. If the battery 

 terminals are reversed the deflections of the galvanometer are 

 reversed. 



Thus the zinc pole connected to Bl gives the same result 

 as the negative charge repelled from the plate M by the gutta- 

 percha rod R. Hence a stream of negative charges must be 

 flowing from the zinc through the galvanometer to the copper 

 plate. If the copper plate is attached to Bl and the zinc plate 

 is attached to B2 the reverse takes place. A stream of nega- 

 tive charges flows from the zinc at B2 through the galvanom- 

 eter to the copper plate, deflecting the galvanometer to the 

 red in the same manner that the negative charges flowing in 

 from the ground on to the plate M causes the galvanometer 

 to deflect to the red. 



This experiment shows clearly that the current of elec- 

 tricity flows from the zinc to the copper outside of the cell in- 

 stead of from the copper to the zinc as has always been as- 

 sumed. 



This galvanometer can be used in this way to plot the field 

 existing around a static machine or to examine the nature of 

 a charge upon any rubbed substance. The deflections of the 

 galvanometer are* decided and pronounced, reaching ten to 



. 68 



