1897.] 



on Contact Electricity of Metali, 



529 



minute. Eepeat the test with the cell reversed. If the test is 

 satisfactory with the upper plate high, the insulation of the insulated 

 quadrants in the electrometer and of the lower disc in the Volta- 

 condenser is proved good. If after that the test is not satisfactory 

 with the upper disc at its lowest, we infer that there are vitiating 

 shreds between the two plates, and we must do what we can to remove 

 them ; or, if necessary, we must alter the screw-stop at the top so as 

 to increase the shortest distance between the plates sufficiently to 

 prevent bridges of shred or dust between them, and so to give good 

 insulation. The smaller we make the shortest distance with perfect 



-100 



Fig. 7. 



enough insulation, the more sensitive is the apparatus for the 

 measurement of contact electricity performed as follows. 



§ 11. Eun the slider to zero ; make and keep made the contact at P 

 till the spot of light settles at the metallic zero ; break contact at P, 

 and lift the upper plate slightly. (If you lift it too far, the spot of 

 li^ht may fly out of range.) If the spot of light moves in the direction 

 showing positive electricity on the insulated quadrants (as it does if 

 the lower plate is zinc and the upper copper), connect the cell to 

 make the slider negative (as shown in Fig. 7). Eepeat the experi- 

 ment with the slider at different points on the scale, until you find that, 

 with contact P broken, lifting the upper plate causes no motion of the 

 spot of light. If the compensating action with the slider at the top 



