Contact Electricity of Metals, 



91 



contacts should be between clean and dry polished metals, 

 because if the last connexion on breaking contact is through 

 semi-moist dust, or oxide, or "dirt" (defined by Lord 

 Palmerston to be matter in a wrong place), or if it is any- 

 thing other than metallic, vitiating disturbance is produced. 



§ 10. To make an experiment, first test the insulation with 

 the upper plate held up in its highest position, and after that 

 with it let down to its lowest position, in each case proceeding 

 thus : Holding by hand the wire connected to the slider, run 

 the slider to zero, make contact at P, observe on the screen 

 the position of the spot of light from the electrometer mirror 



Fiar. 7. 



100 



for the metallic zero, and then run the slider slowly to the 

 top of its scale and break contact ; the spot of light should 

 remain steady, or at all events should not lose more than a 

 very small percentage of its distance from metallic zero, in 

 half a minute. Repeat 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 

 disk in the Volta-condenser is proved good. If after that the 

 test is not satisfactory with the upper disk 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 



