248 LECTURES TO SCIENCE TEACHERS. 



electrode connected with the plate A is first connected with 

 the outside casing of the instrument, that is practically with 

 the earth, and the micrometer screw is turned till the index 

 hair is in the sighted position. The earth reading is then 

 taken. Next the electrode is connected with the conductor 

 to be tested. If the conductor is at a different potential 

 from the earth, the attraction of the plate A for the plate 

 G is altered. The micrometer screw is again turned till the 

 index takes the sighted position. From the difference of 

 the two readings the potential of the conductor tested 

 may be deduced. This instrument, however, requires a 

 preliminary determination of constants in a way similar 

 to that which I have already indicated for the quadrant 

 electrometer, in order to enable us to reduce the numbers 

 deduced from the scale-readings to absolute measure. 



The portable electrometer is admirably fitted for obser- 

 vation of atmospheric electricity. As I show you it now it 

 is ready for that purpose. To the electrode of the plate A 

 a long stiff wire is connected, which bears at its point a 

 slow-burning match of blotting-paper, soaked in nitrate of 

 lead, and then dried. When I light the match the heated 

 particles rushing off from it quickly bring the potential of 

 the plate A to be the same as that of the atmosphere just at 

 the point where they are rushing off. By turning the 

 micrometer screw I quickly adjust the position of the plate 

 A till the index is in the sighted position. 



