232 LECTURES TO SCIENCE TEACHERS. 



to prove its truth. Taking away the two half-rings of 

 brass he substituted this ring which I now show you, one 

 half of which is of copper and the other half zinc, the two 

 soldered into one ring. The needle and Ley den jar of the 

 electrometer being completely discharged, the needle was 

 mechanically adjusted so as to hang with its extremity over 

 one of the soldered joints between zinc and copper. The 

 needle and jar were then electrified, first positively and then 

 negatively, and it was found that, when positive the needle 

 moved from the zinc half-ring towards the copper half-ring, 

 and when electrified negatively it moved from the copper 

 half-ring towards the zinc half-ring. Thus the zinc arid 

 copper half-rings in contact behaved as would two insulated 

 half-rings of the same metal, one of which, corresponding 

 to the zinc, is electrified positively relatively to the other. 



