354 ' Observations on Electricity, 



During the motith of October last, I made a number of experi- 

 ments with a view to ascertain the utility of presenting points 

 only upon one side of the plate in the electrical machine. My 

 attention was called to this subject by a singular experiment 

 shown to me by Mr. Daniel Davis, which for some time appeared 

 rather enigmatical, A circular plate of glass was charged by 

 movable coatings, and on removing the coatings, it frequently 

 happened that both sides of the plate when presented to a charged 

 electroscope, exhibited signs of the same species of electricity. 

 After numerous repetitions with a very careful examination, it 

 appeared that only the central portion of the negative side was 

 charged negatively, while a considerable annular space exterior 

 to this, was charged positively ; the redundant positive electricity 

 having forced or spread itself over the edge of the plate. On 

 reversing the experiment and making the redundancy upon the 

 negative side, the negative electricity appeared to pervade both 

 surfaces as did the positive before. Some of our instrument 

 makers have been in the practice of placing the collecting points 

 of the prime conductor only upon one side of the electrical 

 plate, finding that they answered better in many cases, than a 

 row of points upon both sides, although no satisfactory reason 

 has been given for this difference. After witnessing the above 

 experiment, it occurred to me that the difference was owing to 

 the facility with which electricity distributes itself upon glass, 

 especially if it be not entirely clear and dry. Experiment fully 

 confirmed my anticipations, and I was surprised to find to what 

 extent the plate might be discharged by the application of a con- 

 ductor to any part of its charged surface. The prime conductor 

 having been removed, the plate was turned several times and the 

 silk flap thrown back leaving both sides of the plate exposed in 

 a highly charged state. The hand was then laid upon the plate 

 at some distance from the edge and quickly withdrawn. On ex- 

 amining the plate not only the parts under and contiguous to the 

 hand were discharged, but the whole of that portion directly op- 

 posite to the hand on the other side of the plate was found dis- 

 charged to the same degree, although the distance over the edge 

 of the plate was in some cases fifteen inches. It will be found 

 that single or only two points on each side of the plate and near 

 its circumference will succeed better than numerous points upon 

 one side. 



