Mr. Laming on the primary Forces of Electricity* 487 



the centre of gravity of all the moveable parts, that it might 

 be placed no more below the points of support than was ab- 

 solutely necessary ; by which means the instrument was ren- 

 dered so sensible that a weight = ^^^th part of a grain moved 

 the index through 4^^°; the :j\jth part moved the index through 

 8|^°; the ^'^jths moved it through 12^°; and the ^^^th through 

 16° ; and as the scale might be read off' to quarters of a degree, 

 a force = ys^'g^th part of a grain was of course appreciable 

 by its means. When used this electrometer was suspended 

 with its circular plate over an uninsulated disc of gilded wood, 

 by a wire cemented at about its middle into the axis of a glass 

 tube, the latter sliding through an air-tight collar in the top 

 of an exhausted receiver about 6f inches in diameter, and 

 it was placed in connection with the inner coating of a Ley- 

 den jar exposing about two square feet of coating, and whose 

 electrical charge was estimated by means of Mr. Harris's 

 unit jar. The following results were thus obtained. 



Table A. 



Experiments showing that in highly rarefied air, the distance 

 being constant, the force of electrical attraction varies as 

 the square of the quantities of plus electricity. 

 Distance of attracting surfaces = '5 of an inch. 



Comparative Degrees moTcd Same by 



Quantities. through. Calculation. 



1 ... 1- ... 1 



2 ... 4-25 ... 4 



3 ... 9-5 ... 9 



Table B. 

 Further experiments confirming the same fact. 

 Distance of attracting surfaces = 1 inch. 



