290 Mr. Faraday's Experimental Researches in Electricity, 



the electrometer, it was easy, by observing the line which this 

 dot made with the lines of the two graduations just referred 

 to, to ascertain accurately the position of the ball. The upper 

 end of the glass thread was attached, as in Coulomb's original 

 electrometer, to an index, which had its appropriate graduated 

 circle, upon which the degree of torsion was ultimately to be 

 read off. 



1184. After the levelling of the instrument and adjustment 

 of the glass thread, the blocks which determine the place of 

 the carrier- ball are to be regulated (1181.) so that, when the 

 carrier arrangement is placed against them, the centre of the 

 ball may be in the radius of the instrument corresponding to 

 0° on the lower graduation or that on the side of the electro- 

 meter, and at the same level and distance from the centre as 

 the repelled ball on the suspended torsion lever. Then the 

 torsion index is to be turned until the ball connected with it 

 (the repelled ball) is accurately at 30°, and finally the gra- 

 duated arch belonging to the torsion index is to be adjusted so 

 as to bring 0° upon it to the index. This state of the instru- 

 ment was adopted as that which gave the m(5st direct expres- 

 sion of the experimental results, and in the form having few- 

 est variable errors ; the angular distance of 30° being always 

 retained as the standard distance to which the balls were in 

 every case to be brought, and the whole of the torsion being 

 read off at once on the graduated circle above. Under these 

 circumstances the distance of the balls from each other was 

 not merely the same in degree, but their position in the in- 

 strument, and in relation to every part of it, was actually the 

 same every time that a measurement was made ; so that all 

 irregularities arising from slight difference of form and action 

 in the instrument and the bodies around were avoided. The 

 only difference which could occur in the position of anything 

 within, consisted in the deflexion of the torsion thread from a 

 vertical position, more or less, according to the force of re- 

 pulsion of the balls ; but this was so slight as to cause no in- 

 terfering difference in the symmetry of form within the in- 

 strument, and gave no error in the amount of torsion force 

 indicated on the graduation above. 



1185. Although the constant angular distance of 30° be- 

 tween the centres of the balls was adopted, and found abun- 

 dantly sensible, for all ordinary purposes, yet the facility of 

 rendering the instrument far more sensible by diminishing 

 this distance was at perfect command; the results at different 

 distances being very easily compared with each other either 

 by experiment, or, as they are inversely as the squares of the 

 distances, by calculation. 



