8 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



conductor that it neither receives nor gives a charge freely, and so, after contact with 

 a charged conductor, is liable to be in an uncertain condition. Again, it is difficult 

 to turn pith so smoothly as to leave the ball, even when gilt, sufficiently free from 

 irregularities of form, as to retain its charge undiminished for a considerable length 

 of time. When, therefore, the balls are finally prepared and gilt they should be ex- 

 amined, and being electrified, unless they can hold their charge with very little dimi- 

 nution for a considerable time, and yet be discharged instantly and perfectly by the 

 touch of an uninsulated conductor, they should be dismissed. 



1183. It is, perhaps, unnecessary to refer to the graduation of the instrument, 

 further than to explain how the observations were made. On a circle or ring of 

 paper on the outside of the glass cylinder, fixed so as to cover the internal lower ring 

 of tin foil, were marked four points corresponding to angles of 90° ; four other points 

 exactly corresponding to these points being marked on the upper ring of tin foil 

 within. By these and the adjusting screws on which the whole instrument stands, 

 the glass torsion thread could be brought accurately into the centre of the instru- 

 ment and of the graduations on it. From one of the four points on the exterior of 

 the cylinder a graduation of 90° was set off, and a corresponding graduation was 

 placed upon the upper tin foil on the opposite side of the cylinder within ; and a dot 

 being marked on that point of the surface of the repelled ball nearest to the side of 

 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. 



1 184. 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 electrometer, 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 graduated arch belonging to the torsion index is to be adjusted so as 

 to bring 0° upon it to the index. This state of the instrument was adopted as that 

 which gave the most direct expression of the experimental results, and in the form 

 having fewest 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 instrument, and in relation to every part of 

 it, was actually the same every time that a measurement was made ; so that all irre- 

 gularities arising from slight difference of form and action in the instrument and the 



