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 tinfoil, were marked four points corresponding to angles of 90 deg.; four other points exactly corresponding to these points being marked on the upper ring of tinfoil 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 instrument and of the graduations on it.  From one of the four points on the exterior of the cylinder a graduation of 90 deg. was set off, and a corresponding graduation was placed upon the upper tinfoil 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.

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 deg. 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 deg., and finally the graduated arc belonging to the torsion index is to be adjusted so as to bring 0 deg. 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 deg. 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 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 repulsion of the balls; but this was so slight as to cause no interfering difference in the symmetry of form within the instrument, and gave no error in the amount of torsion force indicated on the graduation above.