558 BEPOUT — 1881. 



15. On the Absolute Sine Electrometer. By Professor G. M. Minchin, M.A. 



This is an electrometer in which use is made of the attracted disk and guard- 

 plate principle. 



The continuous plate and the guard plate are two brass plates, each about a 

 foot square, ihey are kept at a constant small distance apart (being parallel to 

 each other) by means of four ivory axes or pins driven through them near their 

 corners. The attracted disc is a square of aluminium, its side being 3 centimetres 

 lono- • it is suspended by means of two ^^■ ollaston platinum wires, each about 

 7 inches long, from a metallic piece at the top edge of the guard plate. The two 

 plates can move, as a rigid body, round a fixed horizontal axis attached to the 

 continuous plate. This system can be tilted out of the vertical position by means 

 of a micrometer screw wliose axis works horizontally against the lower portion of 

 the continuous plate. 



The disk (which is, of course, always in metallic contact with the guard plate) 

 is, in the vertical position of the plates, flush with the guard plate. This flush- 

 ness is obtained by means of four small adjustable screws fixed to the guard plate 

 at the back of the disk ; when the system of plates is tilted out of the vertical, 

 the disk will remain in the flush position, unless it is attracted ofi" the points of 

 these four screws by some force. 



Any motion of the disk is observed by means of a microscope rigidly attached 

 to the back of the guard plate ; the focal length is :^inch. 



If the continuous plate is connected by a wire with one electrified body, while 

 the disk and guard plate are connected with another, the disk will be drawn ofT 

 the screw-points, unless the system of plates is sufficiently tilted out of the 



vertical. 



If M' = weight of disk, ^ = angle of deviation of plates from vertical, rf = distance 

 between the plates, E = difference of potential of the sources in connection with 

 the two plates, S = area of disk, the equation of equilibrium of the disk when it is 

 just out of contact with the screws is 



= w sin 6. 

 OTra" 



The force measured is thus proportional to the sine of the angle of deflection 

 from the vertical — whence the name of the instrument. 



Usually the sine and the circular measure will be equal, since the deflections 

 will not amount to many degrees. 



In the instrument (which has been constructed by Mr. Groves of Bolsover 

 Street) d^'^lQ millimetres, w = -2568 grammes, the vertical distance between the 

 axis of suspension of the plates and the point of the tilting micrometer screw = 15 

 inches, the diameter of the head of this screw is 3 inches, its circumference is 

 divided into 1 ,000 equal parts, and the pitch is 55-inch. Hence the equation of 

 equilibrium (employing the absolute measures of weight and length) is 



E = -0456v/m, 



where wt is the number of divisions turned through on the micrometer screw-head 

 from the vertical position of the plates. 



Since it is diflacult to know when the vertical position is attained exactly, a 

 diff"erential method is resorted to. An observation is made with n cells and then 

 with n' of the same cells, and if, when the disc is just attracted off' the screws, the 

 readings of the micrometer screw-head are «i and 7n', we get 



, / m — 711' 



E = .0456., „^^, 



where, of course, m — in' is known, although m and m' are separately unknown. 



There are four adjustments which must be carefully attended to, the most 

 diihcult being the determination of the distance d. This (on the suggestion of 

 Professor Carey Foster) was determined by taking thiee readings with a sphero- 

 meter at three points of the aperture of the guard plate, before and after the inser- 

 tion of mica washers on the four ivory axes which connect the plates. 



