Prof. Thomson on the Measurement of Electrostatic Force, 235 



telescope at a short distance, it is easy to observe ^fofytik of an inch 

 of its vertical motion. 



I have recently applied this method to reduce to absolute electro- 

 static measure the indications of an electrometer forming part of a 

 portable apparatus for the observation of atmospheric electricity. 

 In this instrument a very light bar of aluminium attached at right 

 angles to the middle of a fine platinum wire, which is firmly stretched 

 between the inside coatings of two Leyden phials, one occupying an 

 inverted position above the other, experiences and indicates the 

 electrical force which is the subject of measurement, and which 

 consists of repulsions in contrary directions on its two ends, pro- 

 duced by two short bars of metal fixed on the two sides of the 

 top of a metal tube, supported by the inside coating of the lower 

 phial. 



The amount of the electrical force (or rather as it should be called 

 in correct mechanical language, couple) is measured by the angle 

 through which the upper Leyden phial must be turned round an 

 axis coincident with the line of the wire, so as to bring the index to 

 a marked position. An independently insulated metal case, bearing 

 an electrode projecting outwards, to which the body to be tested is 

 applied, surrounds the index and repelling bars, but leaves free 

 apertures above and below, for the wire to pass through it without 

 touching it ; and by other apertures in its sides and top, it allows the 

 motions of the index to be observed, and the Leyden phials to be 

 charged or discharged at pleasure, by means of an electrode applied 

 to one of the fixed bars described above. When by means of such 

 an electrode the inside coatings of the Leyden phials are kept con- 

 nected with the earth, this electrometer becomes a plain repulsion 

 electrometer, on the same principle as Peltier's, with the exception 

 that the index, supported by a platinum wire instead of on a pivot, 

 is directed by elasticity of torsion instead of by magnetism ; and the 

 electrical effect to be measured is produced by applying the electrified 

 body to a conductor connected with a fixed metal case round the 

 index and repelling bars, instead of with these conductors them- 

 selves. 



This electrometer, being of suitable sensibility for direct comparison 

 with the absolute electrometer according to the process described 

 above, is not sufficiently sensitive to measure directly the electro- 

 static effect of any galvanic battery of fewer than two hundred cells 

 with much accuracy. Not having at the time arrangements for 

 working with a multiple battery of reliable character, I used a second 

 torsion electrometer of a higher degree of sensibility as a medium 

 for comparison, and determined the value of its indications by direct 

 reference to a Daniell's battery of from six to twelve elements in 

 good working order. This electrometer, in which a light aluminium 

 index, suspended by means of a fine glass fibre, kept constantly elec- 

 trified by means of a light platinum wire hanging down from it and 

 dipping into some sulphuric acid in the bottom of a charged Leyden 

 jar, exhibits the effects of electric force due to a difference of poten- 

 tials between two halves of a metallic ring separately insulated in its 



R2 



