ELECTROMETERS. 217 



afterwards deduce numbers giving differences of electric 

 potential in absolute measure, in accordance with the 

 system of absolute measurement now adopted in every 

 branch of physical science. This I will endeavour to explain 

 more fully a little later. 



The first electrometric measurements were made by 

 Coulomb with the celebrated torsion balance. I have here 

 an instrument similar to that which he used. 1 



The torsion balance was first devised by Mitchell, who 

 used it for the measurement of the force of gravitation 

 between two small bodies. Cavendish also employed it 

 for the same purpose. Coulomb, however, independently 

 reinvented the torsion balance, and, as a preliminary, in- 

 vestigated with great care the laws of torsional elasticity 

 for the suspending fibre. He then used the balance for 

 determining the laws of electric attraction and repulsion ; 

 and afterwards employed it for purely electrometric 

 purposes. 



From the " torsion head " at the top of this tall glass 

 tube which surmounts the main body of the instrument 

 there hangs a vertical wire or glass fibre. Coulomb used a 

 fine silver wire. A fine glass fibre, as used by Faraday, was 

 first proposed by Ritchie, and is always employed now. To 

 the lower extremity of the fibre is attached a very light 

 horizontal bar or lever of shellac, which carries at the 

 extremity of the longer arm of the lever a small pith-ball 

 well gilded, and at the other extremity a counterpoise ; 

 and, in the instruments of Coulomb and Faraday, a vertical 

 disc of paper, or a slip of tissue paper hanging down ver- 

 tically, to " damp out " vibrations and bring the torsion 

 arm quickly to rest after disturbance. The case of the 

 instrument is, as you see, a glass cylinder covered with a 

 circular glass plate, into which is cemented the glass tube 

 that carries the torsion head. Round the glass cylinder 

 and on a level with the torsion arm there is pasted a 

 scale divided into degrees. By means of this scale the 

 position of the torsion arm is read off. 



In the circular glass cover of the cylindrical case a 

 circular hole is cut It is for the purpose of introducing 

 what I shall call the " carrier ball." The carrier ball is a 



1 The diagram represents Coulomb's original instrument. 



