514 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



ment is of necessity such that the force exerted between two 

 given parts of the instrument shall be proportional to the differ- 

 ence of potential between them. This force may be variable 

 and measured by the torsion of a wire, as in Thomson's reflect- 

 ing electrometer ; or it may be constant, and the electromotive 

 forces producing it may be compared by measuring the distance 

 required in each case between the two electrified bodies to pro- 

 duce that constant force. The latter arrangement is adopted in 

 Professor Thomson's portable electrometer, first exhibited at the 

 present Meeting of the Association. The indications of a gauge 

 or electrometer not in itself absolute may be reduced to abso- 

 lute measurement by multiplication into a constant coefficient. 



45. Practical Measurement of Electric Resistance. — The elec- 

 trostatic resistance of a conductor of great resistance (such as 

 gutta percha or india rubber) might be directly obtained in the 

 following manner : — Let a body of known capacity, s (40), be 

 charged to a given potential, P (47), and let it be gradually dis- 

 charged through the conductor of great resistance, r. Let the 

 time, t, be noted at the end of which the potential of the body 

 has fallen to p. The rate of loss of electricity will then be 



P — — * P 



*-. Hence p = P e «• and JL = log e _. Hence 



sr sr p 



r=— ^; (27) 



S log e - 



from which equation r can be deduced if s, t, and the ratio 



P 



— be known ; t can be directly observed ; s can be measured (40) ; 



p 



and the ratio —can be measured by an electrometer (44) in con- 

 stant connexion with the charged body. This ratio can also be 

 measured by the relative discharges through a galvanometer, 

 first, immediately after the body has been charged to the poten- 

 tial P, and again when, after having been recharged to the po- 

 tential P, it has, after a time t, fallen to potential p. (This 

 latter plan has long been practically used by Messrs. Siemens, 

 although the results have not been expressed in absolute mea- 

 sure.) 



Unfortunately, in those bodies, such as gutta percha and 

 india rubber, the resistance of which is sufficiently great to make 

 t a measurable number, the phenomenon of absorption due to 

 continued electrification* so complicates the experiment as to 



* Vide British Association Report, 1859, Trans, of Sec. p. 248, and Report 

 of the Committee of Board of Trade on Submarine Cables, pp. 136 & 464. 



