Prof. Thomson on the Measurement of Electromotive Force. 319 



electrometer corresponding to longer sparks than that, was too strong 

 to be measured with certainty by the portable electrometer, whether 

 from the elasticity of the platinum wire, or from the rigidity of its 

 connexion with the aluminium index being liable to fail when more 

 than 85° or 90° of torsion were applied. So far as it goes, it agrees 

 remarkably well with the other experiments exhibited in Table I., 

 as is shown by the following comparative Table, in which, along 

 with results of actual observation extracted from Table II., are 

 placed results deduced from Table I. by interpolation for the same 

 lengths of spark. 



Table III. Experiments of December 13, 1859, and January 17, 

 1860, compared. 



• The close agreement with one another of the numbers in Col. 4, 

 derived from series differing so much as those in Cols. 2 and 3, 

 and obtained by means of electrometers differing so much in con- 

 struction, constitutes a very thorough confirmation of the remarkable 

 result inferred above from the experiments of the first series, and 

 shows that the law of variation of the electrostatic force in the air 

 required to produce sparks of the different lengths, must be repre- 

 sented with some degree of accuracy by the numbers shown in the 

 last column of either Table I. or Table III. 



The following additional series of experiments were made on pre- 

 cisely the same plan as those of Table II. 



