236 Dr. 0. Lodge on the Electrostatic Force between 



when discharged through them is about 1£ "metres" or 

 *00014 microfarad; i. e. smaller than the ordinary "pint" size. 

 With the help of an adjustable condenser, an instrument for 

 measuring the L of well-insulated coils free from iron 

 suggests itself here. 



Ribbon' Conductors. 



If strips are used instead of round wires for the movable con- 

 ductor, the electrostatic effect has an artificial advantage 

 given it : for take a pair of similar strips, of length Z, breadth 

 b, and distance apart a, the force caused by a current C flow- 

 ing through them with uniform intensity everywhere is 

 easily calculated to be 



4zGHfJb / a log cos a \ 



a \tan a tan 2 a /' 



where a is an angle whose tangent is b/a. 



The quantity in brackets has a maximum value J when 

 a = 0, i. e. when the plates are far apart enough for their shape 

 to be immaterial ; and its value decreases steadily towards 

 zero, viz. ^ir cot a, as a approaches 90° ; the whole becoming 

 ultimately 27r/«tC 1 . CI. 



As for the electrostatic force between strips, I do not know 

 how far we are justified in assuming uniform distribution of 

 density, even if given uniform distribution of current ; but 

 at least when the plates are close together the force will not 

 be very different from 



2 7 r.|J.SV=g.(S 1 PC)* ; 



the value of Si being- — . 



So the ratio of the forces for large close plates is 



K / P6 \ 2 _ / no - °f ohms in impedance of wireV 

 /T'ViW ~ V 120 ira/b ')' 



Hence with strips six times as broad as their distance 

 apart the forces will balance for a steady current when the 

 interposed wire is only 60 ohms resistance. 



Measure of u v." 



In applying an experimental observation of this kind to a 

 determination of the product of the aether constants /jl, K, (and 

 it just strikes me that it is after all only a modification of the 

 method by which Maxwell himself made one of the early deter- 



