480 Messrs. G. C. Foster and 0. J. Lodge on the Flow of 



AP 



Values of ^p(=fA n ) for for four different points on each of 



eight different equipotential lines. 





J"- 2 - 



fl-2. 



fi-i. 



A* - 



fX. 



fc 



f. 



jA 





•200 

 •200 

 •190 

 196 



•335 

 •327 

 •323 

 •333 



•568 

 •568 

 •573 

 •573 



•996 

 100 

 100 

 102 



171 



1-70 

 1-71 

 1-73 



2-94 

 2-95 



298 

 2-98 



5-05 

 513 

 5-18 

 5-53 



8-75 

 9-01 

 8-97 

 8-91 



Means. 



•1965 



= (l-72)" 3 



•3295 



= (W4)~ 2 



•5705 



^Cl-76)- 1 



1005 



1-71 



=(1*1) 



2-96 



= (l-72) 2 



5-22 



= (1-73)3 



8-66; 

 = (1-71) 4 



In the experiments by which the above values were obtained 

 the distance between the poles was 19*45 centims. Taking this 

 as the value of 2a, and 1*724 for fjb, a system of coaxal circles was 

 drawn by the method of § 21, Part I. A tracing of the experi- 

 mental curves was then superposed on these theoretical circles; 

 and the two sets were found to coincide almost exactly. 



Another case tried was that of a circular disk with four equal 

 poles on its edge, two positive and two negative ; the equipoten- 

 tial lines obtained experimentally agreed in all particulars with 

 those drawn by the superposition method of § 35. 



50. The arrangement of fig. 1 serves not only to lay out the 

 equipotential lines on a disk, but also to determine its resistance ; 

 for if the pin C be left in contact with any point of the disk 

 (such as the electrode A for instance), while D is adjusted so 

 that the galvanometer stands at zero, and if C is then moved to 

 another point (say the electrode B) and D again adjusted, the 

 resistance of the part of the disk between the equipotential lines 

 which respectively pass through the two positions of C is the same 

 fraction of the resistance of the branch E A B F, as the resistance 

 of the part of the wire between the two positions of D is of the 

 resistance of the whole wire E F. Accordingly the resistance of 

 the disk is given, in terms of that of unit length of the wire E F, 

 as soon as the ratio of the resistances EABF and EDF is 

 known. In order to determine this ratio, the connexions shown 

 in fig. 1 were somewhat modified, the arrangement finally 

 adopted being that shown in fig. 3. 



One pole of the battery is connected by means of a slider D 

 with some point of the wire E F, while the other can be con- 

 nected at pleasure, on the one hand, with either of the electrodes 

 A and B upon the disk by pressing one of the contact-keys 1 

 and 2, or, on the other hand, with another slider H on the wire 

 E F by pressing the key 3. The galvanometer is permanently- 

 connected with one end E of the wire, and with a fixed point K 



