Fig. 1. 



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



disks, the electrodes being zinc rods inserted through holes in 

 the upper piece of glass. 



46. The results of all these investigations are in such close 

 agreement with calculation that it cannot be considered that any- 

 further confirmation of the general theory is needed. It never- 

 theless seems worth while to point out, and to illustrate by spe- 

 cimens of the results obtained by it, a simple method which 

 serves both for determining the form and distribution of the 

 equipotential lines on a conductor of two dimensions, and for 

 measuring the resistance of the part of it contained between any 

 two given lines. In all except the very simplest cases, it would 

 be much easier to determine the equipotential lines and resist- 

 ance experimentally in this way than by the corresponding cal- 

 culation. 



47. The method referred to is an application of the principle 

 of Wheatstone's bridge, and will be intelligible from fig. 1. The 

 poles of a battery (Z) are connected 

 with the two ends of a graduated 

 German-silver wire, E F, and also 

 with two fixed points, A and B, of 

 the conducting disk on which the 

 equipotential lines are to be traced ; 

 a galvanometer, G, is connected to 

 a sliding contact block D on the 

 wire E F, anil to a contact-pin C, 

 with the point of which the sheet 

 can be explored. Under these cir- 

 cumstances the points C and D 

 will be at the same potential when- 

 ever the galvanometer shows no 

 deflection ; hence if the pin C 

 be moved about on the disk, and a mark be made at every 

 position for which the galvanometer is not affected, all these 

 marks will lie on one equipotential line. By repeating this opera- 

 tion, after shifting the slider D along the graduated wire, another 

 equipotential line can be marked out on the sheet; and any 

 number of lines can be traced, so as to correspond to equal dif- 

 ferences of potential, by shifting the slider through successive 

 equal distances. 



48. In the actual experiments the connexions were not made 

 exactly as shown in frg. 1, but the positions of battery and gal- 

 vanometer were interchanged. This arrangement made it un- 

 necessary to use a separate key for closing the battery-circuit, as 

 the circuit was completed by the contact of the pin C with the 

 sheet ; and since these contacts were only momentary, no sen- 

 sible disturbance can have arisen from unequal heating. The 



