Lateral Diffusion of a Current in a Conductor. 445 



not seen any description of it, and as many details were arranged 

 by me, I think it may be worth while to publish a short account of it. 



The law of the diffusion of the current in a solid conductor is 

 of course perfectly well known. In this paper I merely propose 

 to describe a method of experimentally verifying this known law, 

 and to suggest a modification of it by means of which the (ap- 

 parently) unknown law for the diffusion of the current in a liquid 

 may probably be determined. 



The object of the method was, on a surface of tinfoil through 

 which a current was flowing, to determine a number of groups 

 of points of equal tension, and to draw equipotential lines through 

 them. From these lines the positions of the currents can be 

 easily deduced. 



My plan was as follows : — A sheet of tinfoil of any desired 

 shape was pasted on to a hard mahogany board. At the two 

 points chosen for the electrodes holes were drilled, through 

 which screws were passed so that their heads (about 7 millims. 

 in diameter) rested on the tinfoil ; their stems passed through 

 the board into binding-screws on the underside. A little mer- 

 cury was placed under the heads to make better contact. Wires 

 from a battery passed through a contact-breaker to these screws. 

 Two cells of Grove's battery were at first used ; but the power 

 was afterwards reduced to one, owing to the great heating-effect 

 of two cells. 



The contact-breaker was arranged so that two circuits could be 

 broken or made simultaneously, namely the battery and galvano- 

 meter circuits. A current is produced simply by difference of 

 tension. Therefore, if two poles of a sufficiently sensitive galva- 

 nometer were placed in such a position on the tinfoil that there 

 was no deflection when the current passed, these two points were 

 in the same equipotential line. 



The beautiful reflecting galvanometer belonging to Kin'g's 

 College was used, and was found to be so delicate that, after a 

 little practice, a deviation of less than 1 millimetre from the right 

 position on the tinfoil could be easily detected on the galva- 

 nometer. 



The same plan can be used for determining the lines in 

 liquid conductors by insulating the wires up to their points, and 

 having each galvanometer-terminal fixed on a little stage to slide 

 on the top of the trough, which must be divided so that from the 

 position of the stage the horizontal coordinates of the point of 

 the wire can be determined, while the wire slides through tl 



le 



stage and is divided to give the vertical element. 



lielow is an engraving of half one of the sheets of tinfoil with 

 the lines traced on it. The sheet is divided by a line at right 

 angles to, and bisecting, the line joining the battery-poles. The 



