346 



ELECTKOKINETICS. 



[PT. II. CH. VIII. 



and not to its surface. This of course comes from the fact that 

 the lines of flow diverge in all directions from the electrode 

 instead of remaining parallel. It explains the necessity for large- 

 sized plates for telegraphy or for the earth connection of a light- 

 ning rod. In practice, the conductivity of the earth varying from 

 point to point, the conductivity of the portions near the electrode 

 plays the most important part, so that it is important that the 

 earth : plate be buried in good-conducting material. The problem 

 of the spherical bowl shows that if such a bowl should be made an 

 electrode immersed in an infinite conductor, the other electrode 

 being at a great distance, nearly all the current would flow from 

 the outside of the bowl, the current density being greatest at the 



HP. 



The method of the conformal representation furnishes a means 

 of solution for the case of two-dimensional problems, in particular 

 for the flow of current in a thin plane sheet. Fig. 67 for instance 

 shows the lines of flow in the case of a long ribbon of conductor 

 slit along the axis of U'. 



176. Correction for End of Wire. We shall conclude 

 this subject with the consideration of the practical problem of 

 finding the correction that must be made in the value of the 

 resistance of a uniform wire when it ends in a conductor so 

 large as to be capable of being considered infinite. This is of 

 importance in the case of mercurial standards of resistance, for 

 the tubes end in large cups of mercury. We shall consider a 

 right circular cylindrical conductor ending in a conductor of in- 



FIG. 70. 



