214 The Electrical Met hud uj' Soil Moisture Deteriitinatiun 



and not a necessary pheaomenon, as will be shown from theoretical 

 considerations later on in tliis paper. 



Table V. Resistances at greater Distances. 



* I am \ituible fully l(j account for this lower figure; several subsequent tests about 

 the same place gave results in the neighbourhood of 3600. 



To summarise the results to this point, we find (hat the pot experi- 

 ment appears to favour Briggs' theory, the fact that agreement between 

 calculated and observed resistances, even when the calculation was 

 made from a single triangle, was better in the hexagon than in the 

 square speaks more in favour of (Jardner's limit being correct. On the 

 other hand neither theory explains the increase of resistance at distances 

 exceeding 18", the minima observed in Table II, nor why these minima 

 should appear at greater distances as the depth increases. 



Thk Path of thk Current. 



Experimental. It seemed probable that a more satisfactory result 

 might be attained by casting overboard the idea, really no more than a 

 convenient fiction, of the resistance being due to the soil in the im- 

 mediate neighbourhood of the electrodes and considering the whole mass 

 of soil about them. 



An experiment was devised the object of which was to determine 

 at what depth below the line joining the elet^trodes a conducting layer 

 would appreciably affect the resistance between them. For this purpose 

 a beaker of about 600 c.c. capacity (Fig. 2) was taken and four scales 

 of inches were gummed up the sides of it. Inside this lay a clean per- 

 forated metal plate of nearly the diameter of the beaker so that the 

 curved-in sides of the vessel would just keep it off the bottom. Through 

 the centre of this plate was passed a glass tube carrying a copper con- 

 ductor which was soldered to the underside of the plate. At the top, 

 the tube made connection with a funnel by means of about an inch of 

 rubber tube, but the copper wire passed out at the side and the opening 

 was rendered watertight with sealing wax. Tlie beaker was then filled 

 up to the 2" mark with pure dry sand and five subsidiary electrodes of 



