T. Deighton 211 



a distance of 15 ur more feet. By adding the values of any two of the 

 small electrodes thus found, the resistance is obtained which they would 

 have if 15 feet apart." A table is given showing the actual resistance 

 between the small electrodes at their actual distance apart and the 

 calculated resistance between them when 15' ajDart. He concludes that 

 "when the electrodes are 15 ft. apart 98 per cent, of the resistance is 

 encountered within 9 inches of the surface of each electrode and the 

 intervening 13i ft. of soil causes only 2 per cent, of the total resistance." 

 Briggs (I.e.) states that the resistance is practically confined to volumes 

 of soil not exceeding 6" in diameter with the electrodes as centres. If 

 this be so, there should be no important variation on increasing the 

 distance apart beyond 6"; but both Gardner's results quoted below 

 (Table I), and those of the present author on a more extended scale, 

 show a quite appreciable change beyond this point. 



Table I. Excerpt from Gardner s table. 



Distance Resistance between standard 



apart electrodes* 



5" 928 



8" 934 



12' 968 



18" 979 



* Gardner calls this column (the numbers being given as percentages 92-8 etc.), 

 "Relation of observed to calculated resistance." By making the resistance 1000 ohms 

 at 15' the standard and reducing the other resistances to this the same figures are obviously 

 obtained. 



Gardner's experiments were all made at one depth and the author 

 therefore carried out a series of measurements of the effect of distance 

 apart of the electrodes on the resistance of the soil at varying depths, 



Table II. Effect of Distance Apart of Electrodes on Resistance 

 at various Depths. 



the results of which are given in Table II. There appears to be a tendency 

 for the resistance to fall to a minimum when the electrodes are about 



