T. DeiOxHTON 209 



down to form roughly a paraboloid of revolution, a shape found most 

 satisfactory by Briggs (I.e.) for maintaining contact with the soil. The 

 core was then drilled out for about an inch from the end and the cavity 

 tilled with melted caoutchouc and sealing wax, a mixture which main- 

 tained its position in the cavity and rendered good service for insulating 

 the end of the core throughout the experiments. The opposite end was 

 then scored and an inch of the core drilled out. A thread was tapped 

 on to the inside of the cavity and a copper wire sealed in with fusible 

 alloy. The top was then covered with sealing wax. The whole of the 

 outside of the electrode, except a band one inch wide round the para- 

 boloidal surface at the bottom, which was to act as the electrode surface, 

 was then insulated by painting with two coats of "Duroprene"" and 

 thoroughly dried for two or three days at about 50° 0. As the experi- 

 ments progressed this insulation was found un.satisfactory and in cases 

 where it was necessary to leave the electrodes a considerable time buried 

 another type was adopted which will be described in a later paper. 



There appears to be a theoretical objection to the use of metal 

 electrodes of any kind in direct contact with the soil which has not 

 hitherto been noticed by workers in this field : namely that polarisation 

 is very likely to take place even when an alternating current is employed 

 unless the frequency is very high, since the free metallic ions are not 

 completely returned to the electrode on reversal; and in some cases 

 rapidly form double salts which are not decomposed on reversal. In 

 these cases polarisation may occur even at frequencies of 40,000 per min. 

 Owing to its abiUty to absorb gases a carbon electrode is self-depolarising 

 to a sufficient extent to nullify the first effect and the second does not 

 arise. The phenomenon mentioned is well known to electro-chemists (5). 



Local Variations in Resistance. It was desirable before proceeding 

 further to find whether the resistance was the same under like conditions 

 in different parts of the plot. The method of experiment was simply 

 to push two such electrodes as have been described into the soil of the 

 plot to the desired depth at a definite distance apart; a preliminary test 

 having shown that the contact error in this, while not negligible, did 

 not seriously affect the results. The same pair of electrodes was then 

 moved to another part of the plot and placed at the same depth and the 

 same distance apart. The results for a depth of 7" at 3" interval are 

 shown in Fig. 1, which may be taken to represent the plot, which was 

 about '22' X 30' in extent with the observations entered in the place in 

 which they were taken. Subsidiary experiments proved that the obser- 

 vations were sufficiently close together to justify the insertion of the 



15—2 



