208 The Electrical Method of Soil Moistnre Determination 



In 1899 L. J. Briggs{4) described improved instruments and electrodes 

 and introduced a condenser in parallel witli that arm of the bridge 

 adjacent to the soil resistance, which would throw the small capacity 

 found in the soil and the condenser capacity on opposite sides of the 

 bridge with respect to the telephone receiver. 



I have found a reference to some work by R. 0. E. Davis (Trans. 

 Arner. Electrochem. Soc. 17 (1910), 391-403), in which he is said to have 

 found the resistance of soils within the limits 10-20 per cent, of moisture 

 to be inversely as the moisture content. Unfortunatel)- the original of 

 this paper is not to be obtained here, but the result thus stated is at 

 variance with Whitney's results which experiments to be described in 

 this paper fully confirm. 



The electrical method has never been popular on this side of the 

 Atlantic for several reasons, notably the discrepancy in the results 

 attained by Gardner using the two methods in a parallel series of experi- 

 ments and the failure of the investigators to deal satisfactorily with the 

 question of movement of salts in the soil. 



Preliminary Experiments. 



Apparatus. The experiments which follow were made on a small plot 

 of ground on the south side of the School of Agriculture at Cambridge. 

 It had been dug over a short time previously and was not ideal for the 

 purpose as a thin layer of builder's refuse from the building of the school 

 rendered it less homogeneous than cultivated farm land. It had, however, 

 the merit of being conveniently situated with regard to the laboratory, 

 and the results obtained upon it appear to justify the conclusion that 

 no great error was introduced by this lack of uniform texture. 



For the measurement of the resistance an ordinary post-office pattern 

 Wheatstone's bridge was employed, with an induction coil from which 

 the condenser had been removed to avoid any possibility of polarisation 

 of the electrodes, and a telephone receiver. As a source of energy two 

 small storage cells were used. At first a large coil giving a frequency 

 of something under 30 per second was put in, but this was afterwards 

 changed for a smaller one with a frequency rather over 100 per second, 

 as it was found much easier to estimate the minimum of sound with 

 this. The condenser in parallel with the bridge arm used by Briggs was 

 dispensed with, being found unnecessary for the attainment of the degree 

 of accuracy aimed at. 



The electrodes employed were formed from cored electric arc light 

 carbons 9" long and I" diameter. The tapered ends of these were ground 



