216 Tlie Electrical Method of Soil Moist lar Deteriiii nation 



outside reached the 1|" mark. In the experiment this was observed 

 quite sharply and it was therefore assumed that the liquid surface in 

 the capillaries was sensibly plane. The dampiu^j of the sand above had 

 not been carried so far as to interfere with packing and it was therefore 

 presumed that this condition was equally satisfied above. The resistance 

 unplugged in the standard arm of the bridge was 10,000 ohms, so that 

 the effect awaited as the solution gradually rose was a sudden cessation 

 of sound in the receiver, followed immediately by a sudden increase. 

 This also gave a very sharp indication when tin' water line read on the 

 four scales round the beaker was s(jmething between }/ and \" below 

 the line of the main electrodes, the readings were 3g", 3j", 3", 3^". Thus 

 when the conducting surface reached a level of somewhat more than \" 

 below that of the two electrodes the resistance between them fell by 

 about 10 per cent. From this result it appears probable that the current 

 density which is concentrated in the line joining the two electrodes 

 becomes rapidly less on moving away from this part of the field, though 

 probably not as quickly as the result appears at first sight to suggest, 

 as the conductivity is only increased over a comparatively small part of 

 the region surrounding the line of the electrodes. 



The following experiment, originally performed as a means of con- 

 firming the above result, is described not so much for any value it has 

 in itself, as on account of the justification it seems to supply for the 

 assumptions made in the short mathematical investigation which follows. 

 A tray was taken in which moist sand was laid out in a lamina l" thick. 

 Electrodes were placed 61" apart in this and the width of the conducting 

 layer was gradually cut down by the insertion of wooden partitions on 

 each side of the line of the electrodes at varying distances. The increase 

 in resistance with decrease in width of the conducting layer was quite 

 in accordance with expectations, becoming much greater as the electrodes 

 were approached, while remaining practically unchanged at considerable 

 distances. The results are given later in Table VI. 



Theoretical. It is shown by Mascart and Joubert(fi) that the resistance 

 between two electrodes immersed in an unlimited isotropic conducting 

 medium depends only on the medium and the form and dimensions of 

 the electrodes, and is doubled if the medium is limited by an infinite 

 plane passing through the electrodes, i.e. if the medium extends on one 

 side only of this plane. On account of the comphcated nature of the 

 considerations involved they omit any consideration of the problem 

 when the bounding plane passes to one side of the line of the electrodes. 

 It will suliice lor our purpose however if we can determine in some 



