30 AGRICULTURAL 



The form of apparatus used in this experiment was one 

 designed by the Division of Soils of the U. S. Department of 

 Agrieiilture for taking the temperature of soils and also the 

 moisture. It consists of an electrode and apparatus for meas- 

 uring the resistance. The latter is a special adaptation of the 

 "Wheatstone bridge." (See Bulletin 6 of the Division of 

 Soils.) 



The electrode is a narrow glass tube filled with a solution of 

 common salt. It is in this column of salt solution that the 

 resistance is measured. Its behavior to changes in tempera- 

 ture is the opposite of that of metallic electrodes, that is, the 

 higher the temperature the less the resistance and vice versa. 



In the form of electrode used the tube containing the solu- 

 tion is cemented upon a strip of glass about five inches long 

 and one inch wide. To adapt this for use in the silo, where 

 the pressure is considerable and disturbance is likely from the 

 settling of the ensilage and consequent danger of breaking, it 

 was protected by a wooden casing which left the sensitive por- 

 tion of the electrode uncovered but amply protected ' from 

 pressure. 



The wooden casing was about fifteen inches long to insure 

 its remaining upright when buried in the ensilage, this posi- 

 tion being necessary in the form of electrode used. The wires 

 connecting the electrode with the measuring apparatus were 

 No. 12 copper wire insulated with rubber, the two wires being- 

 united in a single strand by a woven and tarred outer covering. 

 The conducting wires were fastened to the wooden casing in 

 such a way that no strain could be brought to bear upon the 

 electrode connections. The incased electrodes were buried in 

 the ensilage in the upright position near the center of the silo, 

 and the conducting wires led to the wall of the silo where 

 they would be out of the way; a sufficient length of slack wire 

 where it passed through the ensilage was provided so that set- 

 tling might not drag the electrode out of position. This 

 arrangement of apparatus proved entirely satisfactory. 



The entire apparatus as assembled in the laboratory is 

 shown in figure 1. The changing resistances in the elec- 



