Ma2)X>^n(j Soils mid Alkali. 31 



The irrigation water, or the soil, the electrical resistance of 

 which is to be found, is put into the hard-rubber cell with metal 

 electrodes. If the salt content of water is to be determined, the 

 cell is filled even full with the water. If the salt content of soils 

 is to be determined, the soil is made into the condition of a thin 

 mortar with distilled water and the cell filled with this material, 

 gently tappinir the cell on the ground to exclude air bubbles. The 

 top of the soil is then struck off with a knife edge, so that the cell 

 shall be just level full of the saturated soil. The cell is then 

 suspended in the mercury cups attached to the electrolytic bridge 

 and the electrical resistance determined in the following way: 



The telephone receiver is pressed against the ear and the handle 

 of the instrument pressed down, when a buzzing sound will be 

 heard in the receiver. Holding the handle down so as to keep 

 the battery switch closed, the pointer is rotated to either right or 

 left until the position is found at which the note in the telephone 

 receiver is no longer heard or is only indistinctly heard. On 

 rotating the pointer to either side of this position, the sound in 

 the receiver should gradually increase. In case difficulty is found 

 in locating the exact position of balance, it will be found of assist- 

 ance to rotate the pointer rapidly back and forth over the position 

 of least sound, locating points of equal intensity on either side. 

 The mean position between these two points gives the position of 

 balance, and the number opposite the pointer gives the desired 

 reading. 



In case a balance is not obtained with the 1,000-ohm coil of the 

 rotary switch, the 100-ohm and 10-ohm coils should be tried in 

 succession. It is best to choose the coil which will bring the bal- 

 ance as near as possible to the center of the scale, as this is the 

 most sensitive position. 



Having obtained the balance, the resistance is found by multi- 

 plying the resistance of the comparison coil, as shown by the 

 rotating switch, by the number on the scale 0])posite the i:>ointer. 

 Thus, if the comparison coil used has a resistance of 100 ohms 

 and the reading on the scale is 0.92, the resistance in the scale is 

 92 ohms. If the comparison coil is 1,000 ohms and the reading on 

 the scale is 4.5, the resistance would be 4,500 ohms. After taking 

 the resistance in this manner, take the temperature immediately, 



