DETERMINING SOLUBLE SALTS 95 



is ioo and the scale reading 0.92, the resistance of the cup 

 is 92 ohms. When the extra 100-ohm coil is used with the 

 cup, the 100 ohms added must be subtracted from the 

 resistance read on the scale. Thus if the 100 ohms is in 

 series with the cup and the scale reads 1.2, while the 

 comparison coil shows 100 ohms, then the resistance of the 

 cup and coil is 120 ohms. Subtracting the 100 ohms of 

 the coil leaves 20 ohms as the resistance due to the cup. 

 The resistance of the cup contents must be corrected to a 

 temperature of 6o F. To do this, immediately after 

 reading the resistance, a thermometer is stuck into the cup 

 and read after two minutes. The resistance at the tem- 

 perature found is then corrected to 6o F. (by means of 

 Table XII). Having found the resistance of the cup 

 contents, the percentage of salt may be determined for 

 soils by use of Table XIII, and for soil solutions by 

 Table XIV." In making temperature corrections, which 

 must be done before determining the parts per million of 

 salts present, the column containing the temperature of 

 the soil is found. The sum of the resistances of the sep- 

 arate digits corresponding to the resistance at the given 

 temperature of the soil is found and the sum of the resist- 

 ances of the separate digits corresponding to the resistance 

 at the given temperatures is added. "As an example of 

 its use, suppose the resistance to be 1349 ohms at 72 F. 

 On the left-hand side of the table find 72 F., then opposite 

 under the columns marked '1000' will be found 1170 ohms 

 at 6o F. as the value of 1000 at 72 F.; 3000 ohms at 72 F. 

 will be found equal to 3510 at 6o F.; hence 300 is equal 

 to 351 at 6o F., 40 is equal to 46.8 ohms at 6o F., and 9 is 

 equal to 10.5 ohms at 6o F.," and the sum of these re- 

 sistances at 6o F. is equal to 1578.3 ohms, which is the 

 desired resistance. 



