308 SOILS: PROPERTIES AND MANAGEMENT 



ing on the texture and structure of the soil and the amount 

 of moisture present. 



220. Measurement of conductivity. Ordinarily the 

 conductivity of a soil is measured by applying a constant 

 source of heat as quickly as possible and measuring the 

 change in temperature by means of thermometers set 

 in the soil at regular intervals. (See Fig. 48.) The soil in 

 question should be homogeneous in composition and of 

 uniform compaction, and should contain a definite mois- 

 ture content. It should of course be at a temperature 

 equilibrium before the heat is applied. Ordinarily radia- 

 tion and convection currents are diminished somewhat 

 by inclosing the soil in an insulated compartment. The 

 study of heat movement downward instead of laterally 

 is to be recommended, in order that unnecessary air 

 circulation may be avoided to some extent. 



FIG. 48. Longitudinal section of apparatus for the study of heat con- 

 ductivity of soil. (C), water at constant temperature; (0, ther- 

 mometer; (P), copper plates; (F), screw clamp for pressing soil 

 firmly against source of heat ; (r) , skids for soil box. 



221. Effect of texture on conductivity of heat. The 

 conductivity of a soil is affected by a number of factors 



