PHYSICAL METHDDS IN BLOOD EXAMINATION. 



26l 



ab and be are read off directly as bridge wire lengths, 5 is the known com- 

 parison resistance. Hence the unknown cell resistance is given by 



R=S* 



ab 



As S in practice is always taken as 10, 100 or 1000 ohms and ac is always 

 divided decimally, tables are constructed giving directly the value of R 

 for any value of ab read off. In practice the cell R is always kept at a 

 constant temperature, as the conductivity of liquids varies greatly with 



FIG. 28. Simple form of 

 Kohlrausch conductivity cell. 



FIG. 29. Conduc- 

 tivity cell for poor 

 conductors or smaller 

 quantities. 



temperature changes. To maintain this constant temperature the cell is 

 usually immersed in a large water thermostat, so constructed that it may 

 be readily controlled. Forms of cells are illustrated. 



The electrical conductivity of urine is also an important factor which 

 may be found by the same kind of apparatus, and which will be discussed 

 later. The arrangement of apparatus for such work is shown in the 

 adjoining figure. 



Value of the Conductivity for Blood. Expressed in the 

 terms just explained the value of the conductivity of blood 

 serum is about K = 0.012, or expressed in another form very 

 convenient for calculation i2oXio~ 4 . A good part of this 

 conductivity is due to the sodium chloride present. If the 

 chlorides be accurately determined by one of the usual meth- 

 ods of quantitative analysis and the proper conductivity cor- 

 responding to this salt content be calculated, which is possible 

 with a considerable degree of accuracy, and subtracted from 

 the total or observed conductivity a remainder is obtained 



