150 



PHYSIOLOGY 



CHAP. 



in the schema of Fig. 44. Here 1 is the source of the induced current, r the 

 resistance of the liquid to be determined, R a resistance expressed in ohms, 

 T a telephone, x a contact that slides along a metal wire pq, which is kept 

 tense and parallel with a scale divided into 1000 parts and one metre in f * 

 length. 



This arrangement constitutes the so-called Wheatstone Bridge, and no 

 electrical current passes through the telephone T, i.e. it remains silent, when 

 the contact x divides the wire pq into two parts, px and xq, such that the 

 resistances of px, xq, r and R are related thus : 



r : R --=px : xq. 



FIG. 44. Diagram of apparatus used for determining the electrical conductivity of fluids. 



It is easy to find this point x by holding the telephone to the ear and sliding 

 the contact along the wire pq, the above ratio enabling us to calculate the 



, and noting that px + xq = 1000 : 



2 



required resistance, as 



The resistance capacity ((7) of the cell containing the fluid to be examined is 

 found by determining the resistance (2} which it offers with a given solution 



of known conductivity (x), e.g. KC1 ^ and calculating C=x2. In all other 



fluids to be examined in the same cell the specific electrical conductivity 



C 

 is calculated on the basis of this value, = --' The conductivity at 25 



of the serum of healthy human blood (Viola) calculated in ohms (ccxlO 6 ) 

 oscillates between 1128 and 1232. That of the blood is much less, and at 

 the moment of clotting it presents a rapid diminution (Galeotti). 



Besides osmosis and electrical conductivity, we must briefly 

 consider the physiological importance of another physical property, 



