PHYSICAL METHODS IN BLOOD EXAMINATION. 



259 



mainly because of its content of salts. The proteins in abso- 

 lutely pure condition, salt free, are probably non-conductors. 

 Some of them, however, because of their acid character exist 

 in combinations resembling salts and these have a weak con- 

 ducting power. But, because of their high molecular weight, 



FIG. 26. Diagram of Wheatstone bridge connections. A represents a cell or 

 induction coil, ac the bridge wire, S the standard resistance with which com- 

 parison is made, R the conductivity cell containing the substance under ex- 

 amination. In most conductivity experiments A is a small induction coil, a 

 telephone, as shown, being employed as the current indicator. 



the part which this conductivity plays in the total conductivity 

 of the blood is very small. As applied to the blood, therefore, 

 conductivity measurements give us an idea of the number of 

 salt molecules present, or inorganic concentration. 



FIG. 27. Simple Wheatstone bridge. The bridge wire of exact length is 

 stretched over the graduated scale. The several wire connections are made at 

 the binding posts lettered. 



In practice conductivity is the reciprocal of resistance, which is the 

 factor actually measured. Resistance is expressed in terms of some 

 standard arbitrarily chosen, and comparisons are usually with the "ohm" 

 as the final standard. The legal ohm is the resistance of a column of 

 pure mercury 106.3 cm - long and I mm. in section at a temperature of 

 o, but for practical use resistance standards of wire are employed. A 



