260 PHYSIOLOGICAL CHEMISTRY. 



series of standard wire resistances running from a tenth, or hundredth 

 of an ohm even, to 1000 ohms or more is generally employed in the form 

 of a resistance " set " or " box." 



In dealing with solutions the unit of conductivity is taken as the 

 reciprocal of the resistance of a substance which, in the form of a column 

 i cm. square and I cm. long (a symmetrical cubic centimeter), has a 

 resistance of I ohm. That is, the conductivity, *, is measured in terms 

 of that of an ideal liquid, one symmetrical cubic centimeter of which has 

 a conductivity of i between opposite faces, or which offers between the 

 same faces a resistance of i ohm. The resistance of liquids is always 

 found in small vessels of glass made in different shapes and sizes accord- 

 ing to the character of the liquid. Small platinum plates are mounted 

 in the vessels and it is the resistance of the column between these which 

 is measured. Before use the resistance capacity of the vessel must be 

 found. This is done by measuring in it, with the plates in fixed position, 

 the resistance of some liquid the conductivity of which has been previously 

 determined by some standard method. The data for several solutions 

 have been very accurately determined and are everywhere used for pur- 

 poses of graduation of conductivity vessels. With such a standard liquid 

 with conductivity K we find in our cell the resistance, R. The resistance 

 capacity, C, is given by the relation: 



That is, C is the resistance which would be found in the vessel if it were 

 filled with a liquid of unit conductivity, and is used as a constant in all 

 following calculations with the same vessel when we wish to find *. R 

 we always find by direct measurement in ohms and with C known we 

 have now : 



C 

 K =R' 



The resistance of liquids cannot be found as is that of a solid by means 

 of the Wheatstone bridge combination and a galvanometer, since under 

 such circumstances liquids suffer hydrolysis with rapid change of resist- 

 ance. In place of the direct current and galvanometer Kohlrausch sug- 

 gested the use of a weak induction current, with a telephone as current 

 indicator. With this arrangement, which is illustrated by the annexed 

 diagram, it is possible to measure the conductivity of the serum or other 

 liquid very readily; a simple form of Wheatstone bridge is shown also. 



ac represents the graduated wire of the Wheatstone bridge, S the stand- 

 ard resistance with which comparison is made, R the cell containing the 

 serum or other liquid under investigation, T the telephone which ceases 

 to buzz when no current passes through bd. This gives the " null " point 

 in the combination and when this is found the following proportion holds : 



ab:bc::S:R. 



