170 THE SIGNS OF LIFE 



and blood, and 10 for solutions of higher conductivity) so that the 

 resistance under observation shall fall between the limits of 100 and 

 1000 ohms. The electrodes, forming the A: arm of aWheatstone bridge, 

 are plunged into the fluid under examination ; a telephone is in the 

 bridge in place of a galvanometer ; the testing currents come from a 

 small induction coil, and the silence point (or its equal " to much " 

 and " too little ") is sought for by alteration of the variable resistance. 

 Readings of resistance are thus easily obtained within an error of 

 I per 100, with the normal temperature = 18. A correction of 

 2 per 100 of the reading is to be made per i. 



rr,, . ,- 7 , . ., Resistance Capacity 



1 he specific conanctivitv = 



Observed Resistance 



The resistance capacity of a given pair of electrodes in a given 

 vessel is ascertained by taking a measurement of resistance through 

 a standard solution of known conductivity. E.g., a pair of electrodes 

 in decinormal KC1 at 18 is found to have a resistance = 200 ohms. 

 The resistance capacity == 200 x 0.01119 = 2.238 (Resistance x 

 Conductivity = Capacity). 



With the same pair of electrodes the resistance of a specimen 

 of serum is found to be 320 ohms. Its specific conductivity 



^ = 0.006994 m ho or 6994 7. 

 320 



The necessary data are given in Kohlrausch and Holborn's 

 tables, from which the following useful empirical rule is taken : 



For weak saline solutions | and under ) the specific conduc- 



\io / 



tivity (in mhos) x 10 == the number of gramequivalents per litre, 

 and -- the specific conductivity x molecular weight == the percentage 

 of salt in solution. 



REFERENCES 



LIPPMANN. " Capillary Electrometer," Comptes Rendus de P Academic des 



Sciences, p. 1407, 1873. 

 MAREV AND LIPPMANN. " Electrometer Records," Comptes Rendus, p. 278, 



1876. 

 BURDON-SANDERSON AND PAGE. " On the Electrical Phenomena of the 



Excitatory Process in the Heart of the Frog and of the Tortoise, as 



investigated Photographically," Journal of Physiology, vol. iv. p 327, 



1883. 



