APPENDIX 1G9 



Measurements of resistance (and of conductance) are best taken 

 by means of a Wheatstone bridge, preferably by Kohlrausch's method, 

 but for many purposes where polarisation of electrodes and of tissue 

 may be disregarded, it is convenient and sufficient to estimate con- 

 ductance directly from the galvanometric deflection. In the case 

 of plants the resistance is generally so great that it is allowable to 

 disregard that of the galvanometer and electrodes, and to at once 

 express deflections in terms of conductance. A carbon megohm 

 (=1,000,000 ohms) attached to the keyboard is a convenient 

 standard of reference, giving at once on the scale or photograph 

 the value of our unit of conductance, i y. 



Correction for the resistance of the galvanometer and electrodes 

 is to be made, when required, as follows : 



Let r l and d^ be the resistance of the galvanometer + megohm, 

 and the deflection by any convenient voltage. 



;'o and d. 2 the resistance of the galvanometer + electrodes, and 

 the deflection by the same voltage (practically we must take y^th 

 or Yjjoth that voltage and multiply by 10 or 100). 



7' 3 and d z the resistance of the galvanometer + electrodes + object 

 of experiment, and the deflection by the same voltage. 



The required resistance, r v of the object examined = r 3 - r. 2 , 



j r-i d., ?', d<, 



and since , and - = -? 



r. 2 ^ r. A d l 



r -2 4- 1 , and 7- 3 Li 



the required resistance r, = -i 1 - -V 1 



and the corresponding conductance = , ,'\ 3 , N- 



If the galvanometer resistance is known, it is, although convenient, 

 not necessary to take a megohm into circuit. 



Kohlrauscli 1 s method is used in this laboratory for the rapid 

 testing of distilled water, of dilute saline solutions, and of blood, 

 serum, urine, etc. 



The electrodes (of platinised platinum) are adjusted to a suitable 

 "resistance capacity" (about -^ for water, i for dilute saline, serum, 



