Mr. I. B. Cooke on the Measurement of Chemical Affinity. 91 



usually classed^ without any very precise definition, as a result 

 of polarization ; and though it is perhaps thoroughly understood 

 by many philosophers, has not, as far as the writer is aware, 

 been clearly explained in any published treatise. 



A few experiments with the resistance tube sufficed to demon- 

 strate the real nature of the phsenomenon. But first, the con- 

 struction of the tube was improved in accordance with the facts 

 above deduced, and a form adopted which permitted the measure- 

 ment of intensities with greater minuteness. A glass tube 13 

 inches long, and ^th of an inch in internal diameter, of perfectly 

 even bore, was graduated through its length on the glass into 

 twentieths of an inch, and these divisions numbered from 1 to 

 240. It was placed in a tall glass jar about 2 inches in dia- 

 meter. A coil of copper riband covered the bottom of the jar, 

 and one end of the riband rose to the surface, and was connected 

 with one wire of the galvanometer. A piece of copper wire, 

 rather longer than the tube and thin enough to move easily in 

 it, was inserted into the tube, and being slightly bent, would 

 remain at any height at which it might be placed. To its upper 

 end was attached a long fine copper wire, which could be con- 

 nected by means of a mercury cup on the table with one of the 

 metals of any cell, the intensity of which was to be tested ; the 

 other metal being connected with the second wire of the galva- 

 nometer. The connexion was always made so as to deposit copper 

 upon the copper coil. The jar was filled with rain-water, to 

 which a small quantity of a solution of sulphate of copper was 

 added ; when it was found that the current of a pair of zinc and 

 copper, of ordinary dimensions in rain-water, produced a mo- 

 mentary deflection of 5° in the needle of the galvanometer, the 

 end of the wire in the tube being raised to the forty-sixth divi- 

 sion. When the wire was placed at a lower elevation, a greater 

 deflection was of course obtained. And a permanent deflection of 

 10° could be maintained for a considerable time without variation, 

 by any pair of zinc and copper exposing more than a square 

 inch of surface. 



As the intensity of the current is equal to the product of the 

 quantity into the resistance, the relative intensities of different 

 circuits would therefore be measured by the product of the de- 

 flection of the needle into the number of divisions contained 

 between the end of the wire and the bottom of the tube, pro- 

 vided the resistance of the tube could be safely taken as the total 

 resistance of the circuit ; or 



1 = 5x46 = 230. 



But inasmuch as the coil of the galvanometer is formed of a very 

 fine long wire, it may be supposed to oppose a sensible resist- 



