l8 ELECTROCHEMICAL INVESTIGATION OF LIQUID AMALGAMS 



ment of a measurement, the plugs were all placed in the box A and all 

 removed from B. The resistance between U and V under these conditions 

 was 10,112.06 ohms, embracing a drop of potential of 1.0184 volts, as 

 given above. By removing plugs from A to the corresponding place in B, 

 in order to keep the drop of potential constant, and manipulating the slider 

 X, it was easy to compensate the potential of the unknown cell by opposing 

 potential tapped from the box A and the slide-wire bridge, since the poten- 

 tials measured never amounted to as much as o.i volt. 



Suppose the total reading of the box A and the slide wire to be a ohms. 



Then the potential of the cell measured would be - x 1.0184 volts. 



10,112.06 



The factor - ?-, being a constant value, its logarithm was found once 

 10,112.06 



for all, and entered into all calculations. 



In following this mode of procedure, the only portion of the resistance 

 which needs very accurate calibration is the box A. The wire MN was 

 65 cm. in length, and was divided by a scale into divisions 6.10 mm. long, 

 each of these corresponding to the millionth of a volt. Since the total fall 

 of potential in the wire was only about o.oooi volt and preliminary calibra- 

 tion showed it to be very uniform in resistance, no correction was deemed 

 necessary for the readings of this scale under the wire. PQ was a three- 

 way switch. When thrown towards P, the standard cell H was balanced 

 against the fall of potential between U and V. When thrown toward Q, 

 the potential was ready to be balanced against a portion of the bridge MN 

 and box A. The galvanometer G of the d'Arsonval variety was manu- 

 factured by the Leeds and Northrup Company, of Philadelphia, and is 

 designated by them as Type H. It was read with a telescope and scale 

 at a distance of 60 cm. 6" was a double-rocker switch, the base of which 

 was a thick plate of ebonite. It was so arranged that the galvanometer 

 was either in the circuit or short-circuited itself. The galvanometer was 

 extremely sensitive, and when short-circuited it returned to zero without 

 any oscillations whatever. The whole potentiometer with the exception 

 of the galvanometer was placed inside of a large glass case with a swing- 

 ing door in order to avoid disturbing effects from changes of temperature 

 and impurities in the atmosphere. 



The apparatus, as described above, was used in the measurements on 

 thallium amalgam cells and was easily accurate to within three or four 

 millionths of a volt. Since thallium under the conditions of the measure- 

 ments was univalent, and consequently gave comparatively large poten- 

 tials, the above accuracy was fully sufficient; but in the case of trivalent 

 indium, which for equal concentrations gives potentials only one-third as 

 large as those of a univalent metal, even greater accuracy was desirable. 



