26 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



and its potential was very constant, though not exactly equal to i volt. Its 

 standardization will be considered on page 29. 



The galvanometer G was a sensitive instrument of the d'Arsonval type. Its 

 resistance was nearly 300 ohms. The glass front, which was imperfect, was 

 removed, and the galvanometer was completely inclosed in a tight wooden 

 box not shown in the diagram. A small window of optical glass was placed 

 in the side of the box opposite the mirror, and the opposite wall was cov- 

 ered with black paper. The influence of drafts and of annoying reflections 

 was thus avoided. Deflections were observed through a telescope, T, com- 

 bined with a graduated scale ; the portion of this, whose reflection was seen 

 in the telescope, was illuminated by a small gas flame, F , placed at a safe 

 distance from any junction of unlike metals. A thousand ohms could be 

 introduced into the galvanometer circuit by means of the switch S 2 when 

 circumstances required it. The whole apparatus was set up in a room of 

 constant temperature. 



The cell for containing the amalgams and the method followed in setting 

 it up have already been described. The terminal wires, with great care to 

 avoid momentary short circuits, were attached to brass binding posts, J x 

 and 7 2 , mounted on an ebonite base clamped to the rim of the thermostat. 

 The resistance in E was now adjusted as previously described, after which 

 the switch S 1 was thrown over toward K 2 . Then the potential of the un- 

 known cell was neutralized within a millivolt by placing the connectors Y 

 and Z on the proper pegs, y and z, of box A. After this approximate ad- 

 justment, the compensation of potential was completed by moving the plati- 

 num contact F along the slide wire MN until on pressing the key K 2 no 

 deflection was noted in the galvanometer. Positive indications could be 

 obtained from this instrument from changes in the position of the slider 

 corresponding to five millionths of a volt. Therefore, no interpolation was 

 necessary to get the fifth place correct to within half a unit. To make 

 assurance doubly sure, a point five millionths of a volt on either side of the 

 zero, giving swings of the galvanometer in opposite directions, was always 

 noted. 



The total value, x, of the slider was determined under the conditions 

 actually prevailing in potential measurement. A certain cell was made to 

 read very nearly 0.03200 nominal volt by suitably adjusting its temperature. 

 The following readings were then obtained : 



Box A. Corrected. 



(1) it = o.03300-|- 0.025 X 7T =0.032984-1-0.025 ;r 



(2) 7T = 0.03200 -{-.a: ?r:=o. 03199 -\-x 



