OF THALLIUM, INDIUM, AND TIN IQ 



As has been previously mentioned, the portion of the bridge wire MN 

 corresponding to one ohm was divided into 100 parts, giving direct read- 

 ings to o.oooooi volt for each 6 mm. of wire. The graduation of the 

 instrument was therefore adequate ; improvement was to be attained only 

 by eliminating all irregularities ; and prominent among these, as every one 

 knows, are thermoelectric effects due to junctions of dissimilar metals. 



Two ways of suppressing thermoelectric effects are available: one, to 

 use only one metal ; the other, to keep the temperature the same through- 

 out. The latter method was in the present case the more convenient. It 

 was at first found that the temperature at the two ends of the glass case 

 containing the potentiometer differed by as much as 0.5. Part of this 

 difference was traced to the proximity of an incandescent light, which 

 was removed ; but there still remained a considerable variation. This was 

 finally overcome by the use of a small revolving fan which was attached 

 to an axle run through one of the corners of the case and driven at high 

 speed by a motor. Thus the air was stirred and kept at the same tempera- 

 ture throughout. Contact of the operator's hand with the bridge slide 

 was obviated by the use of two cords attached to opposite sides of X and 

 passed through small holes in the ends of the case; and the final adjust- 

 ment was made on the bridge with the case closed. In this way another 

 frequent source of irregularity was avoided and the readings were im- 

 proved. The room in which all the apparatus was placed was kept as 

 constant in temperature as possible. 



In seeking for the causes of the yet remaining fluctuations, it was found 

 that the galvanometer was influenced by the proximity of the observer, 

 and even more so by heat-effects due to the operation of the rocker switch 

 5" with the hand. Therefore, the galvanometer was removed some dis- 

 tance from the apparatus and screwed against a very firm wall, the con- 

 nections being made by insulated copper wires incased in glass tubes. The 

 case of the galvanometer was packed in felt and covered with a sheath 

 of copper, a small hole permitting a view of the mirror ; and the instru- 

 ment was read by a telescope and scale placed at a distance of about 

 130 cm. The rocker switch 5 was placed inside the case and operated 

 from outside by means of a long cord, the observer being seated at the 

 telescope some distance away. 



The resistance box A was standardized by substitution. A sensitive 

 Wheatstone bridge was used and the corrections on the various resistances 

 were determined and tabulated exactly as if they were weights. 27 



Only two of the corrections thus found were as much as o.oi ohm, and 

 since each o.oi ohm corresponds to very nearly o.oooooi volt, it is easily 

 seen that all others were negligible. The two in error were the 300 and 



" Richards, Proc. Am. Chem. Soc., 22, 144 (1900). 



