HALL. — CONDUCTIVITY OF MILD STEEL. 285 



"With these data the line given in Figure 7 was, obtained. It is a 

 straight line, the inclination of which , indicates a neutral point in the 

 neighborhood of 400° C. 



Calibration of Copper-German-Silver Junctions. 



It has already been stated that junctions of copper and German- 

 silver were used to measure the difference of temperature of the 

 incoming and outgoing water of the upper chamber. These junctions, 

 held firmly in the plugs p, />, of Figure 1, were tested in the same 

 apparatus and by the same method that had been used with the 

 copper-steel junctions, the ends of the plugs fitting into the short 

 side tubes occupied by rubber stoppers in Figure 6. The results 

 presently to be recorded were obtained with two pairs of junctions 

 called respectively No. 1 and No. 2. The same piece of german- 

 silver wire was used in both pairs. The copper wires of No. 2 were 

 not the same as those of No. 1, but all were taken from the same 

 spool, that is, all had once been parts of one continuous wire. 



The following method was sometimes used in making the junc- 

 tions. The German-silver wire was heated in melted parafRne to the 

 neighborhood of 230° C, the object being to forestall any change of 

 its properties which might otherwise take place in the heat of solder- 

 ing. In order to make sure that this temperature was not exceeded 

 in the soldering, a bath of melted paraffine of about the same tempera- 

 ture was used to heat the soldering "iron." This device was aban- 

 doned after a time, experience seeming to show that it was unneces- 

 sary, and the junctions of Nos. 1 and 2, just mentioned, were not made 

 in this way. The junctions of No. 1 were not coated with copal to 

 protect them against chemical action of the water, but rosin was used 

 in making this pair of junctions, and this left a partial coating which 

 gave considerable protection. Experiments had appeared to show 

 that a protective coating was not necessary. A test for disturbance 

 due to chemical action with this pair of junctions was satisfactory, no 

 evidence of such an efi^ect being found. In making No. 2, however, 

 the junctions were carefully coated with shellac, dried on at a tem- 

 perature of about 80°. All of the conduction experiments of which 

 the results will be given in this paper were made with No. 2, except 

 those of August 13, which were made with a similar pair. 



