HALL. — CONDUCTIVITY OF MILD STEEL. 



287 



concave upward. Line No. 3, a mean between No. 1 and No. 2, is 

 practically a straight line, which, if continued, would indicate a neutral 

 point in the neighborhood of the absolute zero. In subsequent work 

 this stiaight line is assumed to be correct, and data needed for use in 

 the conductivity calculations are taken from it. 



The Electric Circuits. 



Figure 1 shows that the fine copper wires connected with the upper 

 and under copper coatings of the steel disk led out to connections 

 with heavier copper wires on a wooden shelf surrounding the apparatus 

 carrying the disk. Some slight thermo-electric effect could be pro- 

 duced by warming one of these connections, but as the wires from 

 both faces of the disk were connected here in the same way, the con- 

 nections being all equidistant from the disk, the thermo-electric forces 



Fig. 9. 



existing in them must have neutralized each other in the ordinary use 

 of the apparatus. The copper wires leading away from these connec- 

 tions were all of the same length, size, and quality. They terminated 

 in a kind of commutator, or switch-board, shown in Figure 9. The 

 numerous small circles in this figure indicate small holes containing 

 mercury. Holes 1, 2, 3, 4, etc. are connected, by means of the wires 

 just mentioned, with junctions 1, 2, 3, 4, etc., respectively, on the 

 disk. A connector, G, brings the four junctions 1, 4, 7, 10, which 

 are (see Fig. 4) all at the same distance from the centre of the disk, 

 into multiple arc, and takes the currents coming from them to one 



