HALL. — CONDUCTIVITY OF SOFT IRON. 



125 



the change of temperature of the stream of water which flows over the 

 upper surface of the disk. More will be said of this later. The parts 

 surrounded by the water-jacket are carefully and thickly wadded with 

 cotton-wool so as to make a nearly cylindrical body, about 18 cm. in 

 diameter, up to the plugs J x and J 2 . The plugs also are covered with 

 cotton-wool, as well as the slot in the top of the jacket. Around the 

 disk itself the wadding is so thick as barely to allow the jacket to 

 enclose it. 



The copper wires leading out from the plugs J\ and J 2 ran, after 

 July 17, outside the cotton-wool wrapping, but without touching the 

 jacket. 



Determination of the Difference of Temperature 

 of the Two Faces of the Disk. 



As before, this is effected by thermo-electric means, the iron disk and 

 its two copper coatings being used as a thermo-electric couple. As be- 

 fore, thirteen fine copper wires lead 

 off from as many points on the upper 

 coating, and similar wires from cor- 

 responding points on the under coat- 

 ing. Each pair of corresponding 

 wires can be used singly, or all the 

 thirteen pairs can be joined and 

 used in multiple by an arrangement 

 described in a preceding paper. The 

 distribution of wires over either 

 coating is shown in Figure 1, the 

 numerals being alongside the points 

 of attachment to the coating. 



In the arrangement of these points 

 there was an attempt to make the 



various areas represented respectively by the individual points as nearly 

 equal as practicable. Point 13 is intended to beat the centre of the 

 disk ; points 3, 6, 9, and 12, 2.55 cm. from the centre of the disk ; points 

 2, 5, 8, and 11, 3.80 cm. from the centre of the disk; points 1, 4, 7, and 

 10, 4.60 cm. from the centre of the disk. 



No great accuracy is attained in placing these points ; and, in fact, each 

 attachment is rather a line, about 0.5 cm. long, than a point, each such 

 line, except No. 13, crossing nearly at right angles the radius upon which 

 it lies. 



Figure 1. 



