HALL. — CONDUCTIVITY OF MILD STEEL. 301 



except that of August 13. This heavy line is, therefore, the typical 

 difference-of-potential curve for low temperatures, about 27°. The 

 heavy line of Figure 13 is similarly the typical curve for high tem- 

 peratures, near 59°, the curve for August 15 not being used in ob- 

 taining this composite. 



The difference between these two typical curves is noticeable. Each 

 indicates that the greatest difference of potential is near the edge of 

 the disk. Each shows a minimum about two thirds as great as the 

 maximum, but the minimum of the high temperature curve occurs 

 farther from the centre of the disk. The reason for this dijBference is 

 not obvious. The rapidity of the flow of water was about the same 

 for high temperatures as for low, but there was a greater tendency to 

 development of bubbles in the warmer water, and this may have in- 

 fluenced to some extent the course of the water over the surfaces of 

 the disk. 



The low temperature curve of September 2 is in marked contrast 

 with most of the others in its group, showing a relatively great differ- 

 ence of potential at the edge of the disk. It has already been stated 

 that the copper coatings of the disk had been cleared of accumulated 

 dirt shortly before the experiments of vSeptember 2. This dirt had 

 lodged mostly near the edge of the disk, and its removal apparently 

 had just the effect that would have been expected upon the flow of 

 heat in that region. The value of k obtained September 2 accords 

 well with those obtained shortly before the disk was cleaned. 



As to the absolute values found for k, they are very likely in error 

 to the extent of two or three per cent of their own magnitude, pos- 

 sibly more. A source of possible error not yet discussed is found at 

 the ring which encloses the disk (see Fig. 1). The effective diameter 

 of the disk is taken to be the distance through the ring and disk, 

 from the groove on one side to the groove on the other. The ring 

 itself in this grooved part is about 0.08 cm. thick. Its cross-section 

 is about three per cent of the whole conductive cross-section. The 

 material of the ring is very similar to that of the disk, if not quite 

 the same. The uncertainty as to whether the thin part of the ring 

 can properly be assumed to act like an equal portion of the disk 

 itself, comes from the dubious nature of the approach to this part of 

 the ring above and below. The flow into or from it may be partly 

 lateral, through the edge of the disk proper, but it cannot be wholly 

 so, for a narrow strip of the thicker portion of the ring is exposed to 

 the water above and below the disk. It is not likely that the error 

 here is large, but in future experiments a different method of sup- 

 porting the disks will probably be used. 



