HALL. 



CONDUCTIVITY OF MILD STEEL. 



295 



Thus, according to the observations of September 2, the conduc- 

 tivity of the steel at 27° C. is 0.1312. 



The difference between k^ and k^, is large in this case, but it has al- 

 ready been explained that neither is intended to be complete by itself. 

 The mean of the two is necessary to eliminate certain errors which have 

 been discussed. It may well be doubted whether this elimination is per- 

 fect when the difference between k^ and kf, is so large. It will presently 

 appear that the difference was exceptionally large on September 2. 



The following table includes all the results obtained with the appa- 

 ratus in its final condition. It has already been stated that the results 

 of August 13 and August 15 do not agree with those found later, and 

 a possible reason for this has been given, namely, lack of sufficient 

 precaution, in the earlier experiments, against error from disturbing 

 thermo-electric effects external to the conduction apparatus proper. 

 Although the results obtained August 13 and 15 are here given, they 

 are not included in finding the mean values given at the foot of each 

 column. At one time it was suspected that an accumulation of dirt 

 forming near the edge of the upper face of the disk, and checking the 

 flow of heat there, might account for the discrepancy between the 

 earlier and the later results. Accordingly, between August 28 and 

 September 2 the conduction apparatus was dismounted, and the disk 

 was cleaned and replaced. But the result obtained September 2 was 

 in close agreement with most of the others. 



