HALL. — CONDUCTIVITY OF SOFT IRON. 123 



The relation of the values here giveu for k and x to those found by 

 others who have studied the thermal and electrical conductivities of soft 

 iron, I have set forth in the " Physical Review" for May-June, 1900. 



The following details of ray work are perhaps unnecessarily extended 

 and tedious ; but a considerable study of the literature of thermal con 

 ductivity has convinced me that most experimenters in this field have 

 omitted important matters in the printed description of their investiga- 

 tions. It is my hope that those who may have to deal with problems 

 similar to, though not exactly like mine, will find what is here written 

 worthy of their attention. 



The iron used for tlie expei'iments on thermal conductivity was in the 

 form of a disk cut from the end of a five-inch cylinder and turned down 

 at first to a diameter of 10.5 cm. The thickness of the disk was about 

 1.996 cm., the greatest thickness indicated by the calipers being 1.998 cm., 

 and the least 1.995 cm. 



TREA.TMENT OF THE DiSK. 



The disk was coated with copper electrolytically on both faces by a 

 method substantially the same as that previously described. Spots 

 which, because of slight flaws in the surface, appeared not to be taking 

 the copper well from the preliminary cyanide bath, were rubbed with the 

 point of a lead pencil to give them a coating of graphite, after which 

 they speedily became coppered like the rest. In the sulphate bath the 

 convex surface of the disk was protected as before by rubber bands ; but 

 outside these bands was now placed a band of paraffined paper about 

 5 cm. wide, the object of which was to impede the deposit at the edge of 

 the faces of the disk and so make it keep better pace with the rate of 

 deposit at the centre of the faces. As before, it was occasionally neces- 

 sary during the progress of the deposit, which lasted about a week, to 

 remove the disk from the bath in order to break off or file off projecting 

 pimples, or corals, of copper. 



The final coating of copper on each face, after being turned down 

 nearly to a plane, was about 0.2 cm. thick. The whole curved surface 

 was now turned down until the diameter of the disk was 10.00 cm. 



Mounting and Use of Disk and Adjacent Apparatus. 



Figures 1 and 2 of an article already mentioned, " On The Thermal 

 Conductivity of Cast Iron," indicate with accuracy in most particulars 



