HALL. — THERMAL AND ELECTRICAL EFFECTS EN SOFT IRON. G09 



ably less than the fairly well-established value, about 0.00005, of the 

 thermal conductivity of " stagnant " air at ordinary temperatures. 

 Rubner^ found /; = 0.000098 for cotton-wool packed closely, about 

 1 gm. to 8 cu. cm., though he found smaller values for wool and silk. 

 Lees and Chorlton ^ found k = 0.00043 for asbestos-paper. They 

 give no description of this paper beyond its name and the thickness, 

 about 0.3 cm., of the layer composed of it. The mean temperature of 

 this layer was about 87°. The value 0.00043 lies between the two, 

 0.00038 and 0.00050, derived above for the effective k of our layer of 

 asbestos fibre ; but, as it is altogether likely that the asbestos-paper 

 of Lees and Chorlton made a much more dense layer than the one 

 which we used, and would therefore have a greater conductivity proper, 

 it seems probable that there was more or less convective movement of 

 air through the layer of fibre as we used it. 



The coefficient of outflow through this layer at 100° was about L7 

 times as great as that found in our previous work with closely packed 

 cotton-wool at 100°. 



In making allowances for the lateral outflow at temperatures inter- 

 mediate between 100° and 218° we assumed that the coefficient of 

 lateral flow, which may be defined as the fraction of a calm'ie lost per 

 second from unit length of a main bar per degree difference of tempera- 

 ture between main bar and guard-ring, increases at a uniform rate 

 with rise of temperature from 100° to 218°. 



It is assumed, furthermore, that this coefficient is practically inde- 

 pendent of the gradient of temperature which exists in the bars and 

 iu the packing parallel to the length of the bars. This is equivalent 

 to the assumption that the various currents of heat through the pack- 

 ing, whether maintained by conduction proper or by convection, are 

 practically independent of each other. As the currents of air through 

 the packing must be comparatively slow, this assumption is not, per- 

 haps, a violent one, though the doubt attaching to it is sufficient, in 

 view of the large corrections Avhich must be made for lateral flow, to 

 make these corrections the least satisfactory part of our work. But 

 see p. 616. 



Temperature Coefficient of Thermal Conductivity. 



On February 7, and again during the night of March 2-3, 1906, 

 observations were made on the temperature coefficient of thermal con- 



6 Archiv fur Hygiene, 24, 320 (1895). 

 T riiil. Mag., 5th Series, 41, 502 (189G). 



VOL. XLH. 39 



