ON THE THERMAL DIFFUSIVITIES OF DIFFERENT 



KINDS OF MARBLE. 



By B. O. Peirce and R. W. Willson. 



Received May 4, 1900. Presented May 9, 1900. 



Last year we published in the Proceedings of the American Acad- 

 emy of Arts and Sciences an account of some determinations of the 

 thermal conductivities of different kinds of marble, made by the so- 

 called "Wall Method." The horizontal bases of a rectangular prism, 

 of height small compared with the area of a horizontal cross section, 

 were kept for a long time at constant temperatures, the final tem- 

 peratures at two or more points in the vertical axis were determined, 

 and the flux of heat through a definite central portion of the colder base 

 was measured. For the details of the apparatus we refer to that 

 paper. 



In each experiment of one of our sets a rectangular prism 60 cm. 

 square and not more than 6 cm. high was built up of a slab 

 of the material to be tested, enclosed between two other slabs of 

 the same material. Between each two consecutive slabs of the prism 

 was placed a thin metal sheet. This consisted of two rectangular leaves 

 of tinfoil, about 60 cm. long and a little less than 30 cm. wide, 

 placed side by side, and separated by a narrow ribbon thermal element 

 made by butt-jointing, end to end, with the help of silver solder, a 

 strip of German silver, and a strip of copper of the same thickness as 

 the tinfoil. After the edges of the ribbon had been varnished so as to 

 prevent electrical contact, the ribbon and the tinfoil could be placed 

 close together so as to form a continuous sheet of metal 60 cm. 

 square and about 1-10 of a millimetre thick. 



The ribbon thermal elements gave consistent results at all times, 

 provided that the junctions themselves were in contact with the slabs 

 between which they lay. If the tinfoil wings were considerably 

 thicker than the junction-ribbon, or if the junction itself were scraped 

 thin, the reading might be in error by an amount not easy to be 

 accounted for by the mere resistance of the thin air-film on each side of 

 the junction. 



