274 PROCEEDINGS OF THE AMERICAN ACADEMY. 



iuvestigate the same matter recently, go to show that this appre- 

 heusiou was groundless, that different specimens of copper wire now 

 purchased in the American market differ very little among themselves 

 in thermo-electric quality, and differ very little from galvanoplastic 

 copper. 



The ordinary method of depositing copper, from a sulphate solution, 

 is not immediately available for a steel plate, because the solution 

 attaclis steel. On the other hand, the method which uses a cyanide 

 solution is comparatively slow, and requires much time for a thick 

 coatiug. Accordingly, the disk here used was subjected to the 

 depositing current in a cyanide solution for a sufficient time to cover 

 the surface completely, aud was then transferred to a sulphate solu- 

 tion, where the process was continued for many hours, until the coating 

 was about 0.05 cm. thick on each side of the disk. 



The letters rr in the Figure mark a ring of steel, similar in quality 

 to the disk. It was turned a bit too small to receive the disk when 

 both were at the same temperature. The ring was then heated, and 

 the disk, smeared on the edge with a i)itcliy cement, was slipped into 

 it. The contraction of the ring in cooling made a joint so tight that 

 under severe tests no leakage through it has been detected. 



The parts h h and lih' are rings of hard rubber. These rings do 

 not extend quite to the copper coatings of the disk, but leave a line 

 of the steel ring rr, about 0.07 cm. wide exposed, above and below 

 these coatings. These exposed edges have caused trouble through 

 rusting, under the action of the water flowing through the apparatus. 

 No varnish protects them effectually. In future experiments care 

 will be taken to have no iron or steel similarly exjjosed. 



The parts hh and h'V are hollow brass cylinders about 10 cm. in 

 external diameter, fitting into grooves in the hard rubber rings h h aud 

 /^'A' respectively, a small rubber tube at the bottom of each groove being 

 used to make, under pressure, a water-tight packing of the joint. At 

 first these brass rings fitted directly into the ring r^-, the rings hh 

 and h'h' not being- used. But it soon appeared that heat, carried to 

 or from the ring r r by the rings h h and b'b' , each of which in the 

 first arrangement exposed a broad surface to the water, marie the 

 difference in temperature between the top and bottom of the disk 

 much greater near the edge than elsewhere. For it is to be noted 

 that the temperature difference between top and bottom of the disk 

 was in most parts only a small fraction, jierhaps 5%, of the maximum 

 temperature cliflFerence between the water above and the water below 

 the disk. 



