HALL AND AYRES. — HEAT CONDUCTION IN IRON. 30i 



APPENDIX I. 



Measurement of the Thermo-Electric Quality op 

 Short Iron Bars. 



"When it became necessary to determine, relatively to copper, the 

 thermo-electric quality of the cast iron disk thickness-wise, the problem 

 appeared to be one of some difficulty. The thickness of the disk was 

 about 1.8 cm. The thickness of the slab from which the disk had been 

 taken was such that bars 2 cm. long could be cut from it thickness-wise; 

 but to make satisfactory thermo-electric measurements upon a single bar 

 of this length appeared to be impracticable. The device of putting a 

 number of such bars end to end, so as to make a column of considerable 

 length, and placing this column lengthwise between two blocks of copper 

 of different temperatures, seemed a hopeful one; but it had to be put 

 to the proof before it could be used with confidence. 



Accordingly a very soft magnet core rod, about 0.16 cm. in diameter, 

 was taken, and from it were cut one piece 15 cm. long and ten pieces 

 each 2 cm. long. Copper wires were soldered to the ends of the 15 cm. 

 piece, and this piece was then mounted very much as the piece I^L is 

 mounted in Figure 4. The parts exposed to the streams were now, how- 

 ever, some 5 cm. long, about twice as long as the exposed parts in sim- 

 ilar preceding tests. A thin coating of paraffine was now used to protect 

 these parts from the chemical action of the water. 



The ten 2 cm. pieces, after being carefully cleaned and polished at the 

 end surfaces, were placed end to end in a wooden tube, which in all its 

 dimensions was much like the wood of a common pencil from which the 

 graphite has been taken out. The iron, corresponding in position to the 

 graphite of a pencil, projected from the wood about 0.2 cm. at each end. 

 R R in Figure 5 (Plate II) shows in diagonal lines a section of the 

 wooden tube, or rod, the iron witliin being indicated by a heavy black 

 line ; the scale of the figure is \. Water jackets, ^ and J^ in Figure 5, 

 surrounded R R for the greater part of its length. The iron column 

 projecting from R R was pressed between two copper blocks B^ B^ and 

 B^ B^, through which flowed streams of water at any temperature re- 

 quired. The pressure was applied by means of a wooden plunger p, sup- 

 ported in the block 5, and pushed against ^2 -^2 by ^ fairly constant force. 

 The blocks B^ B^ and B^ B„ were of the same diameter as the jackets ji 

 and J2, and all of these objects rested in a slot cut lengthwise in a piece 



