HALL AND AYRES. — HEAT CONDUCTION IN IRON. 305 



spring balance at a point too far down to be shown in place. "With the 

 rubber tube in use the pressure against the end of the column was per- 

 haps 1.8 kilograms. When the lever and balance were used, it was some- 

 times about three kilograms and sometimes less. 



The various jacketing and protecting devices shown in Figure 5 were 

 not all used at first, and in the early experiments on soft iron the e. m. f. 

 obtained from the column of short bars was several per cent less than 

 that obtained from the 15 cm. soldered bar, with a given difference of 

 temperature between the two thermometers. This discrepancy gradually 

 diminished as the method of experimentation was improved, until at last 

 it became little or nothing, as the following numbers, obtained with the 

 system of jacketing shown in Figure 5, will testify. 



The e. m. f. is here given in terms of a purely arbitrary unit. The 

 values in brackets were obtained under conditions of special uncertainty 

 as to resistance. Considering the final trials of the end-to-end short bar 

 method satisfactory, I prdceeded to apply it to cast iron. From the end 

 of the slab that furnished the conductivity disk a slice was cut crosswise, 

 about 10 cm. long, 2.5 cm. wide, and 0.3 cm. thick. This was cut up 

 into 26 parts, and each of these parts was turned down to a thickness 

 of about 0.16 cm.; or, rather, 18 of them were so treated, the other 

 8 being broken at some stage of the operation. They were then boiled 

 for about 20 minutes in a strong solution of caustic potash, partly 

 to free them from oil, partly because the disk had been thus heated 

 before its conductivity was tested. In all of this work an attempt Avas 

 made, and I think a successful one, to keep the bars in the same order 

 with respect to each other that they had before being cut from the slice, 

 so that I could at the end tell what bars had been taken from near the 

 end of the slice and what ones from near the middle. 



When ready for the test of thermo-electric quality, I rubbed the flat 

 ends of each little bar bright with infusorial earth, and wiped them care- 

 fully ; for it is evident that a particle of dirt or of vegetable fibre left 



VOL. XXXIV. — 20 



