Novel Thermo- Electric Phenomena. 315 



tion was here at 16° C. throughout ; the neutral points at the 

 1st, 2nd, and 3rd heatings and corresponding coolings of the 

 couple, would therefore be approximately as follows : — 



Neutral Points of Copper-steel Couple. 



First, Second, Third Heating. 



When heating . . . 328° 283° 268° G. 



When cooling . . . 258° 241° 241° 0. 



It is obvious therefore that the curve representing the 

 thermo-electric force of a copper-steel couple at different 

 temperatures is not the same for a rising as for a falling- 

 temperature. This I have found to be the case with couples 

 formed of several other metals, provided one element of the 

 couple is iron or steel or other alloy of iron. When the 

 other metal is platinum, the difference in the two curves is well 

 seen, though with a platinum-iron couple the difference of 

 E.M.F. in heating and cooling is less marked than with a 

 platinum-steel couple. In the latter case, a considerable area 

 is enclosed by the curves (representing the relation between 

 thermo-electric force and temperature) during heating and 

 during cooling. Hence, at a given temperature, say 500 C, 

 of the hot junction, the E.M.F. of a platinum-steel couple is 

 considerably higher during heating than during cooling. 



The reverse is the case with a couple formed of Hadfield's 

 nickel-manganese-steel and copper, or a couple formed of the 

 same alloy with platinum ; in both these cases at any given 

 temperature, the E.M.F. is lower during heating than during 

 cooling. With a couple formed of the same alloy and iron, 

 as described in the earlier part of this paper, there is also a 

 slightly lower E.M.F. at corresponding temperatures during 

 heating than during cooling; but the difference only exists at 

 certain parts of the scale, and is so small that it could not be 

 shown on the curve as reduced in the text. With the nickel- 

 manganese-steel alloy (containing 19 instead of 25 per cent, 

 of nickel) coupled with iron, the E.M.F. is slightly higher 

 at corresponding temperatures in heating than in cooling up 

 to the level part of the curve, i. e. about 400° C, where the 

 E.M.F. becomes almost the same in heating as in cooling — 

 very slightly lower, however, — and remains so until the curve 

 rises, when the E.M.F. again becomes higher at corresponding 

 temperatures in heating than in cooling up to the highest 

 temperature reached : in this case, therefore, the curves 

 showing the E.M.F. during heating and cooling cross each 

 other twice, first at about 400° and next at about 800° C. 



I hope, in a subsequent paper, to give the results of further 

 investigation which 1 am pursuing on this interesting pheno 



