Barrett — On some novel Thermo-Electric Phenomena. 131 



have found that reducing the nickel in the alloy from 25 to 19 per cent., the 

 other constituents remaining the same, does not destroy the sudden arrest of 

 E. M. F. at about 300° C. ; but the range of temperature where the E. M. F. is 

 nearly constant is less, extending from about 400° to 750° C. 



It has been suggested that the peculiar thermo-electric propert}'^ of this alloy 

 may be connected with the effect observed by Lord Kelvin, the so-called Thomson 

 effect, whereby a kind of electric convection of heat occurs. It is very possible 

 that it may have some connexion with this, and hence with the neutral pointy which 

 occurs in the thermo-electric behaviour of certain pairs of metals, such as copper 

 and iron. It is well known that at a certain critical difference of temperature 

 between the hot and cold junctions of, say, a copper-iron couple, the potential 

 difference due to heat disappears ; as the temperature rises, inversion of the 

 current occurs ; and a second inversion may occur at a still higher temperature. In 

 the phenomenon described in this paper, the E. M. F., it is true, does not fall to 

 zero. If, however, the cooler junction were kept at a temperature of 310° C, this 

 would be the case ; and we should then have a series of three successive small 

 inversions of E. M. F., occurring at 540°, 810°, and 1030° C. In a copper-iron 

 couple the neutral point is 275° C, that is to say, no E. M. F. is produced by heat 

 when one of the junctions is as much above 275° as the other is helow that 

 temperature, the neutral point being the arithmetic mean of the temperatures of 

 the hot and cold junctions. If, therefore, the cold junction in a copper-iron couple 

 be kept at 0° C, the thermo-electric current falls to zero when the hot junction 

 is at 550° C, the current being inverted as the temperature rises beyond this. 



I have noticed that the temperature of the neutral point in a copper-iron, or 

 copper-steel couple, is not the same during the heating as during the cooling of the 

 couple. Moreover, in a couple formed of copper and mild carbon steel, the 

 neutral point becomes lower in successive heatings. Thus, at the /rs^ heating 

 the temperature of inversion was approximately 640° C, and in cooling, 500° C ; 

 in the second heating, 550° C, and in cooling, 465° C. ; in the third heating, 

 520° C, and in cooling, 465° C. The difference between the temperature of 

 inversion in heating and cooling becoming less in each successive heating and 

 cooling. The cool junction 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° C. 



When cooling, . . . 258° 241° 241° C. 



It is obvious, therefore, that the curve representing the thermo-electric force of 



