THERMOELECTRICITY 79 



The cobalt supplied by Andrews was probably far from pure ; for with the 

 rod of pure cobalt obtained by electrolytic deposition on aluminium, the 

 aluminium being afterwards dissolved away, J. G. MacGregor and C. M. Smith 

 found that the cobalt thermoelectric line lay below the nickel line and there- 

 fore further away from the iron line 1 . 



Some of the difficulties encountered in these early days are not described 

 either in Tail's Transactions paper, or in the short laboratory notes which Tait 

 communicated from time to time to the Royal Society of Edinburgh. 



Particularly interesting were the experiments on sodium and potassium, 

 the carrying out of which was entrusted to C. Michie Smith and myself. 

 The metals were prepared for Tait by (Sir) James Dewar, who sucked them 

 in the molten state up glass tubes under the surface of melted paraffin and 

 then allowed the whole to solidify. Each of the sodium and potassium bars 

 was thus enclosed in a glass tube, with solid paraffin ends protecting it from the 

 air. The ends were then slightly melted and platinum wires pushed through 

 the paraffin into the sodium or potassium. Sodium-platinum and potassium- 

 platinum circuits were thus constructed. Each bar was only a few inches long, 

 and as the one end had to be kept cool in running water while the other 

 was gently heated in an oil bath, the manipulation of the experiments was 

 not easy. There was moreover some risk of accident to the eye of the 

 operator who attended to the warmer junction. 



Tait seems to have been led into his thermoelectric work in the hope of 

 testing a theoretic result he had obtained with reference to the " Thomson 

 Effect." Experimentally the work was a following up of much earlier investiga- 

 tions made by Thomson himself, to whom indeed the idea of the thermo- 

 electric diagram was due. What Tait did was (i) to establish for most metals 

 and through a considerable range of temperature the parabolic law for electro- 

 motive force, or the linear law for thermoelectric power, in virtue of which 

 each metal was represented by a straight line on the diagram ; (2)10 show how 

 the " specific heat of electricity " was indicated by the inclination of the 

 thermoelectric line and how the Peltier Effect and the Thomson Effect were 

 represented by areas on the diagram ; and (3) to discover the remarkable 

 changes of sign in the Thomson Effect for iron and nickel. His attempts to 

 measure the Thomson Effect directly were not successful, although he made 

 repeated attacks on the problem. For example, by passing a current first in 



1 Working with a fairly pure specimen of rolled cobalt in 1891, I found that its thermo- 

 electric line lay above the nickel line at temperatures below 100 C. but below it at higher 

 temperatures. 



