416 
Proceedings of the Royal Society 
Monday, 7th December 1874. 
Sir WILLIAM THOMSON, President, in the Chair. 
The Keith Prize for the Biennial Period (1871-73) having 
been awarded by the Council to Professor Tait for his Paper, 
entitled “ First Approximation to a Thermo-Electric Diagram,” 
which has been published in the Transactions, the Medal was 
delivered to him by the President at the commencement of the 
meeting. 
The President said — I have now the pleasing duty of awarding 
the Keith Medal, for 1871-73, to Professor Tait, for a paper 
entitled “First Approximation to aThermo-ElectricDiagram.” This 
paper is published in the number of the Transactions which I hold 
in my hand, being Part 1. of Volume XXVII., for Session 1872-73. 
The Society, considering the remarkable interest which attaches to 
the subject of this paper, and the very important discovery which 
it contains, will, I am sure, pardon me if I take up a little of their 
time by referring to some of the antecedents of Professor Tait’s 
investigation. In the first place, there was the great discovery by 
Seebeck, about the year 1821, of thermo-electric currents. Accord- 
ing to this discovery, if a circuit be formed of two different metals, 
and if the two junctions be kept at different temperatures, an 
electric current will be found to go round the circuit : — that is 
Seebeck’s great discovery. Quickly following upon it was the 
very remarkable discovery made by Cumming, the late professor 
of chemistry in the University of Cambridge, that there is in 
certain pairs of metals an inversion of the thermo-electric cur- 
rent of a very remarkable kind. A few words are necessary to 
make it intelligible. Take a circuit of, let us say, bismuth and 
antimony. Let one of the junctions be kept at an ordinary tem- 
perature. Let the other junction be gradually raised in tempera- 
ture. With a proper instrument for measuring the strength of the 
current flowing through the circuit, it will be found that the 
strength of the current gradually increases as the hot junction is 
made hotter and hotter. That was part of Seebeck’s discovery. 
