336 THE ELECTRO-MOTIVE POWER OF HEAT. [MEMOIR XXIV. 



2. When a current of electricity, flowing from the poles 

 of a battery, is made to traverse a metallic sheet, the 

 whole of it does not pass in a straight line from one pole 

 to the other, but diffuses itself through the metal, di- 

 verging from one point and converging to the other. 

 The greater part of the current is found, however, to 

 take the shortest route. 



3. Combining, therefore, the foregoing observations 

 (1, 2), we perceive that there are certain forms of con- 

 struction which will give to thermo-electric arrangements 

 peculiar advantages. For example, the surfaces united 



by soldering must not be too mass- 

 ive. Let A, Fig. 55, be a semi- ring 

 of antimony, and B a semi-ring of 

 bismuth ; let them be soldered to- 

 gether along the line C H, and at 

 the point H let the temperature be 

 raised : a current is immediately ex- 

 cited ; but this does not pass around 

 the ring A B, inasmuch as it finds a 

 shorter and readier channel through the metals, between 

 H and d, circulating therefore as indicated by the arrows. 

 Nor will the whole current pass round the ring until the 

 temperature of the soldered surface has become uniform. 

 An obvious improvement in such a combination is 

 shown at H in Fig. 56, which con- 

 sists of the former arrangement cut A 

 out alone: the dotted lines : here the 



o 



whole current so soon as it exists is 

 forced to pass along the ring. And 

 because the mass of metal has been diminished at the 

 line of junction, such a pair will change 

 its temperature very quickly. 



One of the best forms for a ther mo- 

 Fig. 57. electric couple is given in Fig. 57, where 



