460 Dr. Draper on the Electro-motive Pcwer of Heat. 



Hence there cannot be any doubt, that the quantities of 

 electricity evolved by compound batteries, at the same tem- 

 perature, are directly proportional to the number of the pairs. 



With some general remarks, arising from the foregoing 

 liubjects, I shall conclude this communication. 



1. It is of importance to remember, that thermo-electric 

 currents traverse metallic masses only on account of differences 

 of temperature existing at different points. 



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, diverging from one point 

 and converging to the other. The greater part of the cur- 

 rent is found, however, to take the shortest route. 



3. Combining therefore the foregoing observations (1.2), 

 we perceive, that there are certain forms of construction 

 which will give to thermo-electric arrangements peculiar ad- 

 vantages. For example, the surfaces united by soldering must 

 not be too massive. Let a, fig. 5, be a bar of antimony, and b 

 a bar of bismuth; let them be soldered together along the line 

 c d, and at the point d let the temperature be raised ; a current 

 is immediately excited; but this does not pass around the bars 

 fl, b, in as much as it finds a shorter and readier channel 

 through the metals, between c and d, circulating therefore as 

 indicated by the arrows. Nor will the whole current pass 

 round the bars, luitil the temperature of the soldered surface 

 has become uniform. 



An obvious improvement in such a combination is shown 

 in fig. 6, which consists of the former arrangement, cut out 

 along the dotted lines : here the whole current so soon as it 

 exists is forced to pass along the bars. 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 very best forms for a thermo-electric couple is 

 given in fig. 7, where a is a semicylindrical bar of antimony, 

 b one of bismuth, united together by the opposite corners of a 

 lozenge-shaped piece of copper c. From its exposing so much 

 surface, the copper becomes hot and cold with the greatest 

 promptitude, and from its good conducting power it may be 

 made very thin without injury to the current. With a pair 

 of bars f ths of an inch thick, and a circular copper plate c, 

 having both surfaces blackened, I have repeated the greater 

 part of those experiments which M. Melloni made with his 

 multiplier. 



4. The currents which circulate in a steel magnet are to 



