Measurement of the Peltier Eject. 457 



The difference of temperature between the cylinders was 

 indicated by four thermocouples of copper and constantan 

 wires. These junctions were laid in grooves and insulated 

 from the cylinders by thin strips of mica. The grooves were 

 filled in with cotton-wool, and this together with the junctions 

 was tied to the cylinders with silk thread. The available 

 galvanometer was of the suspended -coil type having a re- 

 sistance of 45 ohms, a period of about 10 seconds, and a 

 figure of merit of 350 mm. scale-divisions per microampere. 

 The galvanometer, when connected to the thermo-junctions, 

 gave a deflexion of about 10 mm. per 0*01 ampere from one 

 cylinder to the other. 



The difference of temperature between the copper-bismuth 

 junctions may also be observed by interrupting the currents 

 through the rod and heating coil and connecting the coppers 

 with the galvanometer. Stray thermoelectric forces in the 

 use of this method would be relatively more important since 

 the thermoelectric power of this junction is only about one- 

 third of that of the four copper-constantan couples. This 

 method would be suitable with a galvanometer of low re- 

 sistance and short period. The temperature of each cylinder 

 was measured by a copper-constantan junction f, g, soldered 

 to the copper. 



The various leads were insulated from one another in gla^s 

 tubes and passed through a copper tube, p q, which was 

 soldered to the cover of the enclosure. The constant-tempe- 

 rature enclosure was a small rectangular copper vessel with 

 walls 2 mm. thick. This was designed for the small gap 

 between the poles of an electromagnet, and in this position 

 the temperature could be reduced by immersing the copper 

 rod extension r in ice and water. 



The currents through the junctions and the heating coil 

 were both sent through the same copper lead, n, o, and 

 therefore, if necessary, could he easily interrupted simul- 

 taneously. The currents were supplied from two separate 

 batteries of accumulators and could be varied almost inde- 

 pendently. Each of these currents could be measured to 

 about 1 part in 500 by moving coil ammeters. 



In making an experiment the current through the heating 

 coil was kept constant, and that through the junctions was 

 adjusted so that the ther mo-junctions gave a small steady 

 deflexion. The exact compensating current through the 

 junctions was deduced from the change of deflexion of the 

 galvanometer, produced by a small change of current through 

 the junctions. Since both the currents traverse the same 

 copper lead to cylinder, it is necessary to reverse the heating 



Phil Mag, S. 6. Vol. 2L No. 124, April 1911. 2H 



