458 Mr. F. W. Jordan on tie Direct 



current through the coil in order to eliminate its Joule effect 

 in the lead. This was done, and the mean compensating 

 current through the junctions was observed. The unequal 

 emissivities of the two copper cylinders were eliminated by 

 reversing the current through the junctions and passing the 

 heating current through the other coil. 



To obtain the temperature of each junction, one of the 

 thermo-junctions /was connected in series with another and 

 the galvanometer. The second junction was immersed in 

 water, and this was warmed until the galvanometer was 

 un deflected. The temperature of the water, as read on a 

 thermometer, was then the temperature of the copper cylinder 

 and each junction. 



The compensating current through the junctions was also 

 determined by using the copper-bismuth junction as an 

 indicator of the temperature difference between the cylinders. 

 It was found that, in the two methods, the compensating 

 currents differed by 005 ampere approximately for a given 

 heating current. It follows that the insulated thermocouples 

 could be relied upon to indicate the temperature difference 

 between the cylinders. 



It is assumed, in calculating the Peltier coefficient, that 

 the temperature of each copper cylinder is practically uniform 

 throughout, and that possible Peltier and Thomson effects 

 external to the surface of contact are negligible. There is 

 no doubt that the fusing of the bismuth rod to the copper 

 damages the crystalline structure at the end of the rod, and 

 so produces a thin transition layer through which the Peltier 

 effect is distributed. The sum of the Peltier effects across 

 this thin layer is measured in this experiment, and according 

 to the laws of the thermoelectric circuit this total Peltier 

 effect is equal to the Peltier effect between the copper and 

 the crystallized bismuth. 



Let Q=rate of supply of energy by heating coil ; 



= mean compensating current through junctions for 

 a given current in one heating coil ; 



P = Peltier coefficient j 



r = effective resistance of the metals about a junction ; 



£ = rate of loss of energy from a copper cylinder pet- 

 degree excess of temperature above the en- 

 closure ; 



t = excess of temperature of each cylinder above the 

 enclosure. 



Then the following equations hold for a current Cj through 



