374 BRIDGM.AJSr. 



kg. and is 3.8% of the maximum effect at 75°, while the average 

 departure is 2,4%; at 50° the maximum departure is 2.8% of the 

 maximum effect at 50° and the arithmetical average is 1.07%; and 

 at 25° the maximum departure is 3% of the maximum effect at 25°, 

 and the average departure 0.6%. Within these limits of error the 

 relation between e.m.f. and pressure at constant temperature is linear. 

 In passing from curves of e.m.f. at constant temperature to those at 

 constant pressure the maximum readjustment was at 97°, as was to 

 be expected; here it was 12% of the maximum effect. No apprecia- 

 ble readjustment was necessary at the lower temperatures. The 

 following data are to be expected, therefore, to have considerably 

 greater relative accuracy at the lower part of the temperature range. 



The numerical results are shown in Tables XLIII and XLIV and 

 Figures 44 and 45. The effect is negative, increasing regularly with 

 pressure and temperature to —1.31 X 10"^ at 12000 kg. and 100°. 

 The effect is among the smallest found. It is interesting to compare 

 the small effect of pressure on the resistance of constantan and its 

 large effect on thermal e.m.f. with precisely the reverse behavior 

 here. The Peltier heat is negative, increasing with pressure, but at 

 each constant pressure it is nearly independent of temperature. The 

 Thomson heat, on the other hand is positive, at the lower pressures 

 changing but little, but at the higher pressures falling with rising 

 temperature. 



Wagner gives between 0° and 100° and to 300 kg. -8.5 X lO^^^ 

 volts per degree per kg. The value given by interpolation of the data 

 above is —11.6 X 10""^^. The agreement is perhaps as close as could 

 be expected in an alloy from presumably different sources. 



Effect of Tension on Thermo-Electric Quality. 



The general method of measurement is the same as that employed 

 in measuring the effect of hydrostatic pressure on thermo-electric 

 quality, namely to measure the thermal e.m.f. of a couple composed 

 of two branches of the same metal, one of which is under tension, and 

 the other of which is free. The connections are shown schematically 

 in Figure 40. A length of the wire to be measured is doubled on itself, 

 running from A to E to C. At E it is fastened to a heavy block 

 capable of withstanding any load that is to be applied to the wire. 

 At A and C the ends of the wire are fastened to the under sides of the 

 left and right hand pans respectively of two equal arm balances. 



