1 2 Sosma » — Platinum- Rhodium. Thermoeh m en t. 



them are due partly to the platiimin wire and partly to tlie 

 alloy. Element Z, for instance, reads lower than E chietly 

 because the platinum wire of Z, is more impure than that of 

 E ; the effect of this impurity is partly neutralized by an 

 apparently larger amount of rhodium in the alloy wire. This 

 appears from the data in the table below, which show com- 

 parisons between several typical 10 per cent elements. The 

 purest platinum appears to be that of J. If the thermoelec- 

 tric eifect of rhodium is proportional to its percentage from 

 to 1 per cent, then about 0'05 per cent rhodium in the plati- 

 num wire would be sufficient to produce the difference between 

 Z and E. The data are in microvolts. 



E.M.F. ofPt E.M.F. ofEh Difference 



wire against Pt wire against Eh between , 



Element of E at 1500° of E at 1500° elements 



Y + 12 +75 + 63 



Z -1-177 +67 —110 



A + 75 +47 — 28 



F... +7 +1 — 6 



J — 9 +1 +10 



The data of Table III are plotted in fig. 3, which shows the 

 relation between temperature and thermal e.m.f. for various 

 alloys. The 30 per cent and 40 per cent curves represent the 

 data of Holborn and Wien. The curve for pure rhodium 

 represents the mean of the two samples of Holborn and Day. 

 There is no indication of a break in any of the curves, over 

 the entire range of temperature. 



In fig. 4 the data of Table III are plotted to show the rela- 

 tion of the thermal e.m.f. at various constant temperatures to 

 the composition of the alloy wire, the cold junction being in 

 every case at 0°. At all temperatures the e.m.f. increases very 

 rapidly with the first additions of rhodium, and at 20 per cent 

 the value has already reached 81 to 93 per cent of the e.m.f. 

 of platinum against pure rhodium. 



The thermoelectric power, or rate of change of e.m.f. with 



de 

 temperature, -r., is plotted in fig. 5 against the atomic concen- 

 tration of the alloy. The values are in microvolts, against 

 pure platinum. The curves for all temperatures are similar 

 in form and approach the curve for 1755° as an envelope. 



In a recent study of the thermoelectric properties at low 

 temperatures of the alloys of tellurium with antimony, tin, and 

 bismuth, and of antimony with silver, Haken'^' comes to the 

 conclusion that a thermoelectric curve of the form of those in 

 fig. 5 accompanies the formation of a solid solution between 

 the end components, while compounds are marked by sharp 

 maxima or minima. The thermoelectric curves of the systems 



* Verh. Deutsch. Phys. Ges., xii, 229-39, 1910. 



