THE ELECTRICAL RESISTANCE OF CERTAIN ALLOYS. 607 
Table VIII. gives the measurements made with the fourth of these alloys, of 
which 20 per cent. by mass or about 21°61 per cent. by volume is iridium. 
TABLE VIII. 
Resistance. 
Temperature. 
Observed. Calculated. 
16°5° C. 5°756 5°7535 
41:7 5897 58998 
60:0 6:000 6:0043 
76°9 6:095 6:0996 
94:7 6197 611997 
1D 6:290 6:2883 
129°7 6393 63898 
1479 6:487 64871 
This table gives the formula : 
R=5'6563 + :005925¢— 0000020837’, 
whence 
r=1+ -0010475¢— 000000368322’, 
and 
. k=1—-0010475t— 00000141562’. 
With this last formula we may compare that given by Marruressen* for an 
alloy of the same metals 33:4 per cent. of which by mass was iridium, viz.,— 
k=1—:000645392 + 000000599887’. 
It agrees better with the formula of that alloy of ours which contains 15 per 
cent. of iridium than with that of the one containing 20 per cent. 
If we compare the formule for the different platinum-iridium alloys, we find 
that in the case of those containing 6, 10, and 20 per cent. of iridium, there 
seems to be a general dependence of the values of the coefficients upon the con- 
stitution of the alloy. In the formula of the alloy containing most platinum, the 
coefficients both of ¢ and ¢? have the greatest values. In that of the one which 
contains least platinum they are least, and for the intermediate alloy they have 
intermediate values. But neither our alloy containing 15 per cent. of iridium 
nor MATTHIESSEN’S containing 334 per cent. can take places in such a series. 
The relation is therefore probably only apparent. The groups of alloys which 
MATTHIESSEN and VocT have examined show the same peculiarity, as may be 
seen by an inspection of their tablet Usually the rate of change of the resist- 
ance of the alloy increases with the percentage of one of the constituents, 
although exceptions are almost invariable. 
In our observations on the thermo-electric properties of the same alloys of 
* Rep. of Brit. Ass. (1862), p. 137. + Phil. Trans. Roy. Soc. Lond., vol. cliv. (1864), p. 167. 
VOL XXIX. PART II. Oe 
