PYRO-ELECTRIC PROPERTIES OF ALLOYS. 
149 
nickel, iridium, and molybdenum, which fall well outside of this zone of 
error, and for such only have thermo-electric inferences been drawn. 
Electrical resistance and its temperature-coefficient. — 1 have now to touch 
upon the main results of the present chapter, i. e., the relation between 
the resistance (* ) and the temperature-coefficient of resistance (a). The 
results in the digest, Table 34, may be best exhibited in a chart, Fig. 26, 
in which a x 10 3 is represented as a function of s . It is well to remark 
in passing that a has been calculated by a linear formula, and from the 
three observations made at ordinary temperatures at 100° and at 357° two 
values of a may be reasonably deduced, since the measurement at room 
temperature is less liable to error. The equation a—(r— r') / {rt' — rt 1 ) 
furnishes a m and a^ 356 , both of which are inserted in Table 36. s applies 
for 0° C. The assumption of the linear form has been wholly a matter of 
convenience, it being desirable to avoid the excessive computation which 
would have been necessitated by quadratic forms. 
The figure contains both ^ 100 and a^ 56 , the former of which is given in 
heavy or large dots 5 the latter is in comparatively light dots. Vertical 
lines pass through each datum, and at their ends the name of the plati- 
num alloy is inscribed. For the sake of comparison, finally, there has 
been introduced into the chart a curve showing the results which Dr. 
Strouhal and I 1 formerly obtained for iron-carburets, the data them- 
selves ranging as follows : 
Specific electrical resistance and electrical temperature-coefficient of steel. Practical table. 
s 
a 
s 
• 
s 
a 
s 
a 
cm 
cm 2 
cm 
r, 0° 
cm 2 
cm 
— 5O 
cm 2 
cm 
5 0° 
cm 2 
microhm. 
microhm. 
microhm. 
microjim. 
10 
0. 0050 
21 
0. 0033 
32 
0. 0022 
43 
0. 0017 
11 
48 
22 
32 
33 
21 
44 
17 
12 
46 
23 
31 
34 
21 
45 
16 
13 
44 
24 
29 
35 
21 
46 
16 
14 
42 
25 
28 
36 
20 
47 
15 
15 
41 
26 
27 
37 
19 
48 
15 
16 
39 
27 
27 
38 
19 
49 
15 
17 
38 
28 
26 
39 
19 
50 
15 
18 
36 
29 
25 
40 
18 
60 
13 
19 
35 
30 
24 
41 
18 
70 
13 
20 
34 
31 
23 
42 
17 
80 
12 
This curve is dotted in the figure, the alloy curve being given in a full 
line. 
Eeturning to the curve for alloys, it appears that the difference be- 
tween a W() and a 356 is not larger than is quite in keeping with the occur- 
rence of reasonable curvilinear relations between resistance and tem- 
perature, and that for the present purposes, where a general survey over 
the whole group of platinum alloys is to be attempted, a m may be 
accepted as coincident in value with the respective tangents of the 
1 Wied. Aim., vol. 20, 1883, p. 525; Bull. U. S. Geol. Survey, No. 14, 1885, p. 19. 
(803) 
