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February, 1925
Fbrging-Stamping- Heat 'Beating
sistance to oxidation it becomes necessary to increase
proportionately the chromium content.
tent is relatively high. The alloys withstand oxidation
fairly well but they are inferior to the nickel-
The electrical properties of this group of alloys are chromium alloys. Another radical objection observed
given in Table IV. The chemical compositions of the was that the wire under continued heating tended to
wires used in these observations are given in Table V. increase permanently in length.
Typical resistance-temperature curves are plotted in
Fig. 3.
+40
TABLE VI. — VARIATION IN RESISTANCE OF IRON-
CHROMIUM ALLOYS WITH TEMPERATURES
Expressed as the ratio of resistance at the various
temperatures to resistance at 20 deg. C.
Temperature,
deg. Cent. No. 1* No. 2
20 1.000 1.000
300 1.129 1.132
400..- 1.175 1.174
500 1.225 1.235
600 1.278 1.283
700 1.316 1.322
800 1.338 1.341
900 1.357 1.355
1000 1.375 1.365
•Plotted in Fig. 2 as typical of iron-chromium alloy. The specific
resistance at 20 deg. C. of alloy No. 1 was 583 ohms per
mil-foot.
Iron-Chromium Alloys.
The iron-chromium alloys have up to the present
received but little attention. Some of these materials,
however, have been introduced on the market so that
some mention should be made of them. The alloys
contain approximately 7 per cent of iron and 22 per
.. '16
g 1.12
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CO c
CO
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CO W
1 ° 1-04
1.00
Temperature, deg. Cent.
Hi' ^
Hicbro*
ej>
l^V£ hromt
ZOO 400 tOO 800
tf_
1000
FIG. 3 — Showing the variation of electrical resistance of
nickel-chromium and nickel-iron-chromium alloys at various
temperatures.
Nichrome III.—Values plotted are Alloy No. 11, Table III;
chromium, 15.70 per cent.
Nichrome IV.—Values plotted are for Alloy D, Table III;
Chromium, 18.95 per cent.
Nichrome.—Values plotted are for Alloy No. 7, Table IV; for
chemical composition see Table V.
Nichrome II.—Values plotted are for Alloy No. 1, Table IV;
for chemical composition see Table V.
cent of chromium, the high content of chromium being
necessary to overcome the tendency of the iron to
oxidize. The silicon content sometimes runs as high
as 2 per cent. From the manufacturer's standpoint the
material is hard to draw, especially if the silicon con­
67
1000
Temperature, deg. Cent.
FIG. 4 — Showing thermal electromotive forces of various
metals and alloys against standard platinum.
Observations on the electrical properties of two of
these wires are given in Table VI. The chemical compositions
of the two wires are as follows:
Iron, per cent
Chromium, per cent.
Nickel, per cent
Silicon, per cent
Carbon, per cent
No. 1
75.10
22.06
0.87
1.67
0.18
No. 2
74.62
22.23
1.17
1.24
0.65
The values obtained for wire No. 1 are plotted in
Fig. 2.
Group C.—Materials With Special Properties.
The third class of alloys to be considered includes
those materials that have some special properties
which render them useful under restricted conditions.
The fact that pure nickel has a remarkably high temperature
coefficient of electrical resistance is made use
of in the' construction of resistance thermometers.
Again, the fact that advance and manganin have remarkably
low temperature coefficients is applied in the
construction of precision resistances.
Electrical Properties of Pure Nickel.
Two samples of nickel wire were obtained directly
from electrolytic sheet. A section cut from the sheet
was rolled to wire. The material was therefore not
contaminated by impurities which usually enter during
melting. From these wires the following results were
obtained:
olytic Nickel
No. 1
No. 2
Specific
Resistance,
Microhm cm.
7.55
7.60
Temperature
Coefficient,
per deg. Cent.
0.00559
0.00553
Temperature coefficient is expressed in ohms per deg. C. per
ohm at 20 deg. C, over the range from 20 to 50 deg. C. The
coefficient for No. 1 is equivalent to a value of 0.00629 per ohm
at 0 deg. C.