Volume I Part 16 (1/2)

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PURE METALS

TEMPERATURE COEFFICIENTS

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CENTIGRADE

FAHRENHEIT

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Silver (annealed)

0.00400

0.00222

Copper (annealed)

0.00428

0.00242

Gold (99.9%)

0.00377

0.00210

Aluminum (99%)

0.00423

0.00235

Zinc

0.00406

0.00226

Platinum (annealed)

0.00247

0.00137

Iron

0.00625

0.00347

Nickel

0.0062

0.00345

Tin

0.00440

0.00245

Lead

0.00411

0.00228

Antimony

0.00389

0.00216

Mercury

0.00072

0.00044

Bis.m.u.th

0.00354

0.00197

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_Positive and Negative Coefficients._ Those conductors, in which a rise in temperature produces an increase in resistance, are said to have positive temperature coefficients, while those in which a rise in temperature produces a lowering of resistance are said to have negative temperature coefficients.

The temperature coefficients of pure metals are always positive and for some of the more familiar metals, have values, according to Foster, as in Table V.

Iron, it will be noticed, has the highest temperature coefficient of all. Carbon, on the other hand, has a large negative coefficient, as proved by the fact that the filament of an ordinary incandescent lamp has nearly twice the resistance when cold as when heated to full candle-power.

Certain alloys have been produced which have very low temperature coefficients, and these are of value in producing resistance units which have practically the same resistance for all ordinary temperatures. Some of these alloys also have very high resistance as compared with copper and are of value in enabling one to obtain a high resistance in small s.p.a.ce.

One of the most valuable resistance wires is of an alloy known as _German silver_. The so-called eighteen per cent alloy has approximately 18.3 times the resistance of copper and a temperature coefficient of .00016 per degree Fahrenheit. The thirty per cent alloy has approximately 28 times the resistance of copper and a temperature coefficient of .00024 per degree Fahrenheit.

For facilitating the design of resistance coils of German silver wire, Tables VI and VII are given, containing information as to length, resistance, and weight of the eighteen per cent and the thirty per cent alloys, respectively, for all sizes of wire smaller than No. 20 B. & S. gauge.

Special resistance alloys may be obtained having temperature coefficients as low as .000003 per degree Fahrenheit. Other alloys of nickel and steel are adapted for use where the wire must carry heavy currents and be raised to comparatively high temperatures thereby; for such use non-corrosive properties are specially to be desired. Such wire may be obtained having a resistance of about fifty times that of copper.

TABLE VI

18 Per Cent German Silver Wire

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No.

B. & S.

DIAMETER

WEIGHT

LENGTH

RESISTANCE

GAUGE

INCHES

POUNDS PER FOOT

FEET PER POUND

OHMS PER FOOT

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21

.02846

.002389

418.6

.2333

22

.02535

.001894

527.9

.2941

23

.02257

.001502

665.8

.3710

24

.02010

.001191

839.5

.4678

25

.01790

.0009449

1058.

.5899

26

.01594

.0007493

1335.

.7438

27

.01419

.0005943

1683.

.9386

28

.01264

.0004711

2123.

1.183

29

.01126

.0003735

2677.

1.491

30

.01003

.0002962

3376.

1.879

31

.008928

.0002350

4255.

2.371

32

.007950

.0001864

5366.

2.990

33

.007080

.0001478

6766.

3.771

34

.006304

.0001172

8532.

4.756

35

.005614

.00009295

10758.

5.997

36

.005000

.00007369

13569.

7.560

37

.004453

.00005845

17108.

9.532

38

.003965

.00004636

21569.

12.02

39

.003531

.00003675

27209.

15.16

40

.003145

.00002917

34282.

19.11

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Inductive Neutrality. Where the resistance unit is required to be strictly non-inductive, and is to be in the form of a coil, special designs must be employed to give the desired inductive neutrality.

Provisions Against Heating. In cases where a considerable amount of heat is to be generated in the resistance, due to the necessity of carrying large currents, special precautions must be taken as to the heat-resisting properties of the structure, and also as to the provision of sufficient radiating surface or its equivalent to provide for the dissipation of the heat generated.

Types. _Mica Card Unit._ One of the most common resistance coils used in practice is shown in Fig. 117. This comprises a coil of fine, bare German silver wire wound on a card of mica, the windings being so s.p.a.ced that the loops are not in contact with each other. The winding is protected by two cards of mica and the whole is bound in place by metal strips, to which the ends of the winding are attached. Binding posts are provided on the extended portions of the terminals to a.s.sist in mounting the resistance on a supporting frame, and the posts terminate in soldering terminals by which the resistance is connected into the circuit.

TABLE VII

30 Per Cent German Silver Wire

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No.

B. & S.

DIAMETER