TABLE VII

30 Per Cent German Silver Wire

No. B. & S.
Gauge		Diameter
Inches		Weight
Pounds per Foot		Length
Feet per Pound		Resistance
Ohms per Foot
21.02846.002405415.8.3581
22.02535.001907524.4.4513
23.02257.001512661.3.5693
24.02010.001199833.9.7178
25.01790.00095131051..9051
26.01594.00075441326.1.141
27.01419.00059831671.1.440
28.01264.00047432108.1.815
29.01126.00037612659.2.287
30.01003.00029823353.2.883
31.008928.00023664227.3.638
32.007950.00018765330.4.588
33.007080.00014886721.5.786
34.006304.00011808475.7.297
35.005614.0000935810686.9.201
36.005000.0000741913478.11.60
37.004453.0000588516994.14.63
38.003965.0000466821424.18.45
39.003531.0000370027026.23.26
40.003145.0000293734053.29.32

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.

Fig. 117. Mica Card Resistance
[View full size illustration.]

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 spaced 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 assist in mounting the resistance on a supporting frame, and the posts terminate in soldering terminals by which the resistance is connected into the circuit.