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

+---------+----------+-----------------+---------------

-+---------------+
|   No.   |          |                 |                |               |
| B. & S. | DIAMETER |     WEIGHT      |     LENGTH     |  RESISTANCE   |
| GAUGE   |  INCHES  | POUNDS PER FOOT | FEET PER POUND | OHMS PER FOOT |
+---------+----------+-----------------+----------------+---

------------+
|   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       |
+---------+----------+-----------------+----------------+---

------------+

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.