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.

TABLE VII

30 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  |    .002405      |     415.8      |     .3581     |
|   22    |  .02535  |    .001907      |     524.4      |     .4513     |
|   23    |  .02257  |    .001512      |     661.3      |     .5693     |
|   24    |  .02010  |    .001199      |     833.9      |     .7178     |
|   25    |  .01790  |    .0009513     |    1051.       |     .9051     |
|   26    |  .01594  |    .0007544     |    1326.       |    1.141      |
|   27    |  .01419  |    .0005983     |    1671.       |    1.440      |
|   28    |  .01264  |    .0004743     |    2108.       |    1.815      |
|   29    |  .01126  |    .0003761     |    2659.       |    2.287      |
|   30    |  .01003  |    .0002982     |    3353.       |    2.883      |
|   31    |  .008928 |    .0002366     |    4227.       |    3.638      |
|   32    |  .007950 |    .0001876     |    5330.       |    4.588      |
|   33    |  .007080 |    .0001488     |    6721.       |    5.786      |
|   34    |  .006304 |    .0001180     |    8475.       |    7.297      |
|   35    |  .005614 |    .00009358    |   10686.       |    9.201      |
|   36    |  .005000 |    .00007419    |   13478.       |   11.60       |
|   37    |  .004453 |    .00005885    |   16994.       |   14.63       |
|   38    |  .003965 |    .00004668    |   21424.       |   18.45       |
|   39    |  .003531 |    .00003700    |   27026.       |   23.26       |
|   40    |  .003145 |    .00002937    |   34053.       |   29.32       |
+---------+----------+-----------------+----------------+---

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

Differentially-Wound Unit. Another type of resistance coil is that in which the winding is placed upon an insulating core of heat-resisting material and wound so as to overcome inductive effects.  In order to accomplish this, the wire to be bound on the core is doubled back on itself at its middle portion