cients, 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 space.

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

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