NON-INDUCTIVE RESISTANCE DEVICES
It is often desired to introduce simple ohmic resistance into
telephone circuits, in order to limit the current flow, or to create
specific differences of potential at given points in the circuit.
Temperature Coefficient. The design or selection of resistance
devices for various purposes frequently involves the consideration of
the effect of temperature on the resistance of the conductor employed.
The resistance of conductors is subject to change by changes in
temperature. While nearly all metals show an increase, carbon shows a
decrease in its resistance when heated.
The temperature coefficient of a conductor is a factor by which the
resistance of the conductor at a given temperature must be multiplied
in order to determine the change in resistance of that conductor
brought about by a rise in temperature of one degree.
TABLE V
Temperature Coefficients
+---------------------------+-----------------------------+
| PURE METALS | TEMPERATURE COEFFICIENTS |
+---------------------------+--------------+--------------+
| | CENTIGRADE | FAHRENHEIT |
+---------------------------+--------------+--------------+
| 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 |
| Bismuth | 0.00354 | 0.00197 |
+---------------------------+--------------+--------------+
_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 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 |
| 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.
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 to form two strands, and
these are wound simultaneously on the core, thus forming two spirals
of equal number of turns. The current in traversing the entire coil
must flow through one spiral in one direction with relation to the
core, and in the opposite direction in the other spiral, thereby
nullifying the inductive effects of one spiral by those of the other.
This is called a _non-inductive winding_ and is in reality an example
of differential winding.
_Lamp Filament._ An excellent type of non-inductive resistance is the
ordinary carbon-filament incandescent lamp. This is used largely in
the circuits of batteries, generators, and other sources of supply to
prevent overload in case of short circuits on the line. These are
cheap, durable, have large current-carrying capacities, and are not
likely to set things afire when overheated. An additional advantage
incident to their use for this purpose is that an overload on a
circuit in which they are placed is visibly indicated by the glowing
of the lamp.
[Illustration: Fig. 117. Mica Card Resistance]
[Illustration: Fig. 118. Iron-Wire Ballast]
Obviously, the carbon-filament incandescent lamp, when used as a
resistance, has, on account of the negative temperature coefficient of
carbon, the property of presenting the highest resistance to the
circuit when carrying no current, and of presenting a lower and lower
resistance as the current and consequent heating increases. For some
conditions of practice this is not to be desired, and the opposite
characteristic of presenting low resistance to small currents and
comparatively high resistance to large currents would best meet the
conditions of practice.
_Iron-Wire Ballast._ Claude D. Enochs took advantage of the very high
positive temperature coefficient of iron to produce a resistance
device having these characteristics. His arrangement possesses the
compactness of the carbon-filament lamp and is shown in Fig. 118. The
resistance element proper is an iron wire, wound on a central stem of
glass, and this is included in an exhausted bulb so as to avoid
oxidation. Such a resistance is comparatively low when cold, but when
traversed by currents sufficient to heat it considerably will offer a
very large increase of resistance to oppose the further increase of
current. In a sense, it is a self-adjusting resistance, tending
towards the equalization of the flow of current in the circuit in
which it is placed.