Cyclopedia of Telephony and Telegraphy Volume I Part 16

+---------------------------+-----------------------------+ 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 Bis.m.u.th 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 s.p.a.ce.

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 s.p.a.ced 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 a.s.sist 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.

[Ill.u.s.tration: Fig. 117. Mica Card Resistance]

[Ill.u.s.tration: 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 gla.s.s, 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.

CHAPTER XII

CONDENSERS

Charge. A conducting body insulated from all other bodies will receive and hold a certain amount of electricity (a charge), if subjected to an electrical potential. Thus, referring to Fig. 119, if a metal plate, insulated from other bodies, be connected with, say, the positive pole of a battery, the negative pole of which is grounded, a current will flow into the plate until the plate is raised to the same potential as that of the battery pole to which it is connected. The amount of electricity that will flow into the plate will depend, other things being equal, on the potential of the source from which it is charged; in fact, it is proportional to the potential of the source from which it is charged. This amount of electricity is a measure of the capacity of the plate, just as the amount of water that a bath-tub will hold is a measure of the capacity of the bath-tub.

Capacity. Instead of measuring the amount of electricity by the quart or pound, as in the case of material things, the unit of electrical quant.i.ty is the _coulomb_. The unit of capacity of an insulated conductor is the _farad_, and a given insulated conductor is said to have unit capacity, that is, the capacity of one farad, when it will receive a charge of one coulomb of electricity at a potential of one volt.

Referring to Fig. 119, the potential of the negative terminal of the battery may be said to be zero, since it is connected to the earth. If the battery shown be supposed to have exactly one volt potential, then the plate would be said to have the capacity of one farad if one coulomb of electricity flowed from the battery to the plate before the plate was raised to the same potential as that of the positive pole, that is, to a potential of one volt above the potential of the earth; it being a.s.sumed that the plate was also at zero potential before the connection was made. Another conception of this quant.i.ty may be had by remembering that a coulomb is such a quant.i.ty of current as will result from one ampere flowing one second.

The capacity of a conductor depends, among other things, on its area.

If the plate of Fig. 119 should be made twice as large in area, other things remaining the same, it would have twice the capacity. But there are other factors governing the capacity of a conductor. Consider the diagram of Fig. 120, which is supposed to represent two such plates as are shown in Fig. 119, placed opposite each other and connected respectively with the positive and the negative poles of the battery.

When the connection between the plates and the battery is made, the two plates become charged to a difference of potential equal to the electromotive force of the battery. In order to obtain these charges, a.s.sume that the plates were each at zero potential before the connection was made; then current flows from the battery into the plates until they each a.s.sume the potential of the corresponding battery terminal. If the two plates be brought closer together, it will be found that more current will now flow into each of them, although the difference of potential between the two plates must obviously remain the same, since each of them is still connected to the battery.

[Ill.u.s.tration: Fig. 119. Condenser Plate]

Theory. Due to the proximity of the plates, the positive electricity on plate _A_ is drawn by the negative charge on plate _B_ towards plate _B_, and likewise the negative electricity on plate _B_ is drawn to the side towards plate _A_ by the positive charge on that plate.

These two charges so drawn towards each other will, so to speak, bind each other, and they are referred to as _bound charges_. The charge on the right-hand side of plate _A_ and on the left-hand side of plate _B_ will, however, be free charges, since there is nothing to attract them, and these are, therefore, neutralized by a further flow of electricity from the battery to the plate.

[Ill.u.s.tration: Fig. 120. Theory of Condenser]

Obviously, the closer together the plates are the stronger will be the attractive influence of the two charges on each other. From this it follows that in the case of plate _A_, when the two plates are being moved closer together, more positive electricity will flow into plate _A_ to neutralize the increasing free negative charges on the right-hand side of the plate. As the plates are moved closer together still, a new distribution of charges will take place, resulting in more positive electricity flowing into plate _A_ and more negative electricity flowing into plate _B_. The closer proximity of the plates, therefore, increases the capacity of the plates for holding charges, due to the increased inductive action across the dielectric separating the plates.

Condenser Defined. A condenser is a device consisting of two adjacent plates of conducting material, separated by an insulating material, called a _dielectric_. The purpose is to increase by the proximity of the plates, each to the other, the amount of electricity which each plate will receive and hold when subjected to a given potential.

Dielectric. We have already seen that the capacity of a condenser depends upon the area of its plates, and also upon their distance apart. There is still another factor on which the capacity of a condenser depends, _i.e._, on the character of the insulating medium separating its plates. The inductive action which takes place between a charged conductor and other conductors nearby it, as between plate _A_ and plate _B_ of Fig. 120, is called _electrostatic induction_, and it plays an important part in telephony. It is found that the ability of a given charged conductor to induce charges on other neighboring conductors varies largely with the insulating medium or dielectric that separates them. This quality of a dielectric, by which it enables inductive action to take place between two separated conductors, is called _inductive capacity_. Usually this quality of dielectrics is measured in terms of the same quality in dry air, this being taken as unity. When so expressed, it is termed _specific inductive capacity_. To be more accurate the specific inductive capacity of a dielectric is the ratio between the capacity of a condenser having that substance as a dielectric, to the capacity of the same condenser using dry air at zero degrees Centigrade and at a pressure of 14.7 pounds per square inch as the dielectric. To ill.u.s.trate, if two condensers having plates of equal size and equal distance apart are constructed, one using air as the dielectric and the other using hard crown gla.s.s as the dielectric, the one using gla.s.s will have a capacity of 6.96 times that of the one using air.

From this we say that crown gla.s.s has a specific inductive capacity of 6.96.

Various authorities differ rather widely as to the specific inductive capacity of many common substances. The values given in Table VIII have been chosen from the Smithsonian Physical Tables.

TABLE VIII

Specific Inductive Capacities

+-----------------------+------------------------+ DIELECTRIC REFERRED TO AIR AS 1 +-----------------------+------------------------+ Vacuum .99941 Hydrogen .99967 Carbonic Acid 1.00036 Dry Paper 1.25 to 1.75 Paraffin 1.95 to 2.32 Ebonite 1.9 to 3.48 Sulphur 2.24 to 3.90 Sh.e.l.lac 2.95 to 3.73 Gutta-percha 3.3 to 4.9 Plate Gla.s.s 3.31 to 7.5 Porcelain 4.38 Mica 4.6 to 8.0 Gla.s.s--Light Flint 6.61 Gla.s.s--Hard Crown 6.96 Selenium 10.2 +-----------------------+------------------------+

This data is interesting as showing the wide divergence in specific inductive capacities of various materials, and also showing the wide divergence in different observations of the same material.

Undoubtedly, this latter is due mainly to the fact that various materials differ largely in themselves, as in the case of paraffin, for instance, which exhibits widely different specific inductive capacities according to the difference in rapidity with which it is cooled in changing from a liquid to a solid state.

We see then that the capacity of a condenser varies as the area of its plates, as the specific inductive capacity of the dielectric employed, and also inversely as the distance between the plates.

Obviously, therefore, in making a condenser of large capacity, it is important to have as large an area of the plate as possible; to have them as close together as possible; to have the dielectric a good insulating medium so that there will be practically no leakage between the plates; and to have the dielectric of as high a specific inductive capacity as economy and suitability of material in other respects will permit.

Dielectric Materials. _Mica_. Of all dielectrics mica is the most suitable for condensers, since it has very high insulation resistance and also high specific inductive capacity, and furthermore may be obtained in very thin sheets. High-grade condensers, such as are used for measurements and standardization purposes, usually have mica for the dielectric.

[Ill.u.s.tration: Fig. 121. Rolled Condenser]