Copyright
Wirt Robinson.

The elements of electricity online

. (page 44 of 46)
Online LibraryWirt RobinsonThe elements of electricity → online text (page 44 of 46)
Font size
QR-code for this ebook


energy of a condenser is ^ V 2 K. The signaling distance therefore

varies with two factors, K, the capacity, and V 2 , the square of
the potential in the aerial. The capacity may be increased by
increasing the number of wires in the aerial, but, as shown above,
this increase is far from being in proportion to the number of
wires added. The most promising method is to increase the



HIGH POTENTIAL.



569



difference of potential across G. This can be done by separating
the knobs more widely, for, in order to throw a spark across this
wider gap, the aerial must be charged to a higher potential.
This solution, however, involves another difficulty for as we
widen the gap G, we greatly increase the resistance and we have
seen that in order that the oscillations may be produced by the
spark, R 2 must be less than L/K (Par. 687). If, therefore, R
be increased too much, the discharge is no longer oscillatory.




Fig. 370.

701. Coupled Circuits. There still remains a way by which
the potential in the aerial may be increased without increasing
the resistance across the spark gap. This is by applying to the
oscillatory circuit the principle of the step up transformer. One
form of this arrangement is shown diagrammatically in Fig. 371.
The battery, key and induction coil are just as described above
but in addition there is shunted around the spark gap G a circuit
containing a condenser D (usually a battery of Ley den jars),
and a coil F. This coil is the primary of an air core, step up trans-
former, the secondary of which is the coil H in series with the
aerial AE. In the actual apparatus, the coil H is within the coil
F, although separated from it by considerable space. For the
sake of clearness they are represented in the diagram as entirely
separate. When the key K is closed, the high voltage of the
secondary of C causes the condenser D to receive a large charge



570



ELEMENTS OF ELECTRICITY.



and therefore when a spark occurs at G, a large current oscillates
back and forth. As this current flows through F, it induces a
corresponding oscillatory current in H, the voltage of this last
being greater than that in F in proportion to the ratio of the
number of turns in H to those in F. The voltage in the aerial
is therefore stepped up and the waves which it sends forth have
so much the more energy.




Fig. 371.

The arrangement just described is said to be inductively-
coupled. Sometimes F and H are parts of one continuous coil,
that is, they constitute an auto-transformer (Par. 652). In this
case the apparatus is said to be direct-coupled. Owing to the
very high frequency of the oscillations, hysteresis prevents the
use of iron cores in these coils.

Commercial wireless telegraph plants no longer employ the
battery and induction coil as described in the preceding paragraphs
but use alternating current generators of from 2 to 5 kilowatts
capacity and supplying 220 volts at 500 cycles. The current
from these generators is stepped up by suitably-designed
transformers.

702. Tuning of Coupled Circuits. The aerial (Fig. 371) has
a natural period, that is, there is an electric wave of a certain
length which will produce in it resonance. If the waves in the
circuit FD can be made of this length, resonance will be set up in
the aerial and it will radiate a maximum amount of energy.
This circuit contains capacity in the shape of the condenser D,



HIGH POTENTIAL. 571

and inductance in the coil F. In Par. 696 it was shown that the
length of a wave in an oscillatory circuit varies as 2nr\/LK', we
may therefore vary the wave length by varying either the induc-
tance or the capacity or both. Condensers of variable capacity
are of frequent use for this purpose. In the diagram, however,
the inductance is represented as variable. The wire from D to F
connects to F by means of a clip and may be slid up or down so
as to embrace fewer or more turns of the coil. To determine
when resonance is secured, a hot wire ammeter (Par. 463) is in-
serted in the aerial either above or below H and the inductance
of F is varied until the ammeter shows a maximum current.

703. Branley's Coherer. Having seen how electric waves may
be produced, we shall now show how they may be detected at
a distance.

The receiving circuit, like the transmitter already described,
has an aerial, in fact, by means of a shifting switch uses the same
aerial as the transmitter at the same station. The waves from a
distance strike this aerial and produce in it electrical oscillations,
but these are usually very feeble. The first efforts were therefore
directed to produce a sensitive relay (Par. 412) which would
operate under these feeble oscillations and close a delicate switch,
thereby throwing in on some recording apparatus an auxiliary
source of current.

A B



CM:




Fig. 372.

The first successful solution of this problem was made by a
device due to Branley. In 1890 he found that metallic filings
placed between two metal plugs in a glass tube, the plugs consti-
tuting the terminals of an electric circuit, were ordinarily non-
conductive or at least of very high resistance, but were rendered
conductive by electric oscillations in their vicinity. If, after
being so rendered conductive, the filings were jarred, their original
high resistance was restored. No entirely satisfactory explana-
tion of this phenomenon has been given. His discovery resulted
in a piece of apparatus, the coherer, which in the hands of Marconi
took the form shown in Fig. 372. It consists of a slender glass
tube in which are two metal plugs A and B separated by a nar-



572



ELEMENTS OF ELECTRICITY.



row space in which are loosely piled rather coarse filings of a
mixture of 95 parts nickel and 5 parts silver. The wires connect-
ing with the plugs are sealed into the tube and the tube itself is
then exhausted and sealed. The exhaustion of the tube is to pre-
vent the filings from becoming tarnished.

704. Operation of Receiving Circuit. The operation of the
receiving circuit will be understood from the following: The in-
coming electric waves produce oscillations in the aerial A (Fig.
373) which render the coherer a conductor. This enables the




Fig. 373.

battery B to send a current through the coherer and the relay.
This current is very feeble, being less than one-thousandth of an
ampere, but is sufficient to cause the relay to operate and thus
throw in the battery C on the Morse sounder, or on whatever
recording apparatus may be used in its place. It also throws
this battery in on the buzzer D, an apparatus identical in operation
with the bell described in Par. 410. The small hammer of this
buzzer beats against the coherer, jarring the filings sufficiently
to cause them to decohere and restoring the resistance of the
coherer so that it is in readiness to receive the next succeeding
oscillations. These oscillations, however, in trains of ten or
twelve (Par. 689), follow each other with such rapidity that the
relay is kept closed and the sounder does not release its armature
until the operator at the sending station opens his key. The
sounder therefore repeats the dots and dashes made at the send-
ing key.

705. Use of Telephone and Detectors. The relay and coherer
as described above have given way to much more sensitive forms



HIGH POTENTIAL.



573



of receiving apparatus with corresponding increase in signaling
distance.

In place of the relay, sensitive telephones are now employed.
Pierce states that while it requires about 1/200 of a volt to operate
a relay, a 540 cycle alternating E. M. F. of 8 millionths of a volt
will produce an audible sound in such a telephone.

Instead of the coherer, many more sensitive forms of detectors
have been devised. These are of a number of classes, only two
of which we shall mention.

In 1896 General H. H. C. Dunwoody (Class of 1866, U. S. M. A.)
discovered that a crystal of carborundum (Par. 488) inserted in
an electric circuit served as a detector of electric oscillations.
Many other crystalline substances, such as sulphide of molyb-
denum, oxide of zinc, silicon, etc., have since been found to
possess the same property. Pierce by some beautifully conceived
and brilliantly performed experiments has shown that the action
of the crystals is to rectify the oscillatory currents in some way
analogous to the operation of the mercury arc rectifier (Par. 654).
He therefore designates them as "crystal rectifiers."

Their operation may be understood from the following: In the
diagram (Fig. 374), A represents a metal point pressing upon a
crystal B, and T is a telephone shunted around AB. Suppose
that the oscillatory currents in the aerial may pass upward
through AB but not downward (although
this direction is immaterial). The suc-
cessive oscillations follow at intervals
which may be less than a millionth of a
second, and the currents flowing upward
but not downward, a charge accumulates
in the antennae. When the oscillations
cease, this charge flows down through the
telephone. The extreme rapidity of the
oscillations prevent them from causing
vibrations in the diaphragm of the tele-
phone, and even if they did cause such
vibrations, their frequency would be far
beyond anything that the ear can detect.
The downward-flowing charges however follow in accordance
with the intervals between the sparks at the sending station, and
thus produce an audible note.





574 ELEMENTS OF ELECTRICITY.

Fessenden has devised an electrolytic detector which also has been
shown to be a rectifier. It differs from the arrangement shown in
Fig. 374 in that B is a small platinum cup containing dilute acid,
and A is a platinum wire, a thousandth of an inch or less in diame-
ter and barely touching the acid in B. There is also inserted in
the telephone circuit a single cell which sends a feeble but steady
current through the telephone. The oscillatory current in the
aerial causes the current through the telephone to vary and thus
produce a sound.

706. Tuning of Receiving Circuits. The receiving circuit is
not quite so simple as represented in Fig. 374 but is usually a
coupled circuit (Par. 701) and contains both a variable condenser
and a variable inductance. By varying one or both of these, the
circuit may be adjusted so as to be in resonance with the parti-
cular waves which are being received. If waves of different lengths
are arriving, the circuit may be tuned to resonance with those of
one length to the exclusion of those of other lengths. This natur-
ally suggests that where two wireless stations are endeavoring to
communicate and are being disturbed by signals from other
stations, confusion may be avoided and perhaps privacy secured
if by pre-arrangement the two stations concerned should use
waves of different lengths from those used by the other stations.
This involves the ability of the stations to ascertain the length of
wave which they are emitting and to adjust their apparatus to
emit waves of the desired length. This information is furnished
by several forms of wave meters, instruments carrying a graduated
scale from which, when they have been adjusted to resonance,
may be read direct the length of the corresponding wave in the
exciting circuit.

The standard wave length now used in communicating with
vessels at sea is 425 meters.

707. Distance Attained by Wireless Telegraphy. The distance
to which wireless signals may be sent is constantly being increased
and with more powerful sending apparatus, loftier antennae and
more sensitive receiving instruments, there appears to be no reason
why eventually they may not be exchanged between diametri-
cally opposite points on our globe. Within the present year (1913)
signals have been exchanged between the station at Arlington,
Virginia, and Gibraltar, Panama and Alaska.



HIGH POTENTIAL. 575

Several theories have been advanced to explain why it is pos-
sible to send these signals around considerable arcs of the earth's
circumference. According to some, the radiations proceed in
straight lines but at a height of about 50 miles in the atmosphere
reach a point where the pressure is so reduced that the air is a
conductor just as in the Geissler tubes (Par. 670). Since, as we
have already seen (Par. 690), these waves can not penetrate a
conductor, they are reflected from this tenuous stratum and thus
by successive reflections pass around the arc.

According to others, the waves leave the grounded aerial and
slide along over the surface of the earth like an immense inverted
U. The better the conducting surface, the better the transmis-
sion of the waves. This is corroborated by the fact that signals
can be exchanged at a much greater distance over salt water than
over land.

Finally, it is a fact not yet explained that these signals can be
sent nearly, if not quite, twice as far at night as they can during
the day and that the conditions for attaining long distance are
especially unfavorable at sun rise and at sun set.



INDEX.



577



INDEX.

References are to Paragraphs.



Absolute measurement of current,
374, 546

of resistance, 542, 546
Absolute temperature, 256
Absolute unit of current, 355, 536, 546

of E. M. F., 426, 537

of electric power, 496

of resistance, 427, 546

of self-induction, 433
Absolute zero of temperature, 289
Accumulator, 237

chloride, 241

reactions, 244
Acheson, 488
Actinium, 679

Adaptation of generator to w,ork, 582
Adjustment of mariner's compass, 183
Advantages of electro-magnet, 405

of multipolar machines, 574, 639

of Edison battery, 253
Aerial, 699, 701, 702
Aging of magnets, 167
Air, dielectric strength of, 93
Air condenser, 86, 87
Alpha rays, 679
Alternating current, 554, 606

compared with direct, 656

graphic representation of, 555, 618,
627

rectification of, 556, 653

transformation of, 648

value of, 612, 613
Alternating current motors, 657

classes of, 658

Alternating E. M. F., graphic repre-
sentation, 555, 618, 627

composition of, 611
Alternation, 608
Alternator, compound, 638

di-phase, 644



Alternator, field excitation of, 637

inductor, 640, 643

polyphase, 640, 644, 645

single phase, 640, 644

tri-phase, 645, 646, 647
Alternators, 636, 649

classes of, 640

usually multipolar, 639

with revolving armatures, 641

with revolving field, 642
Aluminum, manufacture of, 489
Amber, 12

American telegraph system, 413
Ammeter, 455, 457, 459, 467, 471, 472,
474, 702

classes, 462

connection of, 457, 459

millivoltmeter as, 474

resistance of, 457, 459

Weston, D. C., 467
Ammeter shunt, 465, 466
Amount of induced charge, 31
Ampere, 181, 344, 345, 346, 360, 371
Ampere defined, 228, 232, 307, 448,,

544, 546

Amperes, virtual, 612
Ampere turns, 388
Analogues of electric potential, 70
Analogy between cells and pumps, 33S
Angle of lag, 609

of lead, 570, 60
Anode, 220
Antenna, 699
Apparent power, 635
Application of electrolysis, 233, 234,

235

Arago, 428
Arc, electric, 485

enclosed, 521

flaming, 522



578



INDEX.



Arc lamp, 515

constant current, 520

constant potential, 519

magnetite, 523
Arc lamp mechanism, requirements

of, 517

Arc lights, efficiency, 524
Armature, 124, 560
Armature core, 560, 565
Armature reaction, 570, 587
Armatures, classes of, 566
Arrhenius, dissociation theory of, 268
Artificial magnets, 109
Astatic combination, 366, 368
Atomic character of electricity, 280,

685

Attracted disc electrometer, 101, 102
Attraction, electric, 15, 16, 17, 30

magnetic, 112, 120
mutual, 114
takes place through intervening

bodies, 113

Auto-transformer, 652, 701
Avogadro's law, 256
Ayrton, 381, 445, 454
Back E. M. F., 297, 593, 594, 595
Balance, Coulomb's torsion, 52, 100,

127, 132

Ballasting coil, 508, 519, 527
Ballistic galvanometer, 384
Base, electrolysis of, 223
Battery defined, 191

De La Rive's floating, 370

development of power in, 49

lead, care of, 248
objections to, 249
troubles of, 247

storage, charging, 245, 246, 415,

582

Bauxite, 489
Becquerel rays, 679, 680
Bell, electric, 410
Bell telephone, 440
Beta rays, 679
Bichromate cell, 205
Bifilar suspension, 127, 382
Biot's experiment, 39
Bipolar generator, 561



Blow out, magnetic, 485
Bolometer, 534
Boys, Vernon, 534
Branley, 703
Bridge, meter, 325

slide wire, 325

Bridge, Wheatstone, arrangement of
resistances, 315

evolution in form, 316

measurement of resistances by, 317,
318, 319, 320, 321

principle of, 313, 314

resistances measured by, 324

with reversible ratios, 322
Brushes, 553, 560, 568

shifting of, 570, 571, 598
Brush holders, 568
Bunsen's cell, 204
Bus bars, 579
Buzzer, 704
C. G. S. system, 10
Calcium carbide, manufacture of, 488
Calculation of E. M. F. of generator,
578

of flux of magnetic circuit, 401
Calibration of galvanometer, 454
Calorie, 11, 478
Canal rays, 676
Candle power, 509
Capacity, 79, 607, 625

E. M. F. and current curves in case
of, 627

of plate condenser, 89

of sphere, 80

of spherical condenser, 88

of wires, 699

practical unit of, 95

and resistance, 629

inductance and resistance, 630
Capacity, electric, 45, 79, 83, 88, 95
Capacity, dielectric, 31, 90, 92

determination of, 91
Capacity reactance, 628
Carbon, variation of resistance with
pressure, 285

with temperature, 289
Carbons for arc lamps, 516
Carbon filament, 505



INDEX.



579



Carborundum, manufacture of, 488

use as detector in wireless, 705
Care of lead batteries, 248
Cathode, 220
Cathode rays, 671, 672, 680

effect of electric field on, 674

effect of magnetic field on, 673
Cell, bichromate, 205

Bunsen's, 204

Clark's standard, 212

conventional sign for, 214

Daniell's, 206, 427, 544, 546

defined, 201

dry, 210

Edison-Lalande, 208

electrolytic, 220

elements of, 193, 194

gravity, 207

Grove's, 203

Leclanche, 209

Plante, 240

primary, 201

secondary, 237, 238

simple, 193

chemical action in, 195
dissociation theory applied to,
279

standard, need of, 211

Weston's standard, 213

voltaic, requirements of, 200
Cells, 191

analogy with pumps, 338

classification of, 202

E. M. F. of, 200

great variety of, 201

grouping of, 334

in multiple, 339

in parallel, 336, 337

in series, 335, 337

internal resistance of, 294

reversibility of, 236
Centimeter, 7
Characteristic defined, 583

external, 585

internal, 585

magnetization, 584

of series generator, 585

of shunt generator, 587



Charge, bound, 33

carried by corpuscle, 675

confined to surface, 38, 39, 68

distribution of, 40

division of, 45

electric, 18

free, 33

induced, amount of, 31
distribution of, 29

of storage battery, indications of,
246

on conductor, 37, 68

on non-conductor, 36

on surface exerts no force on in-
terior point, 67

residual, 87

surface density of, 41, 65, 66
Charges, induced, separation of, 32

variation of electric force with, 54
Charging Edison battery, 252

storage battery, 245, 246, 582

from A. C., 655, 656
Charles' law, 256
Chart, isoclinic, 173

isodynamic, 174

isogonic, 170

Chemical action in simple cell, 195
Chloride accumulator, 241

reactions, 244
Choke coil, 621, 622, 650
Circlet of cups, Volta's, 191
Circuit, no current unless complete,
216

divided, 293

division of current in, 300
Circular coil, field at center, 354

field on axis of, 354
Circular measure of wires, 296
Circular mil, 296
Clark's standard cell, 212
Classes of A. C. motors, 658

of alternators, 640

of armatures, 566

of D. C. motors, 598

of electrical machines, 550
Classification of ammeters and volt-
meters, 462

of cells, 202



580



INDEX.



Clutch for arc lamps, 518
Coercive force, 398
Coherer, 703, 704
Coil, choke, 621, 622, 650

induction, 438
use of condenser with, 439

resistance, 311

rotating in magnetic field, 551
Collector rings, 553
Commercial unit of electric power,
496

of electric work, 496
Commutation, 556, 571
Commutation plane, 570
Commutator, 556, 560, 567
Commutator segments, 567
Comparison of A. C. and D. C., 656
Compass, mariner's, 182

adjustment of, 183
Composition of alternating E. M. F.s,

611
Compound alternator, 638

generator, 563, 588
Condenser, 83, 86, 94, 96

energy of, 97

in A. C. circuit, 626

location of charge in, 87

use with induction coil, 439

work expended in charging, 96
Condenser, plate, capacity of, 89

spherical, capacity of, 88
Conditions affecting wireless teleg-
raphy, 707
Conductance, 292
Conductivity, 292

of gases, 667, 680
Conductor defined, 19
Conductors and non-conductors, 19

table of, 20

Conductors carrying currents, force
exerted between, 361, 362

in parallel, resistance of, 293

in series, resistance of, 286
Connection of transformers, 651
Consequent poles, 165
Constant current arc lamp, 520
Constant potential arc lamp, 519
Contact series, Volta's, 187



Contact theory, Volta's, 188
Control of field of machines, 564

of light, 513

of speed of shunt motor, 600
Controlling force, 146

method of weakening, 366
Conventional sign for cell, 214
Converter, rotary, 653

synchronous, 653
Cooper-Hewitt, 527
Copper, refining by electrolysis, 233
Core of armature, 560, 565

of solenoid, effect upon field, 390
Core transformer, 649
Corpuscles, 672, 681, 682, 685

mass of, 684

nature of charge carried by, 675

velocity of, 683

Cost of power from primary cells, 343
Coulomb, 38, 42, 52, 53, 100, 123, 128,

132
Coulomb, the, 56, 228, 536

defined, 228
Coulomb's first law, 115, 123

second law, 128, 133

torsion balance, 52, 100, 127, 132
Counter E. M. F., 297, 593

in motor, 593, 594

reading of voltmeter across, 595
Coupled circuits, 701, 702
Coupling of generators, 581
Critical frequency, 631

resistance, 586
Crookes, 671
Crookes' dark space, 670

tube, 670, 671, 672, 675, 676, 677,

678, 682
Cryolite, 489
Crystal rectifiers, 705
Gumming, 529
Curie, 679

Current, absolute unit of, 355, 356,
536, 546

alternating, value of, 612, 613

direction of flow of, 217

displacement, 690

division in divided circuit, 300

eddy, 428



INDEX.



581



Current Continued.

electric, 215
effects of, 215, 444
mechanical production of, 423
work done by, 476

equality at every cross section, 229
I Foucault's, 429

measurement of, by tangent gal-
vanometer, 374

none unless circuit complete, 216

practical unit of, 228, 307, 448, 546

production by rotating coil, 553

rotation by magnetic pole, 351

units of, 536

Curves of magnetization, 394
Cutting of lines of force, 424, 425
Cycle, 608

of magnetization, 398
Cylinder machine, 48
D. C. generator, essential parts, 560
Daniell's cell, 206, 423, 427, 544, 546
Damping, electrical, 379, 430, 467, 471
D'Arsonval galvanometer, 378, 467
Davy, 223, 270, 515
Dead beat instruments, 379
Declination, annual change in, 179

diurnal change in, 178

magnetic, 169, 177

secular change in, 177
Decomposition, chemical, 257

of water, 218
Deflection of needle, right hand rule

for, 345

De La Rive's floating battery, 370
Delta connection, 646
Density, electric, 692
Depolarizers, 199
Detector, crystal, 705

electrolytic, 705
Detectors in wireless telegraphy, 703,

704, 705
Detonator, 483
Diagrams, electric, 5~

of parallel series grouping, 342

vector, 610
Dial bridge, 323
Diamagnetics, 122
Diamagnetism, 122, 402



Dielectric, 55

Dielectric capacity, 31, 90, 92

determination of, 91
Dielectric coefficient, 90
Dielectric strength, 93
Dimensional formula of resistance,
540

formulae, 539, 547

table of, 547

Difference of potential, 69, 73
Dip, magnetic, 171, 177

secular change in, 177
Di-phase alternator, 644
Dipping needle, 172
Direct coupling, 701
Direct current compared with alter-
nating, 656

transformation of, 648
Direct current motors, classes, 598
Direction of electric field, 59, 61

of field about wire carrying a cur-
rent, 347, 348

of flow of current, 217

of rotation of motor, 604
Discharge through high vacua, 670

through moderate vacua, 668
Discovery of Galvani, 185
Displacement current, 690
Dissociation, 257

by heat, 258

extensive scope of theory of elec-
trolytic, 281

theory applied to simple cell, 279

theory of Arrhenius, 268
Distance attained by wireless teleg-
raphy, 707

Distribution of induced charge, 29
Divided circuit, 293

division of current in, 300

drop of potential in, 312
Division of charge, 45
Drop of potential, 298, 299

in divided circuit, 312

measurement of resistance by, 309,

310

Drum winding, plane development of
576

star development of, 577



582



INDEX.



Drum wound armature, 566, 575, 576,

577

Dry cell, 210



Online LibraryWirt RobinsonThe elements of electricity → online text (page 44 of 46)