WIRELESS TELEPHONY AND ALTERNATING CURRENTS
The developments in wireless communication have been so rapid
during recent years that a more extended account, than that given
in Art. 417 of the apparatus and methods used at the present time,
seems desirable. The study of Alternating Currents is also included
with the idea that it will make the text more complete and of wider
usefulness.
WIRELESS TELEPHONY
=425. The Wireless Telephone.=--One of the most important developments
in wireless communication in recent years has been in wireless
telephony. We realize its possibilities, when we hear of the
achievements of talking across an ocean or between airplanes and the
ground.
The wireless telephone can be best understood by comparing it with the
common telephone. When the latter is in use, a direct current flows
continually through the instrument. (See Arts. 312-316.) When a person
speaks into the transmitter, the sound waves of the voice cause the
diaphragm to vibrate, this action causes rapid changes in the
_resistance_ of the transmitter, which in turn causes the direct current
to fluctuate just in step with the pulses of the voice waves. This
fluctuating direct current passes through the primary of an induction
coil, producing in the secondary an intensified alternating current.
This passes over the line wires to the receiver where it produces
variations in the magnetic field affecting the receiver diaphragm,
causing the latter to reproduce the voice of the person speaking in the
transmitter. Now to make the comparison clear, two facts must be noted
with regard to the wire telephone: first, there must be an action in the
transmitter which causes variations in a current through the instrument;
second, this fluctuating current produces a more intense alternating
current which flows over the line and affects the receiver diaphragm,
producing there sound vibrations of greater intensity than those used at
the transmitter. This added energy comes from the current flowing
through the transmitter. The case is analogous to that of an electric
bell. The armature of the bell vibrates with greater energy than is
required to push the button, the extra energy being derived from the
battery.
=426. The Action of the Wireless Telephone.=--In the wireless telephone
we have a continuous stream of electric waves of high frequency. (See
Fig. 425_A_.) This stream of electric waves corresponds to the current
that flows through the transmitter in the wire telephone. These waves
are of such high frequency that even though we had a receiver diaphragm
vibrating in step with the waves, we could not hear the sound because
the human ear cannot hear a sound which consists of more than about
40,000 vibrations per second. The sound waves act upon this stream of
waves very much, as in the wire telephone, the transmitter acts to
modify the line current. The impulses caused by the voice are much
slower than the electric waves first mentioned and these slower impulses
are reproduced in the receiver. Not only are these slower impulses
reproduced but they are _amplified_, that is, produced with greater
energy than the impulses impressed on the stream of waves. Fig. 425_A_
represents as nearly as is possible in a diagram the continuous stream
of electric waves. Fig. 426_B_, represents the impulses produced by the
sound alone, and Fig. 426_C_, shows how these voice impulses are
impressed on the stream of waves.
[Illustration: FIG. 425.--_A_, unmodified high frequency waves; _B_,
waves of voice frequency; _C_, high frequency waves modified by waves of
voice frequency.]
[Illustration: FIG. 426.--Vacuum tube, transmitting type. (_Western
Electric Co._)]
[Illustration: FIG. 427.--Vacuum tube, receiving type. (_Western
Electric Co._)]
[Illustration: FIG. 428.--Diagram of wireless telephone transmitting
set.]
=427. The Vacuum Tube or Audion.=--The device by which all of this is
accomplished is the _vacuum tube_. (See Fig. 426.) This tube contains
three electrodes. _First_, a _filament_ (_F_, in Fig. 428) which is
heated by a current from a battery (_B_{1}_, Fig. 428) and because it is
heated, sends out a stream of electrons. _Second_, the _plate_ which
forms the anode of the circuit from battery, _B_{2}_. This plate
receives the electrons which are thrown off by the heated filament,
hence a current flows through the circuit of _B_{2}_; the discharge
through the tube depending on the e.m.f. between the filament and the
plate. _Third_, a _grid_ is placed between the filament and the plate
and is connected to the _secondary_ of the induction coil, the primary
of which is connected to the transmitter. When the transmitter diaphragm
is vibrating, the e.m.f. induced in the secondary of the induction coil
causes a variation in the potential of the grid. This means a variation
in the electric field between the filament and the plate. (See Fig.
428.) The changing electric field causes a variation in the discharge of
electrons through the tube; the variation corresponds to the vibrations
of the transmitter diaphragm. This produces a surging current of the
frequency of the sound waves in the primary of the transformer (_T_,
Fig. 428). The secondary of this transformer is connected to the antennæ
(_A_) and the earth (_E_). By means of the transformer, rapid surgings
are set up in the antennæ and these surgings produce a continuous stream
of electromagnetic waves which goes out in space. (Like Fig. 426_C_.)
These electromagnetic waves produce oscillations in the antennæ of a
receiving station. The antennæ transmit the impulses to a _tube_ (Fig.
427) which acts as a _detector_, and makes possible the reproduction of
the sound by an ordinary telephone receiver.
[Illustration: FIG. 429.--View of wireless telephone set.]
The _vacuum tube_ in the transmitting circuit also _amplifies_ the
impulses, that is, the energy of the waves given out is greater than
that of the impulses which produce them, the additional energy being
derived from the battery sending current through the plate and filament.
In operation, the filament and the plate are connected to a battery with
a _condenser_ (_VC_) and an _inductance coil_ (_I_) in the circuit, as
shown in Fig. 428. Photograph of a complete modern wireless telephone
set is shown in Fig. 429.
ALTERNATING CURRENTS
=428. Alternating currents= are of interest to us because of their
general commercial use. To understand the reason for the extensive
application of alternating currents it is necessary to learn the
fundamental principles which pertain to them. The production of such
currents has already been explained in Arts. 300-304. It should be
remembered that the current developed in the armature of a dynamo is
alternating. A dynamo may _deliver_ a direct or an alternating current,
depending on the method of collecting the current from the armature. If
a _commutator_ is used, the machine delivers _direct_ current, if _slip
rings_ are employed, an _alternating_ current is delivered.
=429. The Magnetic Field of an Alternating Current.=--The magnetic field
of a direct current has been considered in Arts. 255-256. It has been
shown to be arranged in circles about the conductor, according to the
_Right Hand Rule_. (See Figs. 229 and 230.) These facts will help one to
understand the following experiment:
If a number of magnetic compasses be arranged in a circle about a
straight vertical wire carrying a direct current, the compass
needles will point out a circle about the wire. (See Fig. 430,
_A_.) If now the current be reversed the compass needles will
reverse themselves and point in a direction just opposite to that
taken at first. (See Fig. 430, _B_.) This will be clear if you
imagine yourself walking around the wire in the direction the
compass needles pointed at first, and then walking around the wire
in the reverse direction. This illustrates what happens in the
field of an alternating current. The field reverses each time the
current reverses.
The magnetic field of an alternating current not only rapidly reverses
itself, but also continually _changes in intensity_. At the instant when
the current reverses, the force of the magnetic field is zero since the
current at that instant is zero. As the current begins flowing and
increases to its maximum intensity, the magnetic field appears and
increases in intensity; and as the current decreases to zero, the
magnetic field changes in a similar manner. The field as it grows in
strength extends farther and farther from the wire, as it decreases in
strength it contracts or draws closer to the wire. Thus the magnetic
field may be said to expand and contract. We may picture the lines of
force as continually moving. In a typical a.-c. circuit, the complete
series of changes takes place in a small fraction of a second, and is
repeated many times over in a second. Contrast this with the magnetic
field of a constant direct current. Here the magnetic field has the same
direction as long as the current flows and does not change in strength.
This comparison is important because most of the differences between
direct and alternating currents depend on differences in the action of
their magnetic fields.
[Illustration: FIG. 430.--Arrangement of compasses about a wire carrying
an alternating current.]
=430. Transformers.=--The transformer has been described in Arts.
309-310. The principle of the transformer may be illustrated by the
following experiment:
A coil having several hundred turns of No. 18 d.c.c. copper wire is
placed over one arm of a "U" shaped iron core (see Fig. 431) and
then connected to a 110 volt a.-c. lighting circuit. Another coil
(_S_) having about 50 turns of No. 22 d.c.c. copper wire is
connected to an electric bell or buzzer, or a low voltage electric
light bulb. When the small coil is held over the other arm of the
"U" shaped iron core, the bell rings or the bulb glows. It is
evident that the electromotive force developed in the small coil
(_S_) is due to the alternating magnetic field surging back and
forth through the iron core. In Fig. 431 the core is "open" since
the magnetic field must pass through the air from one end of the
core to the other. A typical transformer has a _closed core_ to
provide a _closed magnetic circuit_. To secure this, take a
suitable bar of iron and lay across the end of the "U" shaped core,
and notice any change in the induced current produced in the small
coil, due to increased movement of magnetism through the closed
iron core.
[Illustration: FIG. 431.--Diagram of a transformer.]
This experiment illustrates the construction and action of a
transformer. In a commercial transformer, the two windings are on a
closed magnetic circuit. (See Figs. 304 and 305, p. 346.) To keep the
coils insulated, the transformer is placed in an iron "housing" and
covered with oil. These "housings," or transformer cases are generally
attached to poles near buildings in which alternating current is used.
=431. Voltage Relation in a Transformer.=--In the experiment described
above, a bell was rung by an induced current produced in the secondary
coil. The induced e.m.f. was less than the voltage of the primary coil
partly because there was some magnetic leakage, but mainly because
there were fewer turns of wire on the secondary. In a commercial
transformer the magnetic leakage is practically zero. In such a case,
the ratio of the number of turns on the primary coil to the number on
the secondary equals the ratio of the e.m.f. induced in the primary to
the e.m.f. induced in the secondary. Suppose, for example, we wish to
make a bell ringing transformer to use on a 110 volt lighting circuit,
10 volts being required for the bell; the secondary will then need
one-eleventh of the number of turns of the primary. So that if 550 turns
are on the primary, then 50 turns will be needed for the secondary. This
will be a "step-down" transformer. On the other hand, suppose we wish to
"step-up" the voltage as is done in a certain power station where the
voltage of the generators is 6000 volts, the voltage being stepped up to
44,000 by means of large transformers. This means that the secondary
coils have approximately 7-1/3 times as many turns as the primary.
=432. Power Loss in a Transformer.=--When the voltage is "stepped up" in
a transformer, do we gain power? To answer this question we must
remember that electric power does not depend on voltage alone but on the
_product_ of e.m.f. and current intensity. (See Art. 291.) By tests with
a.-c. voltmeters and ammeters, we find that when the secondary e.m.f. is
_greater_ than the primary e.m.f., the secondary current intensity is
_less_ than that in the primary. It is also found that the _power_
developed is less than the power received by the transformer, _i.e._,
the "output" is less than the "input" as we would expect from the law of
machines. The power loss is mainly due to the work required to reverse
the magnetism, that is, to continually reverse the position of the iron
molecules. (See Art. 205.) The energy lost in this manner is known as
"core loss" since it occurs in the iron core. The lost energy appears
as heat. So much heat is developed in large transformers that special
means of cooling are provided. In order to make the heat developed as
small as possible, the cores are "laminated" (see Fig. 305, p. 346),
that is, built up of thin sheets of iron, because if the iron cores were
solid, the changing magnetic fields would induce electric currents in
the iron cores, which would produce an excessive amount of heat with a
correspondingly large power loss.
[Illustration: FIG. 432.--Diagram of "bell-ringing" transformer.]
=433. Choke Coils and Inductance.=--If we refer to Fig. 432 we see that
the primary winding of the bell ringing transformer is connected across
the line. This winding forms a closed circuit whether the bell is
ringing or not. The resistance of this winding is small. Let us assume
it to be one ohm. With a one ohm resistance connected across a 110 volt
line we might expect a current of 110 amperes. This is certainly what we
should get if we were to connect a one ohm resistance across a line
having 110 volts direct. The primary would form a short circuit if the
current were direct. But the fact is that practically no current flows
through the primary winding when the bell is not ringing. Herein lies
one of the important differences between alternating and direct
currents. With an alternating current the primary winding of our
transformer acts as a _choke coil_ and "chokes" down the current almost
to zero. Let us see how this is done.
[Illustration: FIG. 433.--A circuit containing a choke coil.]
Let Fig. 433 represent a choke coil. Since alternating current is used,
the magnetic field is continually changing. Each turn of wire has its
own magnetic field. The lines of force of turn number 1 expand and
contract and as they do so they move across turns 2, 3 and so on. In
like manner the lines of force from each turn of wire move across the
other turns. In other words the coil is cutting its own lines of force.
Now whenever an electric conductor cuts magnetic lines of force an
electromotive force is induced in the conductor. There is then an e.m.f.
induced in the coil by its own magnetic field. This induced e.m.f. on
the whole opposes the applied e.m.f.; in the primary of our bell ringing
transformer the induced e.m.f. opposes the e.m.f. of the line to such an
extent as to reduce the current almost to zero. _Inductance_ is the
action of an alternating current in inducing an opposing e.m.f. in the
coil in which the current is flowing. Since this opposing e.m.f. is
induced in the coil by its own magnetic field this action is also called
_self-induction_. In a transformer the action of the field of the
primary upon the secondary is _mutual induction_; while the action of
the field of the primary in choking the current in the primary itself is
self-induction or inductance. A coil having a single winding and used to
introduce inductance in a circuit is called a _choke coil_. A choke
coil inserted in a lamp circuit in series with the lamps dims the lamps
because it reduces the intensity of the current.
[Illustration: FIG. 434.--Diagram showing graphically an alternating
current with a "lag" of 30° behind its electromotive force.]
Self-induction causes the current to _lag_, that is, the current does
not quite reach its maximum at the instant the voltage reaches its
maximum. Fig. 434 shows graphically an e.m.f. and a lagging current. In
this figure the maximum current is shown following the maximum voltage
at an interval of 30 degrees. In other words the armature in a two-pole
field must turn 30 degrees from the position of maximum voltage before
the current in the coil, where the self-induction occurs, reaches its
maximum.
=434. Reactance and Impedance.=--A choke coil has resistance as well as
inductance. Its resistance can be found by the voltmeter-ammeter method,
using a direct current. (See Art. 278.) Let us take for example the
primary winding of a bell ringing transformer. Using a direct current
and testing the coil with a voltmeter and ammeter we find its resistance
to be, let us say, one ohm. If we connect the same coil across a 110
volt a.-c. line we find the current to be very small, say 0.05 ampere.
The coil now has resistance and _reactance_. Reactance is the effect of
self-induction in hindering the flow of current. It is measured in ohms.
The combined effect of resistance and reactance is called _impedance_.
In the example above, the coil has 110 (volts)/0.05 (ampere) = 2200 ohms
of impedance. In applying Ohm's law to an alternating current circuit,
impedance must be substituted for resistance. Ohm's law as applied to an
a-c. circuit should be stated: "Current intensity equals e.m.f. divided
by impedance", or _I_ = _E_/_Z_. (_Z_ = impedance.)
[Illustration: FIG. 435.--The relation between resistance, reactance and
impedance.]
Impedance, however, does not equal the _sum_ of resistance and
reactance. The relation between these three quantities is similar to
that between the three sides of a right triangle, in which the impedance
represents the hypotenuse, and the resistance and reactance the other
two sides. See Fig. 435 which indicates that Resistance² + Reactance² =
Impedance², or (_R²_ + _X²_ = _Z²_). (_X_ = reactance.) To illustrate
this relation; suppose the primary of a transformer has 10 ohms
impedance and 8 ohms resistance, then the reactance equals 10² - 8² =
6², or the reactance is 6 ohms.
Exercises
1. Find the reactance of a choke coil having a resistance of 10 ohms,
when its impedance is 50 ohms. How great a current flows through this
coil if the terminal voltage is 110 volts?
2. When the bell is ringing, the primary of a bell ringing transformer
has an appreciable current. Suppose this current is 0.2 ampere. What is
the impedance if the voltage of the line is 115 volts? What is the
reactance if the resistance is 1 ohm?
3. The primary of a large transformer has a terminal voltage of 6000
volts and a current of 600 amperes. What is the impedance? If the
resistance is 6 ohms, what is the reactance?
[Illustration: FIG. 436.--A telephone set showing a condenser used in
the circuit of the "ringer."]
=435.--The electric condenser= (see Art. 231) is a very useful device in
a.-c. circuits; _e.g._, in telephone sets used in cities, a condenser is
used in the ringing circuit, as shown in Fig. 436. Alternating current
is required to ring such a bell and a condenser permits an a.-c. current
to act through it, although it entirely prevents the flow of a direct
current. This peculiar action will now be explained.
=436. The action of a condenser= in an alternating current circuit may
be illustrated by the following experiment. Connect twelve, 1 m.f.
(microfarad) condensers, in parallel, and then attach them to a 110 volt
a.-c. line so that an incandescent lamp is in circuit as shown in Fig.
437. The lamp will be found to glow brightly, although there is no
electrical connection between the two sets of condenser plates. If the
same arrangement is connected to a 110 volt direct current circuit, the
lamp does not glow because it is really an open circuit. The lamp glows
on an a.-c. circuit because, although no electricity flows _through_ the
condenser, it does flow _into and out of_ the condenser, surging back
and forth through the lamp with sufficient intensity to cause it to glow
brightly. When the a.-c. current moves one way in the circuit, one set
of plates of the condensers becomes charged positively, the other,
negatively. When the a.-c. current reverses, the charges on the
condenser plates reverse. In the ordinary lighting circuit 120 reversals
take place each second, so that electricity rapidly flows into and out
of the condensers. On removing one condenser after another from the
circuit, the lamp is found to glow less and less, till when but one
condenser is left, no glowing is observed, since one small condenser
does not have sufficient _capacity_.
[Illustration: FIG. 437.--Twelve condensers in circuit with an
incandescent lamp.]
The unit of capacity is the _Farad_. Capacity is defined as the quantity
of electricity per second that flows into a condenser when the voltage
at the terminals changes at the rate of one volt per second. If a change
of one volt per second causes one coulomb to flow per second, that is, a
current of one ampere, the capacity is one _farad_. The condensers used
in the above experiment have a capacity of one microfarad, or one
millionth of a farad.
A condenser, on account of its capacity, causes an a.-c. current to
_lead_ the voltage, that is the current reaches its maximum value before
the voltage does. In this respect a condenser has an effect opposite to
that of the self-induction of a choke coil (the latter causing the
current to "lag"). (See Fig. 435.)
=437. Transmission of Electric Power.=--A field of peculiar usefulness
for a.-c. currents is in the economical transmission of electric power.
This fact is due to the following reasons: (_a_) The loss of electrical
power in a transmission line is due to the production of heat; the heat
produced being proportional to _I²R_, or to the _square_ of the _current
intensity_. Any lessening of the current flow required to transmit a
given power will therefore increase the efficiency of transmission.
(_b_) In order to employ a small current in transmitting a large amount
of power, we must use a very high e.m.f. Such high electromotive forces,
say from 60,000 to 100,000 volts, can be obtained only by the use of
a.-c. transformers, since it is not practicable to build a direct
current generator capable of producing 60,000 volts. In large power
transmission systems, a.-c. generators are used to produce powerful
alternating currents. The e.m.f. is then stepped up to a suitable
voltage (2300-100,000) by transformers and sent over transmission lines
to the various places where the power is to be used; at these places
suitable transformers "step-down" the e.m.f. to a convenient or safe
voltage for use. (See Fig. 442 of a transmission line and Fig. 438 of a
large power transmission system, and Fig. 439 of an a.-c. generator and
power plant.)
[Illustration: FIG. 438.--Diagram of an alternating current high tension
power system. (_A_) Alternator, (_Tu_) water turbine, direct connected
to alternator, (_E_) exciter, (_T_{1}_) step-up transformers in power
station, (_T_{2}_) step-down transformers in substation, (_M_) motor,
(_L_) lamps, single-phase, three-wire system, (_T_{3}_) step-down
transformers delivering three-phase current to rotary converter (_R_)
which delivers direct current to the trolley line.]
=438. Power Factor.=--The _power factor_ is a matter of interest and
importance in the use of a.-c. machines. Its meaning and use may be
learned from the following explanation: In a direct current circuit,
watts equals volts times amperes. In an alternating current circuit,
this equation is true only when the current is "in step" with the
voltage, that is, only when there is no _inductance_ or _capacity_ in
the circuit. If current and voltage are out of step, _i.e._, if there is
_lag_ or _lead_ (see Fig. 434), the product of volts and amperes gives
only the _apparent power_, the ratio between true and apparent power
depending on the amount of lag or lead. This ratio is called the power
factor. In an a.-c. circuit, then, the power equation is: watts = volts
× amperes × power factor, or power factor = true power/apparent power.
The product of volts and amperes is the _apparent power_ and is called
volt-amperes in distinction from the true power or watts. Therefore the
following is true: power factor = true watts/volt-amperes.
[Illustration: FIG. 439.--Power house showing alternators, direct
connected to horizontal hydraulic turbines. Note the direct current
"exciter" on end of shaft of alternator. (_Courtesy of General Electric
Co._)]
=439. Single-phase Currents.=--There are several kinds of a.-c.
currents. One of the most common is the _single-phase_. It is simply the
common a.-c. current used for light and power in the average home, and
uses a two-wire circuit around which the current is rapidly alternating.
Fig. 440 illustrates the changes of e.m.f. in an a.-c. single-phase
current. It may be produced by a single coil rotating in a magnetic
field. The curve of Fig. 440 represents one _cycle_, that is, one
complete series of changes in the electromotive forces. At the end of
the cycle the armature is in the same condition as at the beginning so
far as the magnetic field is concerned. It then begins a new cycle. The
ordinary commercial alternating current has a frequency of 60, that is
60 cycles per second. One rotation produces as many cycles as there are
pairs of poles. For example, if there are 48 poles in the generator
field, one rotation produces 24 cycles.
[Illustration: FIG. 440.--Graph showing the e.m.f. changes of a
single-phase current for one "cycle."]
=440. Three-phase Currents.=--Now suppose we have three coils as in Fig.
441, the coils being evenly spaced, or 120 degrees apart, at _A_, _B_,
and _C_. If the coils are rotated in a magnetic field, each will produce
an electromotive force. The result produced by three such coils is
called a _three-phase_ current. Ordinarily six wires, or three circuits,
would be required to carry the current produced by three separate coils;
for when coil "_C_" is in the 90 degree position, where its e.m.f. is a
maximum, coil "_B_" is 120 degrees past its maximum, and coil "_A_" is
240 degrees past its maximum. The graph (Fig. 441) shows the maximum
points of the three e.m.f's. separated by intervals of 120 degrees. In
practice, however, it is found possible to use _three wires_ instead of
six, as explained in Art. 441.
[Illustration: FIG. 441.--Graph showing the e.m.f. changes of a
three-phase current for one "cycle."]
=441. Three-wire Transmission.=--The currents produced in the three
coils just described undergo precisely the same changes as those
represented in the _graph_ (Fig. 441) for the three electromotive
forces. Careful examination of the graph will show that at any point the
sum of the _plus_ e.m.f's. equals the sum of the _minus_ e.m.f's. In
other words the algebraic sum of the three e.m.f's. is zero. Therefore
if we properly connect a transmission line of three wires to the
generator, the sum of the currents leaving the generator will equal the
sum of the currents returning to it. Since the algebraic sum of the
currents produced by the three coil combination described in Art. 440 is
always zero, it is possible to use three wires on three-phase
transmission lines. Fig. 442 shows a "tower" carrying three, three-wire
transmission lines. Long distance, high tension transmission lines are
generally three-wire lines carrying three-phase a.-c. currents.
[Illustration: FIG. 442.--A "tower" supporting three, three-phase
circuits of a high tension transmission line.]
=442. Alternators.=--A dynamo which delivers alternating current is
known as an _alternator_. Commercial alternators have many pairs of
poles in the field and as a rule the field rotates while the armature is
stationary. The field must be supplied with _direct_ current for the
polarity of each coil in the field must remain unchanged. Usually a
separate "exciter" is used, which is a small direct current generator.
The current from this exciter is fed into the rotating field by means of
slip rings. Fig. 439 shows a d.-c. (direct current) exciter on the end
of the armature shaft of the large alternator.
[Illustration: FIG. 443.--Diagram of a "Series Motor."]
=443. The A.-C. Series Motors.=--The only type of motor that will run on
either alternating or direct current is the _series motor_. The
"universal" motor used in household appliances such as electric fans,
vacuum cleaners, etc., is a series motor. The reason a series motor will
run on either direct or alternating current is because the direction of
rotation of the armature of a motor depends on (_a_) the direction of
the current in the armature, and (_b_) the polarity of the field.
Reversing either of these alone, reverses the direction of rotation of
the armature, while reversing both at the same instant leaves the
direction of rotation unchanged. Fig. 443 is a diagram of a series motor
since the field coils and armature are connected in series. On an a.-c.
line, both field and armature current must therefore reverse at the same
instant. In a shunt motor (similar to Fig. 286) we have a divided
circuit, and the greater self-induction of the field coils causes an
a.-c. current through these coils to lag behind that flowing in the
armature so that the two currents do not reverse at the same instant.
[Illustration: FIG. 444.--Diagram of a gramme ring. It is shown
connected to a single-phase current so as to produce a rotating magnetic
field, similar to that obtained with a three-phase current. (_Ahrens,
Harley and Burns._)]
[Illustration: FIG. 445.--The "stator" of an induction motor.]
=444. The Induction Motor.=--Another common type of a.-c. motor is the
_induction motor_. Its advantage lies in its simplicity. It has neither
commutator nor brushes, the armature having no connection with an
external circuit. If the wires of a three-phase line be connected to a
coil wound in the form of a _gramme ring_, the connections being 120
degrees apart as in Fig. 444, the magnetic field within this coil will
change in the same manner as if a magnet were spinning upon a pivot at
the center of the coil. Suppose the _N_ pole at one instant is at _A_,
in one-third of a cycle it moves to _B_, in another third to _C_, and in
one cycle it makes a complete revolution. Thus we have a _rotating
magnetic field_. If a cup of some non-magnetic metal such as aluminium
or copper be placed on a pivot in the center of this coil, the cup is
cut by the moving lines of force and currents are induced in it.
Because of these currents, the cup has a magnetic field of its own, and
the action of the two magnetic fields is such as to pull the cup around
and cause it to rotate in the same direction as that in which the field
of the coil rotates. The coil represents the stationary part, the
_stator_ (Fig. 445) and the cup the rotating part, the _rotor_, of an
induction motor. While the cup rotates in the same direction, it does
not rotate so rapidly as the magnetic field. If it should it is plain
that it would not cut the lines of force. The difference between the
rate of rotation of the rotor and that of the magnetic field is called
the "slip." The rotating part in small induction motors is frequently
made in a single casting. In large motors, it is built up of heavy
copper bars. Thus, from its appearance the common form of rotor is
known as the "squirrel cage" rotor. (See Fig. 446.)
[Illustration: FIG. 446.--The "rotor" of an induction motor.]
[Illustration: FIG. 447.--Diagram illustrating the principle of the
synchronous motor. The armature coil passes the position shown in the
figure at the instant the current in the line reverses. Thus the
armature keeps with the line current, making one revolution with each
"cycle."]
=445. A synchronous motor= is one that keeps step with the alterations
of an alternating current. The line current is fed into the armature by
means of two slip rings and brushes. The principle of the synchronous
motor is illustrated in Fig. 447. This shows a motor having a two-pole
field. The armature current must be reversed twice in each revolution.
The reversal must take place when the armature winding is perpendicular
to the lines of force of the field. In a direct current motor this
reversal is brought about by the commutator. In a synchronous motor the
armature reaches the 90 degree position at the exact instant at which
the current reverses in the line. Thus in the case of a two-pole motor
the armature must make exactly one revolution for each cycle; it is,
therefore, a constant speed motor. Such motors are frequently employed
in converter stations where alternating current is converted into direct
current by what are called _rotary converters_.
In practice the synchronous motor has a number of pairs of field poles.
It is essentially an alternating current generator running as a motor.
One of the principal uses of the synchronous motor is that of a
converter, receiving alternating current and delivering direct current.
Synchronous motors are also used in transmission lines to aid in
maintaining constant voltage.
Important Topics
The wireless telephone, essential parts, action, arrangement.
Alternating currents, alternating fields.
Transformers, voltage relation of coils, power and core losses.
Self-induction, inductance, and coke coils, uses, applications.
Impedance, reactance, and resistance; relation and effects.
Condensers, uses and applications with a-c. circuits.
Alternating current power transmission; uses, advantages.
Power factor, lag, lead, volt-amperes, true watts.
Single- and three-phase currents; uses and nature of each.
Three-wire transmission systems, alternators, construction, and action.
A-c. motors, series, induction, synchronous.
INDEX
Aberration, spherical, 408
Absolute scale of temperature, 164
Absorptions of gases by solids and liquids, 29
Accelerated motion, 86
Acceleration, 87
Adhesion, 21
Aeroplane, 97
Air, aspirator, 67
brake, 74
cushion, 46
height (of atmosphere), 64
pressure, 56
pump, 66
weight, 56
Alternators, 481
Alternating current, 337, 466
Amalgamation, 273
Ammeter, 291
Ampere, 291
Archimedes' principle, 48
Arc light, 321
Armature, 335
Artesian wells, 44
Audion, 463
Aurora borealis, 453
Balloon, 72
Barometer, 59
Beats, 376
Boiling, laws, 208
point, 207
Boyle's Law, 63
Breezes, land and sea, 181
British thermal unit, 162
Brownian movements, 16
Calorie, defined, 162
Camera, 426
Candlepower, 394
Capillary action, 25
Cartesian diver, 71
Cathode rays, 453
Centrifugal force, 91
Charles' Law, 165
Chladni's figures, 381
Choke coils, 470
Coefficient of expansion, definitions, 170
gases, 167
liquids, 168
solids, 169
Coherer, 449
Cohesion, 21, 33
Color, 435
bodies, 435
complementary, 436
primary, 440
prismatic, 433
theory of color vision, 440
three-color printing, 440
Commutator, 335, 336
Compass, 230, 240
Concave lens, 418
Condenser, 260, 474
Conductors, 246
Conservation of energy, 127
Continental code, 459
compared with the Morse, 452
Convection, 179
currents in nature, 181
draft of a chimney, 180
Convex lens, 416
Cooling, artificial, 210
Corpuscular theory, 442
Coulomb, 290
meter, 291
Couple, 101
Critical angle, 414
Crookes' tube, 456
Crystallization, 28
melting point of some crystalline substances, 203
Daniel cell, 276
Declination, 240
Density, 38, 52
methods for finding, 53
Dew, 192
Dew point, 193
Diffusion of gases, 13
Dipping needle, 240
Direct Current, 337
Dispersion, 433
Distillation, 208
Draft of a chimney, 180
Dry cell, 275
Dynamo, 330, 333
Dyne, 93
Eye, the, 423
action of, in vision, 423
defects of, 425
Ear, the, 386
trumpet, 361
Earth's magnetism, 238
Echoes, 362
Eclipses, 391
Efficiency, 142
engines (tests), 219
machines, 142
Elasticity, 31
Electric bell, 269, 287
charge, distribution of, upon a conductor, 253
circuit, 269
currents, 267
single phase, 479
three phase, 479
effects, 277
induced, 326
discharge in rarefied air, 452
motor, 339
screen, 256
Electrical capacity, 259
fields, 247
Electrification, 243
Electrolysis, 308
laws, 311
practical uses, 311
Electromagnet, 281
Electromagnetic theory of light, 456
Electromotive force, 267
unit of, 295
Electron theory, 252
Electrophorus, 263
Electroplating, 307
Electroscope, 244
Electrostatic induction, 248
Energy, 120
conservation, 127
falling water, 152
forms, 125
human body, 126
Energy, kinetic, 121
potential, 120
transference and transformation, 124
Engines, 213
Engines, gas, 222
steam, 213
turbine, 225
Equilibrant, 81
Equilibrium, 106
neutral, 107
stable, 106
stability, 108
unstable, 107
Erg, 119
Ether, 177
Evaporation, 18
cooling effect, 19, 197
rate, 198
Expansion, coefficient, 168
gases, 167
liquids, 168
peculiarity, in water, 168
solids, 169
water, on turning to steam, 206
Falling bodies, 109
experimental study, 111
laws, 113
Floating bodies, 48
Fluoroscope, 455
Foot candle, 396
Force, 79
dyne, 93
effectiveness, 134
graphic representation, 80
liquid, against any surface, 38
measuring, 79
moment, 99
parallel, 100
resolution, 96
units, 83
Forces, parallel, 100
Franklin's theory of electricity, 252
Fraunhofer lines, 439
Freezing, evaporation, 197, 199
mixtures, 210
Friction, 147,
coefficient, 149
fluid, 150
kinds, 147
Friction, laws, 150
reducing, 148
uses, 149
Galvanometers, 289
Galvanoscope, 269
Gas engine, 222
efficiency of, 224
Gas meter, 75
Geissler tubes, 453
Gravitation, 103
law, 104
Gravity, 88, 104
acceleration due to, 111
cell, 277
center of, 105
Hail, 193
Hearing, 386
Heat, capacity for water, 201
conduction, 173
constants for transmission, 220
convection, 179
effects, 161
engines, 213, 222
equivalent of fuels, 219
fusion, 201
measurement, 200
methods of transmitting, 173
produced by electric current, 318
radiation, 176
sources, 159
units, 162
vaporization, 205
work, 212
Heating of buildings, 182
direct and indirect radiation, 186
hot air, 183
water, 186
plenum system, 187
steam, 186
vacuum steam, 187
vapor steam, 187
Hertzian waves, 262, 449
Hooke's law, 33
Horse power, 123
electric equivalent of, 123
Humidity, 194
Hydraulic press, 42
elevator, 44
ram, 72
Hygrometers, 194
Hygrometry, 191
conditions for saturation, 192
dew point, 193
fog, 193
formation, of dew, 192
humidity, 194
hygrometers, 194
importance, 191
Hypothesis, 3
Images, concave mirrors, 405
construction, 405
definition, 392
plain mirror, 401
small apertures, 391
Impedance, 472
Incandescent lamp, 320
Inclined plane, 143
Inductance, 471
Induction coil, 343
Inertia, 87
Insulators, 246
Intensity of sound, 363
Interference, light, 442
sound, 374
Joule, 120, 319
Laws, boiling, 208
Boyle's 63
Charles', 165
electric action, 243
falling bodies, 113
floating bodies, 48
gravitation, 104
Hooke's, 33
induced currents, 326
intensity of light, 394
Lenz's, 328
liquid pressure, 37
machines, 131
magnetic action, 229
motion, 87
Ohm's, 298
pendulum, 116
reflection, 399
refraction of light, 411
vibration of strings, 378
Lenses, achromatic, 434
effect on light, 417
equation, 421
formation of images, 418
forms, 416
Leclanché cell, 275
Lever, 132
Leyden jar, 261
oscillatory nature of the discharge, 448
Light, compared with sound, 388, 444
electromagnetic theory, 456
intensity, 394
interference, 442
polarization, 445
rectilinear propagation, 389
reflection, 396
total reflection, 413
Lightning, 254
Lines, of force, 233
agonic, 240
isogonic, 240
Liquids, pressure, 36
Local action, 273
Luminous and illuminated bodies, 388
Machines, 129
advantages, 129
cannot create energy, 130
efficiency, 142
law, 131
mechanical advantage, 134
the six simple, 132
uses, 129
Magnetic action, 229
fields, 233, 466
induction, 231, 236
permeability, 237
poles, 229
properties, 230
retentivity, 231
substances, 230
effect of electric current, 279
Magnetism, 228
Magnetism, theory, 232
Major and minor triads, 369
Magneto, 328
Magnetoscope, 230
Magnets, 228
poles, 229
Major scale, 366
Manometric flames, 382
Matter, 4
effect of heat, 5
molecular theory, 5
properties, 34
states of, 4
states of, defined, 5
Mechanical advantage, 134
Megaphone, 365
Melting points, 203
Mercury arc rectifier, 347
Metric system, 8
Microscope, 427
Mirage, 414
Mirrors, 400
concave, 405
convex, 407
parabolic, 409
plane, 401
Molecular motion in liquids, 18
in gases, 13
in liquids and solids, 27
in solids, 31
Molecules, motion, 16
size, 13
Moment of force, 99, 133
Momentum, 87
law of, 92
Motion, 85
accelerated, 86
curvilinear, 88
direction, 86
first law, 87
modes, 85
second law, 92
third law, 93
uniformity, 86
Motor (electric), 339
A. C. series, 482
induction, 483
synchronous, 485
Muffler, 224
Musical instruments, 377
interval, 368
nomenclatures, table, 369
Musical sounds, characteristics 364
Newton's Laws of motion, 87
Nodes, in pipes, 384
in strings, 379
Noise and music, 363
Ohm, 294
Ohm's Law, 298
Opera glass, 428
Optical illusions, 390, 404
instruments, 423
camera, 426
eye, 423
microscope, 427
opera glass, 428
prism field glass, 429
projecting lantern, 427
telescope, 428
Organ pipes, closed, 384
nodes, 384
open, 384
Oscillatory discharge, 448
Osmosis, 19
Outline Review,
current electricity, 325
force and motion, 118
heat, 227
induced currents, 353
light, 446
magnetism and static electricity, 266
sound, 387
work and energy, 158
Overtones, 379
Pascal's principle, 41
Pendulum, compound, 115
laws, 116
simple, 115
uses, 116
Pepper's ghost, 404
Permeability, 237
Phonograph, 383
Physics, definition, 4
Photometer, 394
Photometry, 393
Pitch, 365
Polarization, of light, 445
of voltaic cells, 273
Portraits,
Bell, 431
Edison, 285
Faraday, 331
Galileo, 89
Gilbert, 217
Helmholtz, 397
Huygens, 397
Joule, 217
Kelvin, 331
Marconi, 431
Morse, 285
Newton, 89
Potential, 257
Power, 123
electric, 316
power factor, 476
transmission of electric, 476
water, 152
Pressure, air, 56
atmospheric, 58
Pressure, definition, 37
effect on liquids and gases, 62
law of liquid, 37
Prism field glass, 429
Projecting lantern, 427
Proof-plane, 244
Pulley, 139
Pumps, air, 66
condensing, 67
water (lift, 68, force, 69)
Quality of musical tones, 380
Radiation, 176
sun's, 178
Radio-activity, 457
Radiometer, 177
Radium, 457
Rainbow, 436
Reactance, 472
Reflection, light, 396
multiple, 404
sound, 360
total of light, 413
Refraction, 410
cause, 412
index, 412
light, 410
in plates, prisms, and lenses, 413
Resolution of forces, 96
Resistance, cells in series, and parallel, 302
conductors, 293
in series and parallel, 299, 300
unit, 294
volt-ammeter method for finding, 304
Resonance, 371
Resonator, 373
Resultant, 81
Retentivity, 231
Right hand rule, 279
Rotary converter, 486
Science, definition, 2
Screw, 144
Shadows, 390
Single phase currents, 479
Siphon, 70
Siren, 366
Solidification, change of volume during, 203
Solutions, 27
Sound, compared with light, 388
interference, 374
media, 355
nature, 356
reflection, 360
rule for finding velocity, 367
source, 354
speed, 355
transmission in air, 359
Specific heat, 200
method of determining, 201
Spectroscope, 438
Spectrum, 433
Spherical aberration, 408
Spinthariscope, 458
Stability, 108
Standpipe, 46
Static and current electricity compared, 287
electrical machines, 262
Steam engine, 213
turbine, 225
Storage battery, 312
Stress and strain, 94
Sublimation, 199
Surface tension, 22
Sympathetic vibration, 372
Telegraph, 283
wireless, 450
Telephone, 349
receiver, 349
transmitter, 350
wireless, 460
Telescope, 428
Temperature, 162
absolute scale, 164
Tempered scale, 370
Theory, 3
Thermometer, air, 167
centigrade, Fahrenheit, 163
gas, 167
Thermos bottle, 176
Thermostat, 188
Three-color printing, 440
Three-phase currents, 479
Three wire transmission, 480
Torricelli's experiment, 57
Trade winds, 182
Transformer, 345, 467
uses, 347
Turbine, steam, 225
water, 154
Vacuum cleaner, 76
pan, 210
Velocity, 86
Vibration strings, 378
sympathetic, 372
Visual angle, 424
Viscosity, 20
Volt, 295
Voltaic cell, 270
advantages, 274
amalgamation, 273
local action, 273
polarization, 273
simple, 270
Voltmeter, 295
Water wheels, 152
overshot, 152
turbine, 154
undershot, 153
Watt, 123, 317
Wave theory, of light, 442
Waves, beats, 376
interference, 374
longitudinal, 358
sound, 358
transverse, 358
visible, 357
Wedge, 144
Weight, 104
Wheatstone bridge, 304
Wheel and axle, 136
Wind instruments, 383
Wireless telegraphy, 450
telephony, 460
Work, 119
units, 119
"X" rays 454
Transcriber's Note:
This book uses B.T.U. and B.t.u., electrophorous and electrophorus,
e.m.f. and E.M.F. and this has been left as written.
Hyphenation is also inconsistent, e.g. electro-plated and
electroplated.
On page 324, Exercise number 8 was not used in the original. The
exercises have not been renumbered.