INDUCED CURRENTS
(1) ELECTROMAGNETIC INDUCTION
=296. Current Induced by a Magnet.=--The discovery in 1819 that a
current in a conductor can deflect a magnetic needle or that it has a
magnetic effect, led to many attempts _to produce an electric current by
means of a magnet_. It was not until about 1831, however, that _Joseph
Henry_ in America and _Michael Faraday_ in England, independently
discovered how to accomplish this important result.
At the present time, voltaic cells produce but a very small part of the
current electricity used. Practically all that is employed for _power,
light, heat, and electrolysis is produced by the use of magnetic fields,
or by electromagnetic induction_.
=297. Laws of Induced Currents.=[M]--To illustrate how a current can be
produced by electromagnetic induction:
[M] An induced current is one produced by changing the number of
magnetic lines of force passing through a coil.
Connect a coil of 400 or more turns of No. 22 insulated copper wire
to a sensitive galvanometer. (See Fig. 279.) Now insert a bar
magnet in the coil. A sudden movement of the galvanometer will be
noticed, indicating the _production of a current_. When the magnet
stops moving, however, the current stops, and the coil of the
galvanometer returns to its first position. If now the magnet is
removed, a movement of the galvanometer coil _in the opposite
direction is_ noticed. This action may be repeated as often as
desired with similar results.
Careful experiments have shown that it is the _magnetic field_ of the
magnet that produces the action, and that only when the _number of
lines of force in the coil is changing_ do we find a current produced in
the coil. These facts lead to _Law I_. _Any change in the number of
magnetic lines of force passing through or cut by a coil will produce an
electromotive force in the coil._ In the account of the experiment just
given, _electric currents_ are produced, while in Law I, _electromotive
forces_ are mentioned. This difference is due to the fact that an E.M.F.
is _always_ produced in a coil when the magnetic field within it is
changed, while a current is found only when the coil is part of a
_closed circuit_. The inductive action of the earth's magnetic field
(see Fig. 280), may be shown by means of a coil of 400 to 500 turns a
foot in diameter.
[Illustration: FIG. 279.--The moving magnet induces a current in the
coil.]
[Illustration: FIG. 280.--A current may be induced by turning the coil
in the earth's magnetic field.]
Connect its ends to a sensitive galvanometer and hold it at right
angles to the earth's field. Then quickly revolve the coil through
180 degrees and note the movement of the galvanometer. Reverse the
coil and the galvanometer swings in the opposite direction.
If the magnet in Fig. 279 is moved _in_ and _out_ of the coil at first
_slowly_ and _later swiftly_, _small and large_ deflections of the
galvanometer coil are noticed. The quicker the movement of the magnetic
field the greater are the galvanometer deflections produced. This leads
to _Law II_. _The electromotive forces produced are proportional to the
number of lines of force cut per second._
=298. The magneto= is a device that illustrates the laws of induced
currents stated in Art. 297. The magneto (see Fig. 281), consists of
several permanent, "U"-shaped magnets placed side by side. Between the
poles of these magnets is placed a slotted iron cylinder having a coil
of many turns of fine insulated copper wire wound in the slot as in Fig.
282. The cylinder and coil form what is called an _armature_. The
armature is mounted so as to be revolved between the poles of the
"U"-shaped magnets by means of a handle. As the armature revolves, the
lines of force from the magnets pass through the coil first in one
direction and then in the other. This repeated change in the lines of
force passing through the coil produces an E.M.F. which may be felt by
holding in the hands the two wires leading from the armature coil. On
turning the armature _faster_ the current is felt _much stronger_,
showing that the E.M.F. in the coil increases as the rate of cutting the
magnetic lines of force by the coils increases.
[Illustration: FIG. 281.--A magneto.]
[Illustration: FIG. 282.--A shuttle armature.]
[Illustration: FIG. 283.--The induced current has a field which opposes
the motion of the magnet. The heavy line represents the direction of the
induced current.]
=299. Lenz's Law.=--While one is turning the armature of a magneto if
the two wires leading from its coil are connected, forming what is
called a "short circuit," the difficulty of turning the armature is at
once increased. If now the circuit is broken, the armature turns as
easily as at first. The increased difficulty in turning the armature is
due to the _current_ produced in the coil. This current sets up a
magnetic field of its own that opposes the field from the steel magnets.
This opposition makes it necessary for _work_ to be done to keep up the
motion of the coil when a current is passing through it. This fact is
called _Lenz's Law_. It may be expressed as follows: _Whenever a current
is induced by the relative motion of a magnetic field and a conductor,
the direction of the induced current is always such as to set up a
magnetic field that opposes the motion._ Lenz's Law follows from the
principle of conservation of energy, that energy can be produced only
from an expenditure of other energy. Now since an electric current
possesses energy, such a current can be produced only by doing
mechanical work or by expending some other form of energy. To illustrate
Lenz's Law, suppose that the north-seeking pole of a bar magnet be
inserted in a closed coil of wire. (See Fig. 283.) The current induced
in the coil has a direction such that its lines of force will pass
within the coil so as to _oppose_ the field of the bar magnet, when the
north pole of the magnet is inserted so as to point to the left. That
is, the north pole of the helix is at the right. Applying the right-hand
rule to the coil, its current will then be _counter clockwise_. On
withdrawing the magnet, the current reverses, becoming _clockwise_ with
its field passing to the left within the coil.
A striking illustration of the opposition offered by the field of the
induced current to that of the inducing field is afforded by taking a
strong electromagnet (see Fig. 284) and suspending a sheet of copper so
as to swing freely between the poles. When no current flows through the
magnet the sheet swings easily for some time. When, however, the coils
are magnetized, the copper sheet has induced within it, currents that
set up magnetic fields strongly opposing the motion, the swinging being
stopped almost instantly. The principle is applied in good ammeters and
voltmeters to prevent the swinging of the needle when deflected. The
current induced in the metal form on which is wound the galvanometer
coil is sufficient to make the needle practically "dead beat."
[Illustration: FIG. 284.--The magnetic field stops the swinging of the
sheet of copper.]
=300. The Magneto and the Dynamo.=--Magnetos are used to _develop small_
currents, such as are used for telephone signals, and for operating the
_sparking_ devices of gasoline _engines_. They are therefore found in
automobiles containing gasoline motors. The most important device for
producing electric currents by electromagnetic induction, however, is
the _dynamo_. It is employed whenever large currents are desired. The
principle of this device is similar to that of the magneto except that
it contains an _electromagnet_ for producing the magnetic field.
Since the electromagnet can develop a much stronger field than a
permanent magnet, the dynamo can produce a higher E.M.F. and a much
larger current than the magneto.
[Illustration: LORD KELVIN
"By Permission of the Berlin Photographic Co., New York."
Lord Kelvin (Sir William Thomson), (1824-1907). Professor of Physics,
Glasgow University. Invented the absolute scale of temperature: also
many practical electrical measuring instruments. The foremost physicist
of the latter part of the nineteenth century.]
[Illustration: MICHAEL FARADAY
"By Permission of the Berlin Photographic Co., New York."
Michael Faraday (1791-1867). Famous English Physicist. Made many
discoveries in electricity and magnetism; "Greatest experimentalist of
the nineteenth century."]
=301. The Magnetic Fields of Generators.=--In the magneto, the magnetic
field is produced by _permanent_ steel magnets. In dynamos powerful
_electromagnets_ are used. The latter are sometimes excited by currents
from some other source, but usually current from the armature is sent
around the field coils to produce the magnetic fields. Dynamos are
classified according to the manner in which the current is sent to their
field coils.
[Illustration: FIG. 285.--A series-wound dynamo.]
[Illustration: FIG. 286.--A shunt-wound dynamo.]
[Illustration: FIG. 287.--A compound-wound dynamo.]
_A._ The _series wound dynamo_ (see Fig. 285) is arranged so that _all_
of the current produced by the armature is sent through coils of coarse
wire upon the fields, after flowing through the external circuit.
_B._ The _shunt wound dynamo_ (see Fig. 286) sends a part only of the
current produced through the field coils. The latter are of many turns
of fine wire so as to use as little current as possible. The greater
part of the current goes to the main circuit. If the number of lamps or
motors connected to the main circuit is increased, the voltage is
lessened which weakens the current in the field coils, causing a weaker
field and still lower voltage, producing a fluctuating E.M.F. which is
unsatisfactory for many purposes. This fault is overcome by
_C._ the _compound wound dynamo_. This dynamo has both shunt and series
coils upon its fields. (See Fig. 287.) If more current is drawn into the
main circuit with this dynamo, the series coils produce a stronger field
compensating for the weaker field of the shunt coils, so that uniform
voltage is maintained. The compound wound generator is therefore the one
most commonly employed.
Important Topics
1. Laws of electromagnetic induction (a) conditions, (b) E.M.F., (c)
direction.
2. Devices, (a) magneto, (b) dynamo: series, shunt, compound.
3. Illustrations of the laws.
Exercises
1. Under what conditions may an electric current be produced by a
magnet?
2. Show how Lenz's Law, follows from the principle of conservation of
energy.
3. A bar magnet is fixed upright with its north-seeking pole upward. A
coil is thrust down over the magnet. What is the direction of the
current induced in the coil? Explain.
4. In what two ways may a current be induced in a closed coil?
5. What method is employed in the magneto? In the dynamo?
6. What is the nature of the current produced in the armature coil of a
magneto, that is, is it direct or alternating? Why?
7. What is the resistance of a 20-watt tungsten lamp if the E.M.F. is
115 volts?
8. Find the resistance of a 40-watt tungsten lamp when the voltage is
115? How much heat will it produce per minute?
9. An Edison storage battery cell on a test gave a discharge of 30
amperes. The average voltage was 1.19. What was the resistance of the
cell?
10. Eight storage cells are connected in series. Each has an E.M.F. of
1.2 volts and an internal resistance of 0.03 ohms. What will be the
current flowing through a voltmeter having 500 ohms resistance in
circuit with them?
(2) THE DYNAMO AND THE MOTOR
=302. The Dynamo= may be defined as a machine for transforming
mechanical energy into the energy of electric currents by
electromagnetic induction. Although electromagnetic induction was
discovered in 1821, practical dynamos were not built for about 40 years
or until between 1860 and 1870. The great development in the production
and use of electric currents has come since the latter date. The
principle parts of the dynamo are (a) the _field magnet_, (b) the
_armature_, (c) the _commutator_ or _collecting rings_, (d) the
_brushes_. Fig. 288 shows several common methods of arranging the field
coils and the armature.
[Illustration: FIG. 288.--Several methods of arranging the field coils
and the armature of a dynamo.]
[Illustration: FIG. 289.--A drum armature.]
The field coils vary in number and position. The purpose of their
construction is always to send the largest possible number of lines of
force through the armature. Some dynamos are _bipolar_, or have _two_
poles, others are multipolar or have more than two. In Fig. 288 No. 4
has four poles. The _armature_ of a dynamo differs from a magneto
armature in that it consists of a series of coils of insulated copper
wire wound in numerous slots cut in the surface of a cylindrical piece
of iron. Fig. 289 shows a _side_ view of the iron core of such an
armature. Iron is used to form the body of the armature since the
magnetic lines of force flow easily through the iron. The iron by its
permeability also concentrates and increases the magnetic flux. The best
armatures are made of many thin sheets of soft iron. These are called
_laminated_ armatures. An armature made of a solid piece of iron becomes
hot when revolving in a magnetic field. This is due to electric currents
induced in the iron itself. This heating is largely reduced by
_laminating_ the armature. Why?
[Illustration: FIG. 290.--Armature connected to slip rings producing an
alternating current.]
=303. Methods of Collecting Current from the Armature.=--The electric
currents produced in the armature are conducted away by _special sliding
contacts_. The stationary part of the sliding contact is called a
_brush_. The moving part is a _slip ring_ or a _commutator_. Fig. 290
shows an armature coil connected to slip rings. As the armature
revolves, the coils and slip rings revolve with it. The two ends of the
armature coils are connected to the two rings respectively. Now as the
armature revolves it cuts the lines of force first in one direction and
then in the other. This produces in the coils an E.M.F. first one way
and then the other. This E.M.F. sets up a current which is conducted to
the outside circuits through the slip rings and brushes. Such a current
which repeatedly reverses its direction is called an _alternating
current_. Fig. 291 (1) indicates graphically how the current moves
alternately one way and then the other. Alternating currents are
extensively used for electric _light, heat, and power_. _Direct
currents_ or those going continuously in one direction are however in
much demand especially for _street car service_, _for electrolysis_, and
for _charging storage batteries_.
=304. The Commutator.=--For a dynamo to deliver a _direct current_ it
must carry upon the shaft of the armature a _commutator_. The commutator
is used to _reverse_ the connections of the ends of the armature coils
at the instant that the current changes its direction in the armature.
This reversal of connection when the direction of current changes, keeps
the current in the outside circuit flowing in the same direction. Fig.
291 is a diagram of an armature with a commutator. The commutator is a
_split ring_, having as many parts or _segments_ as there are coils upon
the armature. The brushes touch opposite points upon the commutator as
they slide over the surface of the latter. Suppose that the armature
viewed from the commutator end rotates in a counter-clockwise direction,
also that the currents from the upper part move toward the commutator
and out the top brush.
[Illustration: FIG. 291.--The armature coils are connected to a
commutator producing a direct current.]
As the armature revolves, its coils soon begin to cut the force lines in
the opposite direction. This change in the direction of cutting the
lines of force causes the current to reverse in the coils of the
armature. At the instant the current changes in direction, what was the
upper segment of the commutator slips over into contact with the lower
brush, and the other segment swings over to touch the upper brush. Since
the current has reversed in the coils it continues to flow out of the
upper brush. This change in connection at the brushes takes place at
each half turn of the armature, just as the current changes in direction
in the coils. This is the manner in which the commutator of a dynamo
changes the alternating current produced in the armature coils, into a
direct current in the external circuit. Fig. 292 (1) represents
graphically an alternating current, (2) of the same figure shows current
taken from the brushes of the commutator of a dynamo with one coil on
the armature.
[Illustration: FIG. 292.--Graphic representation of (1) an alternating
current; (2) a pulsating current; (3) a continuous current.]
[Illustration: FIG. 293.--DeLaval multi-stage turbine and gear driving
750-kw., 750-r.p.m., 600-volt direct-current generator.]
A practical dynamo, however, has many coils upon its armature with a
corresponding number of segments upon the commutator. (See Figs. 289 and
293.) As each coil and commutator segment passes a brush, it contributes
an impulse to the current with the result that armatures with many coils
produce currents that flow quite evenly. (See Fig. 292, 3.)
The current represented in Fig. 292 (2) is called a _pulsating_ current.
[Illustration: FIG. 294.--A wire carrying a current across a magnetic
field is pushed sideways by the field.]
=305. The electric motor= is a machine which transforms the energy of an
electric current into mechanical energy or motion. The _direct current
motor_ consists of the same essential parts as a direct current dynamo,
viz., the field magnet, armature, commutator and brushes. Its operation
is readily comprehended after one understands the following experiment:
Set up two bar electromagnets with unlike poles facing each other about
an inch apart. A wire connected to a source of current is hung loosely
between the poles as in Fig. 294. The circuit through the wire should
contain a key or switch. If a current is sent through the electromagnets
and then another is sent through the wire, the latter will be found to
be pushed either up or down, while if the current is reversed through
the wire it is pushed in the opposite direction. These results may be
explained as follows:
Consider the magnetic field about a wire carrying a current (See Fig.
295.) If such a wire is placed in the magnetic field between two
opposite poles of an electromagnet (Fig. 296), the wire will be moved
either up or down. The reason for this is shown by the diagram in Fig.
297. Here a wire carrying a current and therefore surrounded by a
magnetic field passes across another magnetic field. The two fields
affect each other causing a crowding of the force lines either above or
below the wire. The wire at once tends to move sideways across the field
away from the crowded side. In the figure, the wire tends to move
downward.
[Illustration: FIG. 295.--The magnetic field about a wire carrying a
current.]
[Illustration: FIG. 296.--The magnetic field between two unlike poles.]
[Illustration: FIG. 297.--The crowding of the lines of force above the
wire, pushes it downward.]
In a practical motor, the wires upon the armature are so connected that
those upon one side (see Fig. 298), carry currents that pass in, while
on the other side they pass out. To represent the direction of the
current in the wires, the following device is employed; a circle with a
cross (to represent the feather in the tail of an arrow) indicates a
current going away from the observer, while a circle with a dot at its
center (to represent the tip of an arrow) indicates a current coming
toward the observer.
[Illustration: FIG. 298.--The crowding of the lines of force causes the
armature to revolve in a clockwise direction.]
In Fig. 298 the north pole is at the left and the south pole at the
right. The field of the magnets therefore passes from left to right as
indicated in the figure. Now in the armature the currents in the wires
on the left half of the armature are coming toward the observer while
those on the right move away. Applying the right-hand rule, the magnetic
lines will crowd _under_ the wires on the left side of the armature
while they will crowd _over_ the wires on the right side. This will
cause a rotation up on the left side and down on the right, or in a
_clockwise_ direction.
[Illustration: FIG. 299.--View of a one-half horse-power motor.]
If the current in the armature is reversed (in on the left and out on
the right), the lines of force will crowd the armature around in the
opposite direction or _counter clockwise_. The rotation of the armature
will also be reversed if, while the current in the armature is unchanged
in direction, the poles of the magnet are changed thus reversing the
magnetic field.
The motorman of a street car reverses the motion of his car by reversing
the direction of the current in the _armature_ of the motor.
[Illustration: FIG. 300.--The frame and electromagnet (at left), front
bracket and brush holder (at right) of the motor shown in Fig. 299.]
[Illustration: FIG. 301.--The armature of a motor.]
=306. Practical motors= have many coils upon the armature with a
corresponding number of segments upon the commutator. A large number of
coils and commutator segments enables some one of the coils to exert its
greatest efficiency at each instant, hence a steady force is provided
for turning the armature which causes it to run smoothly. Fig. 299
represents a 1/2 horse-power motor ready for use while Fig. 300 shows
the frame and poles and the front bracket and brush holder, and Fig. 301
represents the armature.
Important Topics
1. The dynamo, four essential parts, action (a) for alternating
currents, (b) for direct currents.
2. The electric motor: (a) essential parts, (b) action.
Exercises
1. _Why_ is an alternating current produced in the armature of a dynamo?
2. _How_ is this current produced? Give careful explanations.
3. What is the result of Lenz's law as applied to the dynamo?
4. Apply the first two laws of electromagnetic induction to the dynamo.
5. What is the power of a dynamo if it produces 40 amperes of current at
110 volts?
6. How much power must be applied to this dynamo if its efficiency is 90
per cent.?
7. A motor takes 10 amperes of current at 220 volts; what is the _power_
of the current in _watts_? If this motor has an efficiency of 95 per
cent., how many horse-power of mechanical energy can it develop?
8. Explain why reversing the current in the armature of a motor reverses
the direction of rotation.
9. Find the cost of running a washing machine using a 1/2-horsepower
motor 2 hours if the cost of the electricity is 10 cents a kilowatt
hour.
10. A 1/8-horse-power motor is used to run a sewing machine. If used for
3 hours what will be the cost at 11 cents a kilowatt hour?
(3) THE INDUCTION COIL AND THE TRANSFORMER
=307. The Induction Coil.=--Practically all electric currents are
produced either by voltaic cells or by dynamos. It is frequently found,
however, that it is desirable to change the E.M.F. of the current used,
either for purposes of _effectiveness_, _convenience_, _or economy_. The
_induction coil_ and the _transformer_, devices for changing the E.M.F.
of electric currents, are therefore in common use. _The induction coil_
(see Fig. 302) consists of a _primary_ coil of coarse wire _P_ (Fig.
303) wound upon a core of soft iron wire, and a _secondary_ coil, _S_,
of several thousand turns of fine wire. In circuit with the primary coil
is a battery, _B_, and a current interrupter, _K_, which works like the
interrupter upon an electric bell. The ends of the secondary coil are
brought to binding posts or spark points as at _D_.
[Illustration: FIG. 302.--An induction coil.]
The current from the battery flows through the primary coil magnetizing
the iron core. The magnetism in the core attracts the soft-iron end of
the interrupter, drawing the latter over and breaking the circuit at the
screw contact, _K_. This abruptly stops the current and at once the core
loses its magnetism. The spring support of the interrupter now draws the
latter back to the contact, _T_, again completing the circuit. The whole
operation is repeated, the interrupter vibrating rapidly continually
opening and closing the circuit.
[Illustration: FIG. 303.--Diagram showing the parts of an induction
coil.]
=308. The Production of Induced Currents in the Secondary Coil.=--When
the current flows through the _primary_ it sets up a magnetic field in
the _core_. When the current is interrupted, the field disappears. The
increase and decrease in the field of the core induces an E.M.F. in the
secondary coil, in accordance with the first law of electromagnetic
induction. The E.M.F. produced depends upon (a) the number of turns in
the secondary, (b) the strength of the magnetic field and (c) the rate
of change of the field. The rate of change in the field is more rapid at
the break than at the make. When the circuit is closed it takes perhaps
1/10 of a second for the current to build up to its full strength while
at a break the current stops in perhaps 0.00001 of a second, so that the
induced E.M.F. is perhaps 10,000 times as great at "break" as at make.
To increase the suddenness of the "make" and "break," a condenser is
often connected in the primary circuit, in parallel, with the
interrupter. (See Fig. 303, _C._) This condenser provides a place to
hold the rush of current at the instant that the interrupter breaks the
circuit. This stored up charge reinforces the current at the make
producing a much more sudden change in the magnetic field with a
corresponding increase in the E.M.F. The induced currents from induction
coils are sometimes called _faradic currents_ in honor of Faraday who
discovered electromagnetic induction. They are used to operate sparking
devices upon gas and gasoline engines and in many devices and
experiments in which high-tension electricity is employed.
[Illustration: FIG. 304.--The transformer has a closed core; the
induction coil, an open core.]
[Illustration: FIG. 305.--The laminated iron core of a transformer.]
[Illustration: FIG. 306.--Cross-section of the transformer shown in Fig.
305 showing the magnetic field around the primary and secondary coils.]
=309. The Transformer.=--This is like the induction coil in that it uses
a _primary_ and a _secondary_ coil, and an iron core to carry the
magnetic field. (See Fig. 304.) They differ in that the transformer has
a _closed_ core or one forming a continuous iron circuit, while the
induction coil has an _open_ core, or one in which the magnetic field
must travel in air from the north to the south poles of the core. The
transformer must always be used with an _alternating_ current while the
induction coil may use either a direct or an alternating current.
Further, the _induction_ coil always produces a higher E.M.F. while the
transformer may produce an E.M.F. in its secondary coil that is either
higher or lower than the one in the primary. The former is called
"_step-up_" while the latter is a "_step-down_" transformer. The
alternating current in the primary coil of the transformer produces an
_alternating magnetic flux_ in the iron core. This iron core is
_laminated_ (see Fig. 305) to prevent the heating that would result if a
solid core were used. The alternating magnetic flux induces in the
secondary coil an E.M.F. in accordance with the following rule. The
ratio of the _number_ of _turns_ in the _primary_ to the _number_ of the
_turns_ in the _secondary_ coil equals the ratio of the electromotive
forces in these respective coils. If the secondary coil has 8 turns
while the primary has 4, the E.M.F. of the secondary will be just twice
that of the primary. Or, if in the primary coil of the transformer Fig.
306 is an E.M.F. of 110 volts, in the secondary will be found an E.M.F.
of 220 volts.
[Illustration: FIG. 307.--A commercial transformer.]
=310. Uses of Transformers.=--In electric lighting systems, dynamos
often produce alternating currents at 1000 to 12,000 volts pressure. It
is very dangerous to admit currents at this pressure into dwellings and
business houses, so that transformers are installed just outside of
buildings to "step-down" the high voltage currents to 110 or 220 volts.
The lighting current that enters a house does not come directly from a
dynamo. It is an induced current produced by a transformer placed near
the house. (See Fig. 307.) In a perfect transformer the efficiency would
be 100 per cent. This signifies that the energy that is sent into the
primary coil of the transformer exactly equals the energy in the
secondary coil. The best transformers actually show efficiencies better
than 97 per cent. The lost energy appears as heat in the transformer.
"The transfer of great power in a large transformer from one circuit to
another circuit entirely separate and distinct, without any motion or
noise and almost without loss, is one of the most wonderful phenomena
under the control of man."
=311. The mercury arc rectifier= is a device for changing an alternating
current into a direct current. It is frequently used for charging
storage batteries where only alternating current is supplied by the
electric power company. It consists of an exhausted bulb containing two
carbon or graphite electrodes marked _G_ in Fig. 308 and a mercury
electrode marked _M_. It is found that current will pass through such a
bulb only from the graphite to the mercury but not in the reverse
direction. In operating the device, the secondary terminals of an
alternating current transformer _T_ are connected to the graphite
terminals of the rectifier. A wire connected to the center of the
secondary of the transformer at _C_ is attached to the _negative_
terminal of the storage battery _SB_. The _positive_ terminal of the
battery is connected to the mercury electrode of the rectifier tube
through a reactance or choke coil _R_. This coil serves to sustain the
arc between the alternations. _Sw_ is a starting switch, used only in
striking the arc. It is opened immediately after the tube begins to
glow.
[Illustration: FIG. 308.--Diagram of a mercury arc rectifier.]
Important Topics
Transformer, induction coil, mercury arc rectifier, construction,
action; uses of each.
Exercises
1. Does the spark of an induction coil occur at "make" or at "break?"
Why?
2. What must be the relative number of turns upon the primary and
secondary coils of a transformer if it receives current at 220 volts
and delivers current at 110? Also show by diagram.
3. Would the transformer work upon a direct current? Why?
4. Explain why the interrupter is a necessary part of the induction coil
and not of the transformer.
5. If a building used eighty 110-volt incandescent lamps, what would be
necessary to light them if they were joined in series? Why would this
not be practical?
6. If a 16-candle-power lamp requires 0.5 ampere upon a 110-volt circuit
what current and voltage will be needed to operate 12 such lamps in
parallel?
7. What will it cost to run these lamps 4 hours a night for 30 days at
10 cents per kilowatt hour?
8. If a mercury arc rectifier uses 5 amperes of current at 110 volts
alternating current to produce 5 amperes of direct current at 70 volts,
what is the efficiency of the rectifier?
9. Compute the heat produced in a 40 watt tungsten lamp in 1 minute.
10. Compute the heat produced in a 60 watt carbon incandescent lamp in 1
hour.
(4) THE TELEPHONE
=312. The Electric Telephone.=--This is an instrument for reproducing
the human voice at a distance by an electric current. The modern
electric telephone consists of at least four distinct parts (see Fig.
312); viz., a _transmitter_, an induction coil, an electric battery, and
a _receiver_. The first three of these are concerned in sending, or
_transmitting_ over the connecting wires a fluctuating electric current,
which has been modified by the waves of a human voice. The receiver, is
affected by the fluctuating current and reproduces the voice. It will be
considered first, in our study.
=313. The telephone receiver= was invented in 1876 by Alexander Graham
Bell. It consists of a permanent steel magnet, U shaped, with a coil of
fine insulated copper wire about each pole. (See Fig. 310.) A disc of
thin sheet iron is supported so that its center does not quite touch
the poles of the magnet. A hard rubber cap or ear piece with an opening
at its center is screwed on so as to hold the iron disc firmly in place.
[Illustration: FIG. 309.--The simplest telephone system. It consists of
two telephone receivers connected in series on a circuit. It will work,
but not satisfactorily.]
_The action of the receiver_ may be understood from the following
explanation: The electric current sent to the receiver, comes from the
secondary coil of the induction coil; it is an alternating current,
fluctuating back and forth just in time with the waves of the voice
affecting it at the transmitter. This alternating current flows around
the coils on the poles of the permanent magnet. When this current flows
in one direction, its magnetic field assists the field of the permanent
magnet, strengthening it. This stronger magnetic field draws the thin
iron disc in front of the poles of the magnet a little closer to them.
When the current in the coils flows the other way, its magnetic field
weakens the field of the steel magnet, and the disc is drawn back by the
force of its own elasticity. Thus the disc of the receiver vibrates with
the alternations of the current, and reproduces the same sounds that
were spoken into the transmitter.
[Illustration: FIG. 310.--A telephone receiver. This receiver has a
permanent horseshoe magnet with a coil about each pole.]
=314. The Telephone Transmitter.=--The telephone receiver just described
has great sensitiveness in reproducing sound, but it is not satisfactory
as a transmitter or sending apparatus. The _transmitter_ commonly used
is represented in cross-section in Fig. 311. In this figure, back of the
mouthpiece, is a thin carbon disc, _D_. Back of this disc is a circular
compartment containing granular carbon, _g_. The wires of the circuit
are connected to the carbon disc and to the back of the case containing
granular carbon. The circuit through the transmitter also includes a
voltaic or storage cell and the primary coil of an induction coil. (See
Fig. 312.)
[Illustration: FIG. 311.--A telephone transmitter.]
[Illustration: FIG. 312.--Telephone instruments at one end of a talking
circuit.]
=315. The action of the transmitter= is explained as follows: When the
sound waves of the voice strike upon the carbon disc, the latter
vibrates, alternately increasing and decreasing the pressure upon the
granular carbon. When the pressure _increases_, the electrical
resistance of the granular carbon is _lessened_, and when the pressure
upon it is _decreased_, its resistance _increases_. This changing
resistance causes fluctuations in the electric current _that_ correspond
exactly with the sound waves of the voice affecting it.
=316. A complete telephone system= operating with a local battery is
shown in Fig. 312. A person speaking into the transmitter causes a
fluctuation in the electric current in the transmitter as described in
Art. 315. This fluctuating current passes through the primary coil of
the induction coil _Ic_. This fluctuating current produces a fluctuating
magnetic field in its core. This fluctuating field induces an
_alternating_ current in the secondary coil which alternates just as the
primary current fluctuates, but with a much higher E.M.F. than the
latter. The alternating current passes to the receiver which reproduces
the speech as described in Art. 313. The line circuit includes the
secondary of the induction coil, the receiving instrument and the
receiver of the sending instrument so that the voice is reproduced in
both receivers. An electric bell is placed at each station to call the
attention of parties wanted. The movement of the receiver hook when the
receiver is lifted, disconnects the bell and closes the talking circuit.
The latter is opened and the bell connected when the receiver is hung up
again.
[Illustration: FIG. 313.--Diagram of a telephone system as used in a
large exchange.]
In cities and towns, the telephone system in use differs from the one
described in usually having one large battery placed in the central
exchange, instead of dry cells at each instrument. (See Fig. 313.) Also
the operator at _central_ is called by simply taking the receiver from
the hook instead of being "rung up" by the subscriber. The operations of
the transmitter, induction coil and receiver, however, are the same in
all telephones.
Important Topics
1. Receiver: parts, action.
2. Transmitter: parts, action.
3. Induction coil, bell, line wires, etc.
4. Action of the whole device.
Exercises
1. State three important electrical laws or principles that are employed
in the operation of the telephone. What is the application of each?
2. Connect the binding posts of a telephone receiver with a sensitive
galvanometer and press on the diaphragm of the receiver; a deflection of
the galvanometer will be noticed. Release the diaphragm and a reflection
in the opposite direction is seen. Explain.
3. Is the current passing through the transmitter the one going to the
receiver of the instrument? Explain.
4. Does the receiver at the telephone used by a person repeat the speech
of the person? Explain.
5. How many 0.5 ampere lamps can be used with a 6 ampere fuse?
6. Why is it necessary to have a rheostat connected in series with a
stereopticon or moving picture machine while a rheostat is not used with
arc lights out doors?
7. How many candle power should a 60 watt carbon incandescent lamp give,
if its efficiency is 3.4 watts per candle power?
8. Three incandescent lamps having resistances of 100, 150, and 240
ohms, respectively, are connected in parallel. What is their combined
resistance?
Review Outline: Induced Currents
Induced currents; 3 laws, illustrations.
Construction, action, and uses of--magneto, dynamo, induction coil,
transformer, motor, telephone. Mercury arc rectifier.
Terms--primary, secondary, for coils and currents, armature, commutator,
slip ring, brush, rectifier, open core, series, shunt, and compound
connections for dynamos.