but never requires mercy, or at least rarely receives it. It comes
nearer than any other machine to paying back all that you put into it.
It is most economical when working up to its full capacity. I recommend
that you look it over carefully and after a few minutes tell me what
you have seen in it.
[Illustration: Fig. 19]
The boys said that it looked just like a dynamo. We must not forget
that it is a dynamo, but is here used as a motor by sending an electric
current through it. This fact, that a dynamo might be driven by an
electric current and serve as a mover of other machinery, was first
publicly exhibited in 1873 at the Vienna Exhibition, and by many
believed to have been discovered by accident at that exhibit. But why
does it look like a dynamo? It has a field whose magnetism is produced
by an electric current sent through coils of wire, and it has an
armature whose magnetism is likewise produced by the electric current.
If it were used as a dynamo, where would it get the electric current to
magnetize its field? From its own moving armature. Is it adapted for
direct current? Yes. It has a commutator and brushes. Is it shunt- or
series-wound? Shunt-wound, as shown by diagram in Fig. 20.
[Illustration: Fig. 20]
Suppose we treat the machine as a dynamo. Bring the ends of the line
wire together, thus, as we say, closing the circuit. By some external
force let us cause the armature to rotate and under the influence
of the magnetic field it will generate an electric current, part of
which will pass through the field and part through the line circuit.
We may adjust the relative amount of wire in field and line so that
any portion of the current we choose will pass through the field. The
amount of current it will generate depends, (1) upon the strength of
the field and (2) upon the speed of the armature. Its field, although
never entirely without magnetism, is very feeble at first, and hence
in the first instance a very small current will be generated in the
moving armature. This, however, will strengthen the field slightly, and
as the field is strengthened the armature will generate more current,
and thus by a mutual reaction the machine gradually "builds up" to full
strength.
[Illustration: Fig. 21]
When now we use the machine as a motor, an electric current must be
sent along the line wires in the opposite direction (Fig. 21) from
which it would come out of the machine when acting as a dynamo. It will
then be noticed that, although the direction of the current through
the field is the same, whether the machine is used as a dynamo or a
motor, the direction through the armature, when used as a motor, is the
reverse of that when used as a dynamo.
You may perhaps be able to notice that the amount of wire on the field
is considerably more than that on the armature. Now if you will trace
the wires carefully you will find that there is provided a way of
supplementing the wire of the armature with some more wire in what
is called the rheostat, Fig. 22. This wire, or portions of it, is
introduced into the armature circuit when the machine first starts.
When, however, the machine has started and the armature is moving
within the influence of a magnetic field, it plays the part of a dynamo
at the same time that it is acting as a motor. Two conflicting and
opposite electro-motive forces therefore exist in the armature at the
same time. In Fig. 22 the arrow _a_ represents the direction of the
electro-motive force which is impressed upon the armature, and the
arrow _b_ represents the counter-electro-motive force which the moving
armature develops.
[Illustration: Fig. 22]
This counter-electro-motive force, which develops while the machine is
in motion, makes it unnecessary to hold back the current longer by the
extra resistance of the rheostat and hence that is usually cut out.
Being used only for starting purposes and looking like a box, it is
generally called the "starting box." If now it was intended that this
motor should run at a constant speed, as is often the case, no other
governor would be needed than this counter-electro-motive force, for
whenever the machine begins to go faster, on account of reduced load,
its counter-electro-motive force increases as the speed and holds in
check the impressed electro-motive force. This acts very perfectly as
a governor, and motors operate with notoriously constant speed under
variable loads. But, of course, in this present instance the motor is
required to work at a variable speed. It must pump air slowly for the
soft passages of music, and it must work the pump to its utmost for the
very strong passages.
[Illustration: Fig. 23]
To understand how an electric motor may pump an organ and have its
speed automatically controlled, let us examine the diagram in Fig.
23. The motor _m_ causes the shaft _S_ to revolve, carrying the crank
_C_ around with it. The rod _r_ causes _a b_, the lower side of the
bellows, to rise and fall, this side being hinged at _b_. The side _b
c_, is fixed. When the side _a b_ is pushed upward by the crank rod the
valve _f_ closes and the air in the compartment _h_ pushes open the
valve _g_ and enters the compartment _j_. The upper side _d e_, of this
compartment rises as it is filled with air. Weights _K_, _K_, _K_,
rest on the top of this and air ducts lead from this compartment to
the pipes of the organ. The keys of the organ operate air cocks which
open and close the air ducts connected with the organ-pipes. A chain
connected with _e_ passes around the axle of the wheel _l_ and has a
weight _W_ upon its lower end. The wheel _l_ carries a strip of brass
_n_, which slides over metal points _p_, _p_, _p_, etc. The successive
points are connected by coils of wire to furnish resistance. This
series of coils is called a rheostat. The wires _t_ and _u_ form a loop
from the armature of the motor and connect this rheostat in series with
the armature. _u_ is connected with the brass strip _n_. Notice that
when the compartment _j_ is full of air and the side _d e_, is lifted
to its greatest height the strip _n_ is moved to the lowest point _p_,
and the electric current must pass from _u_ through all the resistance
of the rheostat in order to get back to the armature by the wire _t_.
This makes the motor go very slowly. When _d e_ sinks down, the strip
_n_ moves to the upper points _p_, and the resistance is reduced step
by step, enabling the motor to quicken its speed and pump faster as
more air is required.
Small motors in order to be effective must travel at high speed. This
motor when moving at its highest speed makes 1,800 revolutions per
minute. The bellows on the other hand needs to be large and move slowly
in order to be efficient. Hence the motor is not in reality connected
directly to the shaft _S_, but causes the shaft to revolve by means of
a series of pulleys and belts. The pulley on the motor is three inches
in diameter. It is connected by a flat leather belt with a wheel thirty
inches in diameter. When the motor therefore, makes 1,800 revolutions
per minute this wheel makes 180 revolutions per minute. The axle of
this wheel carries a small cog-wheel three inches in diameter and it
is connected by a chain belt with a cog wheel on the shaft _S_ (Fig.
23). Thus this shaft revolves thirty times per minute, that is, the rod
_r_ rises and falls each second. A pull of one pound on the rim of the
motor pulley will cause a pull of sixty pounds on the cogs of the wheel
upon the shaft _S_. If the second belt were leather, a sixty-pound pull
would cause it to slip on the smaller pulley. Hence the second belt is
a steel chain and the wheels have cogs, or sprockets, like a bicycle.
[Illustration: Fig. 24]
The organist before beginning to play closes a double-pole,
single-throw switch (Fig. 24), which sends the electric current to the
motor.
The motor pumps air until the bellows is full, and if the organist
delays playing, the strip of brass _n_ (Fig. 23) is carried below the
lowest point _p_, thus cutting off the current and stopping the motor.
As soon as he uses some of the air in the bellows, however, _n_ rises
and makes contact with the points _p_ and the motor starts.
This suggests that a somewhat similar thing is accomplished under
electric cars which have air brakes. An electric motor pumps the air
and compresses it in a tank. When the pressure reaches a certain
point, say sixty pounds per square inch, it automatically shuts off
the electric current from the motor which works the pump. But when the
motorman uses some of the air to apply the brakes to the wheels, and
the pressure in the tank falls below sixty pounds, the electric current
is again automatically turned on to the motor.
Of course if an electric motor can operate a pump to compress air it
may also work a pump to exhaust air. This is what is done in a vacuum
cleaner. The electric pump as it is called (which means a pump worked
by an electric motor), exhausts some of the air from a compartment in
the machine, and the atmosphere pressing in through nozzle and hose
carries dust from rugs and furniture with it into the compartment. The
best vacuum cleaners will produce a pressure of seven or eight pounds
per square inch, about half an atmosphere. This will remove dust from
the warp and woof of a rug better than our greatest hurricanes can when
the rugs are hung upon a line. There are three kinds of air pumps in
use with vacuum cleaners: (1) bellows, (2) rotating disk or fan, (3)
piston.
To milk cows by electricity is simply to apply the vacuum-cleaner idea
to the process, and, in general, doing things by electricity usually
means doing them by some machine that is made to go by an electric
motor. This then is the first key to the Electrical Show, and if you
will remember to look first for the motor it may remove much of the
mystery from some of the exhibits. In many cases it is not necessary
to have a complete electric motor, but simply an electro-magnet to do
the work. In booth No. 56 you will find a piano played by electricity.
Its keys are moving, but no hands strike them. There is no ghost at
work here. A little strip of iron has been placed upon the under side
of each key and a small electro-magnet is placed under that. It is
only necessary that wires should run from these electro-magnets to
two dry-battery cells and to push buttons, and a person far away may
play the piano. In reality, however, it is not a person but a roll of
punctured paper that opens and closes the electric circuits to these
various magnets underneath the keys.
It often happens that you see a person playing a pipe organ with his
keyboard far removed from the organ itself. In this case the keys
simply act as push buttons to close the electric circuit through
electro-magnets placed in the organ itself. These electro-magnets
operate the air valves of the various pipes.
[Illustration: Fig. 25]
You call at some apartment house where there is no hall boy, but a
row of push buttons labelled with the names of the tenants. You push
a button and the door which was locked opens apparently of its own
accord. To say that the door opens by electricity is only to add
mystery. What does happen is that an electric bell up in the apartment
rings in response to your push of the button, and in reply the tenant
pushes a button and the door is unlatched by an electro-magnet
concealed in the door casing (Fig. 25).
So I would say that the first key to the Electric Show or to the
multitude of electrical appliances which you meet in life is the
electro-magnet. Consider the motor as one illustration of its use.
If you are really to understand the Electric Show you should go
twice. I advise going with this key alone first and note down all the
applications of electro-magnets which you can find there. When you have
done so I shall be glad to have your report.
VI
APPLICATIONS OF THE ELECTRO-MAGNET
It became quite the rage now among the boys to find as many uses of
electro-magnets as possible. These were reported and explained to the
club and a list kept. This list included:
1. Dynamo.
2. Magneto.
3. Ammeter.
4. Wattmeter.
5. Motor.
6. Electric piano and organ players.
7. Electric door openers.
Already noticed in the preceding pages, and the following:
8. _The Electric Spinner_ (Fig. 26).--A toy full of instruction. The
standard is a steel magnet which produces a magnetic field. Inside of
this is an electro-magnet which serves as an armature. Plainly visible
on its shaft is a commutator to which the electric current from a dry
cell is sent. This causes the armature to revolve and carry with it a
series of colour disks which may be adjusted so as to show what tint or
shade results from mixing colours in various proportions.
[Illustration: Fig. 26]
[Illustration: Fig. 27]
[Illustration: Fig. 28]
9. _The Electric Engine_ (Fig. 27).--This toy, with one dry battery
cell, develops power enough to run several other toy machines. The
diagram in Fig. 28 will make its plan of operation plain. _B_ is the
battery cell, _c_ the electro-magnets, _a_ an armature of iron. By a
rod this armature is connected with a crank on the axle which carries
the fly wheel _f_. Another crank, _d_, upon the same axle serves like
a push button to close the electric circuit at the right instant. The
wire _g_ from the battery cell encircles the electro-magnet _c_ and
then is connected to the iron base of the toy. When the crank _d_
touches the conductor _e_, which is a spring, the electric current
passes around the magnet, the magnet pulls the iron armature _a_, and
this gives an impulse to the wheel _f_ whose momentum carries it around
during that portion of the revolution when _d_ is separated from _e_
and _a_ is receding from the magnet.
It is customary to say that the circuit is closed through the base of
the machine, but this language requires interpretation. It means that
a way is provided for the electric current to pass through the base. A
person who is expert in language but not in electricity might expect us
to say "the circuit is open through the base."
[Illustration: Fig. 29]
10. _The Telegraph Sounder_ (Fig. 29).--This was a toy half a century
ago, but since the days of Samuel Finley Breese Morse it has become of
vast commercial importance. The Western Union Telegraph Company in 1909
had 211,513 miles of poles and cables, 1,382,500 miles of wire, 24,321
offices, sent 68,053,439 messages, received $30,541,072.55, expended
$23,193,965.66, and had $7,347,106.89 in profits. In the United States
more than 93,000,000 and in the world at large more than 600,000,000
messages are sent annually, and there are men still living who scoffed
at Morse's ideas as _impracticable_.
It is interesting to contemplate what would happen to the Stock
Exchange, to the newspapers, to the railroads, to the congressman
addressing his constituents from the floor of a legislative chamber, to
business in general, if the world were deprived of the telegraph.
A few years ago a telegraph despatch was sent from New York to San
Francisco, Tokio, London, and back to New York, 42,872 miles, in three
minutes less than an hour. Electricity can travel around the world in a
fraction of a second, the time was consumed in repeating the message.
I once sent a message from New York to New Haven to announce that I
was coming, and afterward took my train and reached New Haven in time
to receive my own message and pay the messenger boy. But I have never
lost faith in the beneficent results of Morse's labours.
Morse (1791-1872) was an artist and the first President of the
National Academy of Design. He was likewise a professor in New York
University and constructed his first experimental telegraph line upon
the University campus in 1835. His first public line was built from
Washington to Baltimore in 1844. The Western Union Telegraph Company
was incorporated in 1856. Of course the work of Morse rested upon that
of Oersted, in Copenhagen, who, in 1819, discovered electro-magnetism,
and upon that of Joseph Henry of Albany, who in 1827 first insulated
the wires.
[Illustration: Fig. 30]
The application of the electro-magnet to producing telegraphic signals
will be understood by referring to Fig. 30. _B_ is the generator of
an electric current--sometimes a battery and sometimes a dynamo. One
wire from this goes to the earth, _E_. The other wire goes through a
key, which, like a push button or a switch, serves to open or close
the circuit. This is normally closed when not in use. Through this
the current passes around the electro-magnet _S_, which attracts the
armature _a_, causing it to click against a metal stop, hence it is
called the sounder. From this the current passes along the line wire to
a distant station and there through the sounder and closed key to the
earth. There is likely to be a generator at each station. The current
must run continually through the system. If a battery is employed, the
copper sulphate, or gravity cell, to be described later, is chosen,
because it will endure continued usage better than any other.
The operator, in sending signals, opens the circuit, the magnets cease
to hold down the armatures, and they are raised by springs and strike
against metallic stops above. It is customary to say that the circuit
is completed through the earth. This statement misleads some persons
into imagining an electric current capable of corroding water pipes
and decomposing chemical compounds, passing through the earth between
stations.
[Illustration: Photograph by Helen W. Cooke. Testing the Telegraphy
Outfit]
Perhaps it will help to a better understanding of the truth if we
think of a city pumping water out of the ocean, say to fight fire,
and disposing of it again into the ocean. The ocean currents thus
produced are not likely to be destructive. Indeed, just as we measure
height from the ocean level as zero, so we measure electric pressures
as from the zero level of the earth's electrical state.
[Illustration: Fig. 31]
The key used by telegraphers is represented in Fig. 31. It has
connected with it a switch to keep the circuit closed when the key
is not in operation. The Morse code of signals consists of dots and
dashes, when printed, as follows:
a . -
b - . . .
c . . .
etc.
Operators learn to read the message by the intervals between sounds. A
dot consists of two taps of the sounder with a short interval between,
and a dash consists of two taps with a longer interval between. One tap
of the sounder is caused by its descending upon the metal stop below
and another by its rising against the upper stop.
Telegraph sounders are operated on about a quarter of an ampere of
current if from a battery circuit, or on about one tenth of an ampere
from a dynamo circuit. The dynamo circuit is supplied with more volts
of electric pressure, and hence its power is ample to cause the
armature to strike the metal stops hard enough to be heard by the
operator.
For example a battery circuit may supply to the sounder a current with
these characteristics:
2 volts × .25 amperes = .5 watts,
while a dynamo circuit may give:
6 volts × .1 ampere = .6 watts.
Telegraph line wires are usually bare, the insulation being merely the
glass knobs at the poles. Clean water is a very good insulator but
dirty water is a fairly good conductor. A wet telegraph pole may bring
so much current to earth as to prevent all sounders on the line from
operating. Hence the line is separated from the poles by glass. The
poles are about one hundred and thirty-two feet apart, making forty to
the mile. The wires are usually galvanized iron one sixth of an inch
in diameter. Copper conducts six times as well as iron, and is now
replacing iron in the lines.
Morse laid a submarine telegraph line in New York Harbour and suggested
a cable across the ocean. But that gigantic undertaking had to await
the masterful intelligence of Lord Kelvin and the indomitable will of
Cyrus W. Field. A submarine cable was laid across the Strait of Dover
in 1850. It was cut by the anchor of a fisherman a few hours after
it was laid. The first attempt to lay a submarine cable across the
Atlantic Ocean was made in 1857. Two ships of war, the _Agamemnon_ of
Great Britain and the _Niagara_ of the United States, engaged in this
undertaking. Three hundred miles had been laid when the cable parted
where the ocean was more than two miles deep. William Thomson was on
board the _Agamemnon_ as electrical expert. He went home to study and
improve the methods. The next year, 1858, the _Agamemnon_ and the
_Niagara_ met in midocean each with a portion of the cable on board.
The splice was made, and the _Agamemnon_ started toward Ireland and the
_Niagara_ toward Newfoundland. When six miles apart the cable broke.
The ships met again, made a new splice and again started in opposite
directions. They laid eighty miles and the cable parted a second time.
They met again, spliced and laid two hundred miles when it parted for
the third time. They met a fourth time, made the splice and succeeded
in laying the first cable from Ireland to Newfoundland on August 5,
1858.
In a few weeks the insulation failed and no more messages could be
sent. Seven years were spent in studying the problem, and again in
1865 the _Great Eastern_, a mammoth ship, started to lay the cable.
William Thomson was again on board as the expert. When twelve hundred
miles had been laid the cable parted in deep water. Three times the
cable was grappled and brought part way to the surface and lost again.
The _Great Eastern_ returned to land. The next year, 1866, the _Great
Eastern_, having on board William Thomson (Lord Kelvin), Mr. Canning,
the engineer of the expedition, and Captain Anderson, in command,
laid the cable which has worked successfully ever since. Thomson,
Canning, and Anderson were knighted as a result of their labours. Sir
William Thomson (1824-1907), afterward Lord Kelvin, is credited with
having solved the difficult electrical problems connected with this
enterprise. Cyrus W. Field (1819-1892), born in Stockbridge, Mass.,
helped to secure the many millions of dollars necessary to carry the
work to completion.
There are now seventy-three cables connecting Europe and America, and
two across the Pacific Ocean. Cable rates are: New York to England,
France, Germany, or Holland twenty-five cents a word, to Switzerland
thirty cents a word, and to Japan one dollar and thirty-three cents a
word.
[Illustration: Fig. 32]
The boys were kept very busy now looking up historical and biographical
sketches, as well as working up the many applications of the
electro-magnet. The next to be reported was:
11. _The Relay_ (Fig. 32).--Telegraphing from 3,000 to 10,000 miles
under the ocean is full of difficulties not now to be explained.
Of course when we attempt to telegraph many miles upon land we
find that the resistance of the wire cuts down the strength of the
current so that it will not move the sounder. This, however, is
readily obviated by the relay devised by Morse. It simply serves as
an automatic key to close a circuit. A diagram will make this clear
(Fig. 33). Suppose the line wire to be very long and on account of its
resistance the current is too feeble to operate a sounder. It is likely
to be about .025 ampere where the local sounder may require .25 ampere
or ten times as much. It is easily possible to wind a magnet (Fig. 33),
_R_, such that .025 ampere will close the armature _a_, so that it
may complete a local circuit when it would not make noise enough for
a sounder. _B_ may represent a local battery of any desired strength
which may operate the sounder _S_ of that station as loudly as may be
desired.
[Illustration: Fig. 33]
[Illustration: Fig. 34]
12. _Annunciator_ (Fig. 34).--We live in a fifth-floor apartment.
When we push the button to call the elevator a No. 5 appears in the
annunciator in the elevator car. This tells the elevator boy where
the call comes from. Take out two or three screws and the annunciator
opens, revealing a series of electro-magnets like the one shown in Fig.
35. When an electric current passes around the coil it pulls back an
iron catch and allows a number to drop so as to show through a small
window. The elevator boy, having noted that the call is from the fifth
floor, pushes up the number and the iron catch holds it until the coil
is magnetized again by an electric current.
[Illustration: Fig. 35]
[Illustration: Fig. 36]
[Illustration: Fig. 37]