enters this annunciator (Fig. 36). One wire goes direct to the bell
and the other five reach the bell through the separate coils of the
electro-magnets which control the drops. But how are electrical
connections made between a moving elevator car and the push buttons on
various floors? The diagram in Fig. 37 shows this in elevation. _B_
represents a battery of several dry cells located in the basement. One
wire from it runs direct to the push buttons 1, 2, 3, 4, 5, located
upon the five floors of the house. The other wire from the battery,
together with wires from each of the five push buttons, all run to a
point, _A_, half-way up the elevator shaft. Here the six wires are
gathered into a cable long enough to reach either to the top or the
bottom of the elevator shaft. The other end of this cable enters the
elevator car and runs to the annunciator. The wire from the battery
goes direct to the bell. The wires from the various push buttons go
through correspondingly numbered electro-magnets to the bell. When,
therefore, we pushed the button on the fifth floor, we closed the gap
in the electric circuit at that point. The current came up from the
battery, passed through the button, went down the cable to the car,
went through electro-magnet No. 5, went through the bell, and returned
direct to the battery, thus completing the circuit. Annunciators are
used about buildings to call other attendants, besides the elevator
boy. They are likewise used in burglar alarms to inform the householder
which door or window is being forced. They are used in the fire
department to tell what part of the city the call came from.
[Illustration: Fig. 38]
13. _The Electric Bell and Buzzer_ (Fig. 38).--So common a thing as an
electric bell really belongs to the present generation. Bells were
either novelties or toys when I was your age. They cost then many times
what they do now and then were poorly made. Nobody dared to trust them
for front-door bells. It was necessary to have a card permanently
posted over the push button saying, "If the bell does not ring, knock."
In those days batteries were troublesome to care for, houses were not
wired when built, and no one had learned the art of concealing the
wires neatly.
The buzzer is simply a bell minus gong and hammer. Those shown in Fig.
38 ring well on a single dry cell. A cell costing twelve cents operated
one for two years while it was used as a call bell from dining room to
kitchen, the current required being .15 ampere.
[Illustration: Fig. 39]
[Illustration: Electric Bell]
The connections are shown in the diagram (Fig. 39). Suppose the current
to enter at the binding post _a_, pass around the magnets _b_ and
then to the post _c_. The armature _d_ normally rests against the
post _c_ and the current finds its way along this to the post _e_ and
thence back to the battery. But as soon as the current passes, _b_
becomes a magnet and pulls the armature _d_ away from the post
_c_, thus breaking the circuit, when _b_ ceases to be a magnet and a
spring pushes the armature _d_ back against the post _c_ to repeat the
operation. The armature _d_ carries a hammer which strikes the gong
_f_. If the wire, which is usually connected with the binding post _e_,
is connected with the post _c_, the "clatter" bell is changed to a
"single-stroke" bell, and if the gong and hammer are removed the "bell"
is changed to a "buzzer."
[Illustration: Fig. 40]
In the case of the buzzer, by changing the length of the armature or
by weighting it, we may change the time of its vibrations and its
tone. The connections between battery push button and bell form a
complete circuit. In Fig. 40 _B_ represents a battery, usually of dry
cells, _B'_ represents the bell, and _P_ represents the push button.
The electric circuit is "open," (that is, there is a break in the
conductor) at _P_ until some one "pushes the button," that is, simply
pushes against a spring so as to cause a piece of metal to bridge the
gap in the conductor. Then we say the circuit is "closed."
[Illustration: Fig. 41]
[Illustration: Fig. 42]
Push button devices and switches are innumerable. In every case they
are simply devices for pushing one piece of metal against another
and completing the circuit for an electric current. Every one should
unscrew and examine a few of them, both for the pleasure of seeing how
they work and to learn how to make them work when they sometimes fail.
Not only in bells but in all other instruments where electro-magnets
are used, the magnets are placed in pairs, fastened together upon an
iron base. They are wound so that the free ends are made opposite poles
by the electric current. Like a horseshoe magnet, they form one magnet.
The two poles thus placed are mutually helpful and each is stronger
than it would be if separated from the other.
[Illustration: Fig. 43]
14. _Electric Clocks, Self-winding Clocks, Programme Clocks._--A
pretentious-looking thing which appeared like a dish pan with a glass
bottom was opened by the boys and found to be the simplest of all
clocks. It had an electro-magnet like that in Fig. 44. A strip of iron
acting as an armature across the free ends of this magnet, pushed like
a finger against the cogs of a wheel. This wheel was on the axle of
the minute hand and it had sixty cogs. The electric circuit was closed
through the magnet for an instant each minute and the armature pushed
the wheel ahead one cog. Thus it made one complete revolution in an
hour. A train of four other cog-wheels caused the hour hand to trail
after at one twelfth the speed of the minute hand. This machinery made
simply a small handful in an eighteen-inch stamped-metal "dish-pan"
costing fifteen dollars.
[Illustration: Fig. 44]
A self-winding clock was opened and found to contain two dry battery
cells, an electro-magnet which operated very much like that of a
"clatter" bell, the hammer like a finger poking against the cogs of
a wheel. Once an hour the long hand closed the circuit through the
battery and the magnet and its armature swung back and forth long
enough to give the cog wheel one complete revolution and wind a
spring, which it carried upon its axle. This spring kept the clock
running one hour, until the next winding.
[Illustration: Fig. 45]
The programme clocks which were examined were self-winding clocks, but
were connected by wires to the master clock which corrected them each
hour. Each time the long hand of the master clock came to twelve it
closed an electric circuit through all the clocks in the system. In
each clock the current passed around an electro-magnet and caused it to
pull an armature against a metal stop and set each long hand exactly at
twelve. This master clock is sometimes situated many miles away and may
correct the time for a whole city. Thus a master clock at Washington,
D. C., furnishes standard time to all parts of the United States. The
master clock which we examined also closed the circuit at proper
intervals through a series of programme bells placed in the various
class rooms, and these called and dismissed classes automatically.
15. _Watchman's Time Detector_ (Fig. 45).--This is a device to compel
a watchman to make his appointed trips. Push buttons or switches are
distributed about the building at various points, and it is made his
duty to close the circuits at these points at stated times. When he
does so, the fact is recorded by electro-magnets puncturing, or, in
some way, marking a revolving time card in the clock.
[Illustration: Fig. 46]
16. _Circuit Breakers_ (Fig. 46).--Electro-magnets are used to open
switches and thus protect dynamos and other machines against a larger
electric current than they are able to carry. The switch is held
closed by a spring which, by an adjusting device, may be tightened or
loosened. A dynamo which we examined had its circuit breaker adjusted
so that it would remain closed if any current under 1500 amperes
passed, but if a greater current than that passed it would strengthen
the magnet sufficiently to open the switch and thus break the circuit.
17. _Separating Iron from Ore._--In 1897 Edison first proposed to
use an electro-magnet to separate iron from crushed earth. Fig. 47
represents the process. _E_ is an electro-magnet. _S_ is the stream of
crushed ore containing iron. Gravity would cause all the material to
fall into bin _A_, but the electro-magnet _E_ pulls that portion of the
material which is magnetic to one side so that it falls into the bin
_B_.
[Illustration: Fig. 47]
[Illustration: Fig. 48]
18. _Lifting Magnets._--Electro-magnets are made for use with hoisting
apparatus to save the trouble of manipulating grappling hooks, etc.
They may lift barrels and boxes of iron, the wood of the barrel or
box being transparent, we say, to the magnetic influence. That is, the
magnet will attract iron through the wood just as light will shine
through glass. Such magnets are used to pick up from the bottom of
the sea cases of hardware from wrecked ships. (See the accompanying
illustration, Fig. 48.) In such cases the electric conductors which
lead to and encircle the magnets must be well insulated from the water
of the sea, otherwise the electric current would take the shorter
path from one line wire through the sea water, which is a fairly good
conductor, and back by the other line wire, rather than go the path of
greater resistance around the magnet. Electro-magnets are coming into
use in foundries, etc., for lifting heavy iron castings.
[Illustration: Fig. 49]
19. _Electro-Magnet on Starting Box._--As was explained under _electric
motors_, a starting box is simply a series of resistance coils _r_,
_r_, _r_, _r_, _r_, in Fig. 49. When the motor is not in use the switch
_l_ rests upon the point 1 and no electric current passes. When the
switch is moved to point 2, the current entering at _a_ passes to the
pivot of the switch and up the metal strip _l_ to the point 2, then
around the series of coils, _r_, _r_, _r_, _r_, _r_, to the post _b_
and thence back to the generator. As the switch is moved to the right,
the current passes through less and less of this resistance until, when
it reaches point 7, all the coils of resistance are "cut out," that is,
they are not in the path of the current. Now the motor has reached its
full speed and is developing enough counter-electro-motive force to
protect itself against too much current. Through a shunt, however, a
portion of the current passes from _a_ to _b_ around the electro-magnet
_e_, the two poles of which are presented to the metal strip _l_, which
must be of iron. This magnet holds the switch over so long as the
current is on, but when the current is cut off, by opening a switch in
the line wire, _e_ ceases to be a magnet and _l_ is carried back to
point 1 by a spring. Thus an extra resistance must always be in circuit
when the motor is first started. Those who start motors are expected
to move the lever _l_ of the starting box slowly from point to point,
pausing a second or two on each to give the motor time to acquire
proper speed for its protection. How too great a current would "burn
out" a motor will be explained later.
The motor man handles a lever for starting his car, which works like
that of the "starting box." His "starting box," however, is called a
"controller." Although it accomplishes the same result as the starting
box it has a wholly different and vastly more complex mechanism than
that already described.
The elevator boy, who runs our electric elevator, handles a lever which
also does the same thing through far different mechanism. Indeed, in
his case electro-magnets are used to prevent him from cutting out
resistance too fast if he should move his lever too quickly.
20. _Starting Switches for Electric Elevators._--The motor man has
to be instructed particularly how he should handle the lever of his
controller, and he is trusted to follow his directions to some extent,
however lacking in intelligence and integrity he may be. But the
elevator boy receives scarcely any instructions about his machine, and,
indeed, his machine has been constructed pretty nearly "foolproof." It
will automatically correct his errors of management. If he throws the
handle from one extreme to the other, all resistance cannot be thrown
out instantly, but this is accomplished by a series of electro-magnets
closing one switch after another and thus cutting out resistance
gradually.
21. _Arc Lamp Feed._--As will be explained later, an arc lamp must
have its carbons touching one another when the current is first thrown
on, and then the carbons must be drawn apart from a quarter to half an
inch. The upper carbon is lifted away from the lower one by a portion
of the current passing by means of a shunt around an electro-magnet.
[Illustration: Fig. 50]
[Illustration: Fig. 51]
[Illustration: Fig. 52]
22. _Volt meter._--The volt meter measures the pressure of an electric
current. The volt meter which we examined looked outside like our
ammeter, and when we removed the face it appeared inside like an
ammeter. There was the steel magnet of horseshoe shape to furnish a
field (Fig. 51), and there was an electro-magnet poised between its
poles for an armature. The armature in the volt meter, however, had
wound upon it finer wire and more of it than was the case in the
ammeter. There was no shunt wire in the volt meter as there was in
the ammeter. We connected in series a fluid cell (to be described
later), the ammeter, and the volt meter (Fig. 52). The ammeter shunt
was removed so that all the current went through its armature. The
volt meter needle went to one which was two thirds of the scale (Fig.
53), and the ammeter needle indicated .016. That is, this particular
cell can push sixteen thousandths of an ampere through the resistance
of this volt meter, and .016 ampere passing through the armature of
this volt meter will magnetize it sufficiently to move it against its
spring, say sixty degrees.
[Illustration: Fig. 53]
[Illustration: Fig. 54]
[Illustration: Fig. 55]
We put into the circuit a lot more fine wire for resistance, _R_ (Fig.
54), so that the volt meter needle went only half as far as before,
that is to .5. The ammeter indicated only half as much as before,
that is .008 ampere. We put in resistance enough to bring the volt meter
needle down to .25 and the ammeter indicated one quarter of
the original current. We put in less resistance, bringing the volt meter
needle to .75, and the ammeter indicated three fourths of the
original current. Evidently the volt meter is merely an ammeter with
a different scale marked upon its card. With a pen we marked upon the
card of the volt meter a true ammeter scale (Fig. 55).
[Illustration: Fig. 56]
In order to understand the volt meter, let us turn our attention for a
moment to Fig. 56. I have arranged the water tank _T_ at such a height
above the faucet _F_ that when the faucet is opened one quart of water
will flow in a minute. If I partially close the faucet, making the
opening one half as large (that is, offering twice the resistance to
the flow), half a quart will flow in a minute. If I make the resistance
four times as great only one quarter of a quart will flow in a minute.
It is evident that I could arrange a scale underneath the handle of the
faucet to indicate the quantity of water flowing, just as the ammeter
and volt meter indicate the quantity of electricity which flows. If now
that much is understood, it will be easy to learn how the water faucet
may be used to measure water pressure and the volt meter in like manner
used to measure electric pressure.
Having set the faucet so that a quart will flow per minute, let us put
on a longer tube _p_, and move the tank up to another shelf so that the
distance from the water level in the tank to the faucet is twice as
great as before. Under the increased pressure water runs through the
faucet twice as fast and we now get two quarts per minute.
I purposely placed the tank out of sight behind a partition so that you
might practise judging the water pressure by the flow at the faucet.
We cannot very well talk about pressure in quarts. We might talk about
it in pounds, but if we used this apparatus much we should probably
get into the habit of talking about the pressure from one shelf, two
shelves, three shelves, etc.
In order that the pressure might remain nearly constant during the
experiment we would probably introduce resistance (that is, partially
close the faucet) so that the water level should not fall much. We
might, for example, set the faucet so that half a pint would flow in
a minute when the tank was on the first shelf. Then a pint per minute
would flow when the tank was on the second shelf and one and a half
pints per minute when the tank was on the third shelf, etc. Thus we
should infer the pressure by measuring the quantity.
One more illustration and the case will be clear. To save the trouble
of measuring the quantity of water which flows through the faucet,
suppose I introduce the device represented in Fig. 57. _W_ is a small
water wheel comparable to the armature of the volt meter. It carries a
pointer which moves over a scale just as in the case of the volt meter.
[Illustration: Fig. 57]
It has a spring coiled around its axle which tends to keep the pointer
at _0_, as in the case of the volt meter. The tank is placed upon the
first shelf, the faucet is fixed so that a small amount of water flows
and the needle moves to a certain figure upon the scale. We will mark
this point one and call it "first-shelf pressure." The tank is lifted
to the second shelf and the index moves to another point, which we will
mark two and call it "second-shelf pressure." The tank is lifted to the
third shelf and the index moves to a third point, which we will mark
three and call it "third-shelf pressure," etc.
Ordinarily we measure water pressure with an instrument which allows
no water to run to waste, but in measuring electric pressure by the
volt meter some current must pass through the instrument, just as in
the case of our water-wheel illustration in Fig. 57. We put in large
resistance so as to make this current as small as possible, while we
let enough pass to move the armature.
[Illustration: Fig. 58]
Now let us return to the volt meter itself. By referring to Fig. 55,
we see that it requires .024 ampere to move the needle of the volt meter
clear across the scale, and we have found that one fluid cell was
able to send enough current through the resistance of the armature to
move the needle two thirds of the way across the scale. At this point
we find Fig. 1, which might be read "one-cell pressure." We prefer to
commemorate the name of one of the workers in the field of electricity
and call this pressure a "volt" after Alessandro Volta (1745-1827),
born at Como, Italy. It is the electric pressure which is produced by
one fluid cell of a certain kind. We say, then, that one volt pushes
through the resistance of this armature .016 ampere. Half a volt would
push through the resistance of the armature half as much current or
.008 ampere. At this point we put .5. Thus each of the figures in the
lower row (Fig. 55) shows what part of a volt is required to send
enough current through this particular armature to move the needle to
that point.
[Illustration: Fig. 59]
We found out how much wire was wound upon the armature and put exactly
the same amount in the outside resistance, _R_ (Fig. 59). The needle
now showed that one volt is able to push through twice the resistance
of the armature only half as much current, and the needle stopped
at .008 ampere. If this were to be the resistance in the volt meter
circuit one volt should stand under .008 ampere and two under .016 and
three under .024. It is evident then, that, if we know the internal
resistance of a volt meter, we may make it capable of measuring greater
electrical pressures by adding the proper amount of resistance. By
putting at _R_, (Fig. 59) nine times the internal resistance of the
instrument, thus multiplying the total resistance tenfold, the figures
upon the scale of volts may be read as whole numbers from one to
fifteen. In this case it will require fifteen cells to push the needle
clear across the scale and ten cells to push it two thirds of the
way across. If now we add enough external resistance to multiply the
resistance of the armature a hundred fold it will require 150 volts to
push .024 of an ampere through the armature and pull its needle clear
across the scale. In this case the figures upon the scale of volts are
multiplied by one hundred and read from ten to one hundred and fifty.
Such a scale would adapt this volt meter for use with our 110-volt
lighting circuit. Volt meters are made with a series of such external
resistances, called "multipliers," attached so that they may be easily
thrown into the circuit.
It is evident that we need some term so that we may speak of quantities
of resistance. This need has given rise to a unit of resistance called
an ohm, after George Simon Ohm (1789-1854) born at Erlanger in Bavaria.
Two inches of No. 36 German silver wire, such as is wound upon the
armature of this volt meter, gives one ohm of resistance. There are 125
inches of this wire upon the armature. Its resistance is, therefore,
62.5 ohms, and we may, therefore, say that one volt of electric
pressure can push through 62.5 ohms of resistance .016 of an ampere of
current. Ohm discovered this relationship in 1827, and formulated it
as follows:
volts/ohms = amperes (not, however, using these words).
(1 volt)/(62.5 ohms) = .016 ampere.
62.5) 1.0000 (.016
625
----
3750
3750
----
This is called Ohm's law, as every candidate for college admission will
hear and hear again.
[Illustration: Fig. 60]
Volt meters and armatures for the alternating current have
electro-magnets for their fields as well as for their armatures. Such
instruments are equally well adapted for either direct or alternating
currents. For when the current reverses its direction it reverses in
field and armature alike, and thus a repulsion between like poles is
maintained. Such an instrument, however, cannot respond to as slight
a current as those previously described, since they must consume some
energy in both field and armature.
23. _Telephone Receiver_ (Fig. 61).--It requires a stretch neither
of the imagination nor of the truth to call a telephone receiver an
electro-magnet, although perhaps it has never been called that before.
We took it apart and found that it consisted of a steel-bar magnet _m_
(Fig. 62), with a small spool of wire _w_ around one end of it. The
ends of the wire on the spool run along inside the hard rubber shell
to the two binding posts _a_ and _b_ at the other end. A disk of sheet
iron _S_ is held in the large end of the case very near to, but not
quite touching, the end of the magnet. When an alternating current is
sent through the wire upon the spool it causes rapid changes in the
strength of the magnetic field, if not reversals of the poles of the
field, and the iron disk is made to vibrate, keeping time with the
alternations of the current.
[Illustration: Fig. 61]
[Illustration: Fig. 62]
In this laboratory we have seen that our current has sixty alternations
per second. When it is connected with the receiver the disk, therefore,
makes sixty vibrations per second, and produces a tone which has very
nearly the pitch of C two octaves below the middle C upon the piano.
24. _Spark Coil_ (Fig. 63).--The automobile spark coil which we have
already used is an electro-magnet. The battery sends a current through
wire coiled around an iron core. At one end of this iron core is an
iron armature which is made to vibrate in precisely the same manner as
the armature of an electric bell. This makes and breaks the current and
causes rapid changes in the strength of the field. A rapidly changing
magnetic field may be used to develop electricity in a conductor, as we
have already seen in the case of the dynamo.
[Illustration: Fig. 63]
How it is used in the automobile spark coil will be shown later. It is
sufficient now to mention it as a case of a magnetic field produced by
an electric current passing through a wire coiled around an iron core,
or, in short, an electro-magnet.