wire, silk covered. It offered 75 ohms of resistance, and since it
takes 2-1/2 feet of this wire to furnish 1 ohm of resistance the spool
contains 187-1/2 feet. A thin disc of soft iron .01 inch in thickness
is held by the hard rubber case very near to but not quite touching
this end of the magnet. We drew this disc to one side, as shown in
Fig. 159, and connected the receiver by wires to a magneto. We turned
the crank of the magneto slowly and the iron disk danced up and
down, keeping time with the revolutions of the armature. The magneto
furnished an alternating current, which, when it flowed around the
coil in one direction, strengthened the pole of the magnet, and in the
reverse direction weakened the pole. When the crank was turned so as
to produce twenty to thirty revolutions of the armature per second the
dancing of the disc sounded like the low hum produced by the wing of
a humming bird. When a large, wide-mouthed bottle was brought near to
this the sound was greatly reinforced, as the sound of a bee becomes
louder when he appears at your open window. We next replaced the iron
disc and put on the cap again. We then connected the receiver at _S_
(Fig. 160) and connected two dry cells at _p_. When the primary circuit
was closed the disc vibrated in time with the hammer of the bell making
the same tone. We substituted for the bell a series of buzzers. The
smallest had an armature about one inch long, while that of the largest
was about two inches long. The shorter the armature the faster it
vibrated, and the higher was the pitch of its tone. We arranged these
as shown in Fig. 161. _A_, _C_, _D_, _E_ and _F_ are the buzzers. _B_
is a battery of two cells and _G_, _H_, _I_, _J_ and _K_ are springs of
sheet brass which act as push buttons. By operating upon these springs
with one's fingers, as upon the keys of an organ, it was possible to
represent the tones of a reed organ after a fashion. The armatures are
reeds and they are made to vibrate by electro-magnets. We called it
an electric organ. The telephone receiver was connected at _T_, and
the wires which led to it were lengthened so that the receiver might
be a long distance away. The disc in the receiver kept time with the
armature of each buzzer when it sounded and faithfully reproduced its
sound. But the strangest thing was that when any two buzzers sounded
together, or, indeed, if all five buzzers sounded together, the
receiver responded to them all at the same time, so that a person in
another room or in another house, with the receiver at his ear, might
hear exactly what those did who were in the same room with the buzzers.
The wires from the receiver were connected with the coil in each buzzer
so as to get the induced current, as shown in detail in Fig. 160.
[Illustration: Fig. 160]
[Illustration: Fig. 161]
[Illustration: Fig. 162]
We took a telephone induction coil (Fig. 162) and fastened it to a
board as represented in Fig. 163, _I_. One wire of the primary circuit
was fastened to the binding post _a_. The other wire from the primary
coil passed to the switch _S_ and then to the battery. From the battery
the wire ran to the binding post _b_. _C_ is a steel tuning fork. The
secondary circuit is closed through a telephone receiver. These wires
are extended so that the receiver is too far distant for the tuning
fork to be heard through the air. When the switch _S_ is closed the
tuning fork acts as the interrupter for the primary circuit, and it
interrupts according to its time of vibration. If, for instance, the
fork gives the tone of middle _C_ on the piano it vibrates 256 times
a second. It interrupts the primary circuit 256 times a second. It
induces an alternating current of the same frequency in the secondary
circuit. The diaphragm of the telephone receiver vibrates in perfect
time with the tuning fork and produces the same tone as the tuning
fork. We had a series of tuning forks giving a variety of tones,
which we could substitute one after another in place of this one. The
receiver reproduced accurately the tone of each one of them.
[Illustration: Fig. 163]
[Illustration: Fig. 164]
We took a small induction coil (Fig. 164) _c_ and fastened one end of
the primary circuit to a battery, _B_. The wire at the other end of
the primary circuit was bent into a hook _h_. This hook was adjusted
about a quarter of an inch from the end of the iron core of the coil.
The other wire from the battery was attached to the steel strings of a
piano, _P_. When the coil _c_ was brought over a string and the hook
_h_ was allowed to pass beneath the string and touch it very gently,
the primary circuit was closed through the string, which served as an
interrupter of the current and vibrated according to its tone. The
secondary coil, not represented in the figure, was connected to a
distant telephone receiver, which reproduced the tones of the piano
strings.
Producing a tone is merely a matter of making something vibrate with
the required frequency. It may be a piano string, or a tuning fork,
or a reed of an electric buzzer, or the diaphragm of a telephone
receiver. If it vibrates 256 times a second, it will produce the
same tone as middle _C_ on a piano; if it vibrates 512 times a second
it will produce the _C_ which is an octave above, and if 128 times
a second an octave below middle _C_. The human voice is produced by
vocal cords in the throat, which vibrate with the proper frequency to
give any required tone. But how can we make the human voice act as an
interrupter of the primary circuit? An examination of the telephone
transmitter will supply the answer to this question.
[Illustration: Fig. 165]
[Illustration: Fig. 166]
The boys after taking the transmitter (Fig. 165) apart proceeded to
make one which should answer the purpose as follows: A block of wood
about one inch thick and three inches square (Fig. 166), _A_, was
hollowed out, making a cone-shaped cavity about one half inch deep and
one inch broad. This cavity was filled with small pieces of graphite,
_G_, made by cutting up a lead pencil. An old tin-type, _D_, was laid
over this as a diaphragm and tacked around the edges. A binding post,
_E_, passed through the block, its head being buried in the graphite at
the bottom of the cavity. The binding post _F_ furnished contact with
the tin-type. One dry cell was placed at _B_ and the sensitive ammeter
was connected at _C_. The needle showed that although a small current
was passing it was constantly varying in strength. Tapping upon the
table, walking across the floor of the room, shouting, and particularly
whistling, caused variations in the conducting power of the graphite
and consequently variations in the current strength. This is precisely
the condition we wished to produce in the primary circuit.
[Illustration: Fig. 167]
We next substitute for the ammeter at _C_ the primary and secondary
coil of the telephone. In Fig. 167 _T_ is the transmitter, _B_ is a
battery of two dry cells, _P_ is the primary winding of the coils, and
_S_ is the secondary winding. To this a telephone receiver, _R_ is
connected by wires long enough to reach into another room. A person
holding the receiver at his ear could hear everything said or done in
the room where the transmitter was almost as plainly as though he were
present in the room.
[Illustration: Fig. 168]
Two such transmitters were made and the second one was placed in the
room where the receiver had been, while a second receiver was installed
near the first transmitter. The arrangement is shown in Fig. 168. _T_
is the transmitter at one end of the line and _T'_ the transmitter at
the other end. _B_ and _B'_ are the batteries at each end, _P_ and
_P'_ the primary coils, _S_ and _S'_ the secondary coils and _R_ and
_R'_ the receivers. With this arrangement two persons carried on a
conversation with perfect ease, holding the receivers to their ears,
presenting their mouths to the transmitters and speaking in moderate
tones. _H_ and _H'_ are hooks upon which the receivers are to be hung
when not in use. These hooks act as switches to open and close the
primary circuit. A spring normally pushes the hook upward and closes
the circuit, but while the receiver is hanging upon it the circuit is
open at this point. Thus the battery is saved from running down when
the telephone is not in use.
The wires were finally extended from the mill to the cottage and this
equipment was installed at each end.
It will be noticed that the secondary circuit includes two receivers
and two secondary coils besides the wire of the lines to offer
resistance.
The receivers offer 75 ohms of resistance each. The secondary coils
offer 250 ohms each and the line wires between the mill and the cottage
offer 100 ohms. This makes a total of 750 ohms for the secondary
circuit. But the rapid alternations which are induced in the secondary
circuit impede the electric current ten times as much as the resistance
already mentioned.
When considering alternating currents passing through coils of wire we
are obliged to take into account two kinds of resistance:
1. Ohmic resistance.
2. Impedance.
"You boys understand the resistance to the flow of the electric
current, which we have so often measured in ohms. But I want to show
you that there is another kind of resistance which alternating current
meets. Here is a coil containing 1000 feet of No. 20 copper wire. I
throw on to it, for only an instant, the 110-volt direct current, and
the ammeter reads 11 amperes, showing that it offers a resistance of
10 ohms to the direct current. I now throw on the alternating current,
and the ammeter shows only a small fraction of an ampere. The surging
of the current back and forth induces a counter electro-motive force,
in the successive layers of the coil, which we call _impedance_. In
the experiment which we have just performed _impedance_ is fifty
times as important a factor as ohmic resistance. Impedance depends
chiefly upon the frequency of alternation. The impedance in telephone
circuits is particularly large because of the extremely high frequency
of the alternations produced by the tones of the human voice, these
being usually not far from ten times as rapid as those of alternating
currents in common use.
"We may estimate the total resistance of our telephone circuit as
equivalent to 7500 ohms.
"Our secondary coils have forty times as many turns as the primary
coils, and by means of them the voltage is stepped up to somewhere
near one hundred on open circuit. When closed through the line,
however, the voltage drops down to about ten. The result is that the
actual current which passes between the cottage and the mill when we
telephone is not far from .001 ampere. We may, however, hear a whisper
transmitted by .000001 ampere or less.
"The tone _E´_ which is produced by the tenth key above middle _C_ on
the piano, is the one most readily heard over the telephone. It is
produced by anything which vibrates 640 times per second."
[Illustration: Fig. 169]
We used No. 12 galvanized iron wire for our telephone lines. Two miles
of No. 12 copper wire would offer 16 ohms of resistance. The iron wire
offers about 100 ohms. But this is a trifle when compared with the
total resistance. We used a double metallic circuit so as to avoid the
effects of inductance from our electric lighting circuit.
[Illustration: Fig. 170]
The next thing that we were obliged to consider was some arrangement
for calling persons to the telephone for conversation. We decided
to use magnetos and alternating current bells. Fig. 169 shows the
essential mechanism of the bells. The bell at each end of the line
consists of two gongs _a, b_ and _a´ b´_, with a hammer _c_, _c´_
between them. This hammer is attached to an iron armature _h_, _h´_,
pivoted over the electro-magnets, _m_, _m´_, in such a way that it
rocks back and forth when an alternating current passes through the
lines _d e_, _f g_. The bells at both ends of the line always ring
together, since they are connected in series. A magneto (Fig. 170)
is situated at each end of the line. This, as has been previously
explained, is a generator of electricity, in which the field is
furnished by steel magnet, _M_. The armature _A_ is a coil of wire
whose ends are in contact with the leading out wires _d_ and _c_ by
means of brushes which slide upon rings. The armature is revolved by
hand. The crank and cog wheels employed to produce high speed are not
shown in the figure. By turning the armature rapidly this magneto will
develop 60 volts e. m. f. on open circuit. The magnets of the bells are
wound with a very large number of turns of very fine wire, so that
.025 ampere is sufficient to ring them.
[Illustration: Fig. 171]
Figure 171 shows how the magneto at either end of the line is
introduced into the circuit for the purpose of ringing the bells. _B_
and _B'_ represent the bells, _m_ and _m'_ the magnetos, and _P_ and
_P'_ represent switches. Springs push them upward so that they normally
close the circuit through the bells. When a person at _P_ wishes to
call another at _P'_ he pushes the switch _P_ down so as to bring his
magneto _m_ into series with the bells. When now he turns the crank and
generates the electric current, both bells ring. His own bell serves
the purpose of telling him that the line is operating all right. The
other bell calls the party desired for conversation. As soon as the
operator removes his finger from the switch _P_ the spring throws it
upward again, leaving his bell in circuit, so that he may be called
at any time, but cutting out of the circuit his magneto, which would
introduce unnecessary resistance.
The same wires which carried the current for ringing the telephone
bells carried also the current for operating the telephone receiver.
When the receiver is removed from the hook it releases a twofold
switch. This serves the double purpose of closing the primary circuit
through the local battery and substituting the telephone receiver
circuit for the bell-ringing circuit upon the line.
We used fifty chestnut poles to carry our line between the mill and the
cottage. Each pole had a cross bar, on one end of which the electric
light and power wires were carried and on the other end the telephone
wires. Glass insulators prevented the wires from coming in contact
with the wood of the cross bars. The necessity for this was impressed
upon the boys by something which happened while they were stringing
the wires. The telephone apparatus at the mill had been installed and
the two leading out wires had been connected to it. One of these was
coiled up on the floor, while the other had been strung along upon the
poles for half a mile, but had not yet been attached to the insulators
on the poles. While the boys were lunching at the mill, one of them
gave the crank of the magneto a turn, when, to the astonishment of all,
the bell rang. The circuit had been completed through the damp wood of
the mill, through the damp wood of some of the poles, and through the
earth. After lunch the wire, so far as it had been strung, was fastened
to the insulators upon the poles. But when some one turned the crank of
the magneto the bell still rang. We walked along the line to see where
the difficulty was. We found the end of the line about half a mile from
the mill dangling free from the ground, but touching a tall spear of
grass. When this was moved away from the spear of grass the magneto
could no longer ring the bell. The slight current required to ring
this bell--.025 ampere--had found its way through the spear of grass,
through the woodwork of the mill and through the earth.
We had no sooner got the two telephone wires properly strung and
attached to the hundred glass insulators when a thunder storm came up,
and drove us back to the mill for shelter. Pretty soon the bell rang
and we, supposing that some one at the cottage was trying to call,
went to the instrument, but could get no response, nor could we make
the bell ring. Lightning had sent an alternating current over the line
which rang the bell, but the strength of the current was too great for
our coils of fine wire and one of them was burned out, as we say. In
other words, the wire had melted at the point where it offered the
greatest resistance.
The burned-out coil was replaced, and then we installed lightning
arresters which were of two kinds. The first were simply fuses which
were introduced into the line to protect it against any current too
large for the apparatus to carry, and the second was a plate, _c_ (Fig.
172). These are to be found upon the top of the magneto cases. A wire
is connected with _c_, and its other end is grounded by being connected
with a piece of iron pipe which is driven deep into moist earth.
[Illustration: Fig. 172]
The plate _a b_ is inserted in the line, and the gap between this
and the plate _c_ offers sufficient resistance so that the telephone
circuit suffers no leakage at this point, but lightning has such
extremely high tension that it readily passes across this gap and finds
its way to the earth without damaging the instruments.
We have already noticed that our alternating current dynamo, which
produces 60 vibrations per second in the telephone receiver, causes it
to give a tone very nearly like the _C_, which is two octaves below
middle _C_ upon the piano. _C_ requires 64 vibrations per second. We
may speed up our dynamo so as to make it yield a tone exactly like _C_
or even above it.
Dr. Cahill of Holyoke, Mass., has devised an organ in which alternating
current dynamos produce the necessary number of vibrations for each
tone. The name _telharmonium_ has been proposed for this organ. It
has a separate dynamo for each tone, each dynamo having a frequency
corresponding to the tone required of it. The dynamo, for instance,
which produces middle _C_ makes the electric currents surge back and
forth 256 times a second, and this causes the diaphragm of a telephone
receiver to vibrate 256 times a second, and this sends forth 256 air
waves per second, and when these reach our ears we recognize the tone
we call middle _C_. The frequency of alternation in a dynamo may be
increased by either increasing its speed of revolution or by increasing
the number of coils upon its armature.
Mr. Cahill's great organ looks like a large machine shop with many
counter shafts geared so as to run at different speeds. On each shaft
are a large number of little dynamos whose armatures have various
numbers of coils. The organist, who may be far removed from this
"machine shop," fingers an ordinary keyboard. Each key opens and closes
a switch, thus bringing into action its own dynamo.
If the key which is known as _C_, one octave below middle _C_, is
pressed down, a switch closes the circuit between the telephone and a
dynamo which gives 128 double alternations of current.
The tone which is produced by 128 vibrations per second is the one most
often heard from a man's voice in ordinary conversation.
Another key brings into action upon the same telephone receiver--and
at the same time if desired--a dynamo which gives twice as many
alternations per second and produces the tone most often heard in
female conversation. It is middle _C_.
Another key might bring into action a dynamo which gives 64 vibrations
per second to the diaphragm of the telephone receiver. This would send
forth a tone very nearly like the base note of our 60-cycle alternating
current dynamo.
The following table shows a series of ten tones which might be produced
by the same little piece of sheet iron in a telephone receiver played
upon by ten dynamos at the same time. The whole list of ten tones would
sound well when produced simultaneously. The great mystery is that
the iron disc can vibrate in such a complex manner. It is important
to note, however, that the number of vibrations in each of the upper
tones is a multiple of that of the lowest tone:
2nd octave above C´´--1024 (= 16 × 64)
Middle C G´ -- 768 (= 12 × 64)
E´ -- 640 (= 10 × 64)[A]
1st octave above C´ -- 512 (= 8 × 64)
Middle C G -- 384 (= 6 × 64)
E -- 320 (= 5 × 64)
Middle C C -- 256 (= 4 × 64)[B]
1st octave below G -- 196 (= 3 × 64)
Middle C C, -- 128 (= 2 × 64)[C]
2nd octave below C,,-- 64 (= 1 × 64)
Middle C
[C] The tone most easily reproduced by the vocal cords of a man.
[B] The tone most easily reproduced by the vocal cords of a woman.
[A] The tone which the telephone receiver responds to most readily.
The table covers the range of the human voice, male and female.
All the intermediate tones, with their sharps and their flats, are
produced each by its own separate dynamo.
The insignificant amount of current required to operate a telephone
receiver makes it possible to furnish the music of these dynamos to
many and far distant telephones. This naturally suggests the idea
of having a great musician perform upon the keyboard and have many
auditors scattered about the city in their private homes or even in
many public halls, for the telephone receiver can readily be made
audible to a good-sized audience.
XVIII
ELECTRIC BELL OUTFIT FOR THE COTTAGE
The boys asked me what arrangement of electric bells we needed at the
cottage and so I gave them this problem to work out by themselves:
1. We want a bell in the kitchen to be rung by a push button at the
front door. But there are times when no one is in the kitchen and hence,
2. We want a bell upstairs to make a single stroke whenever the kitchen
bell is rung from the front door.
3. We want a floor push under the dining-room table which will cause
the kitchen bell to ring a single stroke.
4. We want a push button in the dining-room which will cause both bells
to clatter and call people from their beds, from the piazza, the lawn,
etc., to their meals.
This equipment needs only one battery of two dry cells, two bells,
three push buttons and about two hundred feet of wire. It should cost
less than five dollars.
The boys drew many plans and tried many schemes and at last determined
upon the plan shown in Fig. 173.
_P_ is the floor push under the dining-room table. When the circuit is
closed at this point the current leaves the battery from the carbon
pole _c_, passes up and around the magnets of the kitchen bell and back
to the zinc pole of the battery _z_ by way of the push button _P_. All
other circuits are open.
[Illustration: Fig. 173]
_P´_ is the push button at the front door. When the circuit is closed
at this point the current leaves the battery at _c_, passes up to the
right-hand binding post of the kitchen bell and divides, part going
through each bell. The portion of the current which goes through the
kitchen bell passes around the magnets and through the armature to the
left-hand binding post before it can find a path back to the battery.
Hence, the kitchen bell clatters. The portion of the current which
passes to the upper bell goes around its magnets and finds a path back
from the middle binding post to the battery by way of _P´_. Hence the
bell upstairs rings with a single stroke.
_P´´_ is a push button situated upon the wall by the side of the door
which leads from the dining-room to the kitchen. When the circuit is
closed at this point, the current leaves the battery at _c_, passes
up to the right-hand binding post of the kitchen bell and divides,
part of it going through each bell. The portion which goes through the
kitchen bell passes around its magnets and through its armature to the
left-hand binding post, then up to the middle binding post of the upper
bell, through its armature to its left-hand binding post and back to
the battery by way of the push button _P´´_. The other portion of the
current passes directly up to the right-hand binding post of the upper
bell, around its magnets, and through its armature to its left-hand
binding post, thence back to the battery by way of the push button
_P´´_. Hence, both bells clatter and keep time with each other. The
upper bell will ring independently of the lower bell, but the lower
bell is dependent upon the upper one to open and close its circuit,
somewhat as a relay.
Soon after the cottage had been equipped with electric bells I went to
the mill one day and found a push button at the door. Upon going in I
was curious to examine the electric bell outfit of that place and found
what is illustrated in Fig. 174.
[Illustration: Fig. 174]
A switch, _S_, had been attached to the bell. The boys said that when
they felt well they kept the switch upon the left-hand point and the
bell rang as a clatter bell. When they felt a little sick they put the
switch upon the middle point and the bell rang with a single stroke,
but when they felt very sick they put the switch upon the dead point
and the bell did not ring at all.
XIX
USING ELECTRICITY TO AID THE MEMORY
For the sparking equipment of the motor boat we use dry cells which
have an internal resistance of not more than .06 ohm. They will, when
short circuited through the ammeter for only an instant, give 25
amperes.
(1.5 volt)/(.06 ohm) = 25 amperes
When we allow for a slight resistance in the ammeter itself, and for
the drop in voltage, we see that the internal resistance of a cell must
be even less than .06 ohm.
After being used about two months upon the motor boat these cells
develop more internal resistance, and they will then show not more than
six to ten amperes when short circuited through an ammeter. They are
then not reliable for ignition of the engine, but are quite as good as
ever for bell-ringing, and often continue so for more than a year. The
result is that we always have more partly run-down dry cells than we
can use. Seeing them about has stimulated the boys to devise ways for
using them.
The housekeeper is distracted by carrying on so many cooking processes
at one time. She forgets the eggs, and lets them boil five minutes
instead of three because the coffee must percolate twelve minutes, and
she lets the coffee percolate twenty instead of twelve minutes because
the biscuit must bake twenty minutes, and the biscuit are forgotten
because the pies must come out in thirty minutes, and the cake in
forty minutes. All this worries the cook. Harold is a sympathetic boy
and enters into the troubles of others. I had at one time shown him
how to bore a hole in a glass plate in five or ten minutes by using
a round file wet with water. One day he presented the kitchen with a
clock, intended to relieve the burdened memory of the cook. This is
represented in Fig. 175.
[Illustration: Fig. 175]