.5 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an
hour, or
.25 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent
an hour, or
.125 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a
quarter of a cent an hour.
The carbon filament lamps, barring accidents, have a natural life
varying from 600 to 1000 hours of actual incandescence. At the end of
that period the filament has become so thin that it will fall apart by
ordinary usage. It is never profitable, however, to use them for their
whole lifetime. The lamp gradually volatilizes carbon and deposits
it upon the inner walls of the bulb, producing a smoky appearance and
shutting off light. As the filament grows thinner by this process, it
offers greater resistance to the current, and as the amount of current
grows less the proportion of light to current grows rapidly less,
so that at last instead of paying for 3.5 watts of electricity per
candle-power of light one must pay for perhaps seven or eight watts per
candle-power. We pay fifteen cents apiece for 16-candle-power lamps,
and it is economy to renew them about twice a year, if they are burned,
say three hours a day, or a little over five hundred hours. It is
interesting to note that when a direct current is used the evaporation
from the carbon filament always takes place at the negative end alone,
that is, the end from which the current is leaving the lamp. If an
alternating current is used the evaporation goes on from all parts of
the filament alike. This is a case of evaporation from the solid state.
Carbon does not boil below 6,000 degrees, and the filament reaches
about 2,450 degrees.
Tantalum, tungsten, and osmium lamps have metal filaments. These metals
are better conductors than carbon but unlike carbon their resistance
increases as their temperature rises, and their special virtue is that
they are capable of enduring an extremely high temperature without
melting. The wire used in some of these filaments is as small as .002
of an inch, or No. 44. In order to furnish sufficient resistance to
prevent the 110-volt current from melting, they often have a length
exceeding two feet. This is laced back and forth within the small bulb.
At the temperature of bright incandescence their resistance may be
increased as much as fivefold and sometimes becomes about ten ohms to
the inch. Like all metals they are more brittle when cold than hot.
Hence when cleaning such lamps it is advisable to turn on the current
to avoid breaking the filament by jarring. Filaments which are too
fragile to endure the jar of ordinary railway travel, when cold, have
gone through railway wrecks safely when lighted.
It is a general rule that good conductors of electricity grow more
resistant as the temperature rises while non-conductors resist less as
the temperature rises. Hence the insulating material which is used to
cover copper wires fails to protect if highly heated.
If a 110-volt lamp is put into a 220-volt circuit, one might expect
that the lamp would burn out without doing further damage to the
circuit, but this is not the case. As the filament approaches its
melting point, 6000 degrees, it becomes so good a conductor that it
carries current enough to melt a fifteen ampere fuse. It is, therefore,
the fuse that protects the circuit and not the burning out of the lamp.
The bulb containing the highly heated carbon vapour would conduct the
current as an arc lamp does.
[Illustration: Fig. 99]
23. _Arc Lamp._--We fastened two electric light carbons to the ends of
copper wires connected for the 110-volt current. A rheostat, _R_ (Fig.
99), in circuit, was set at 6.5 ohms. One lower carbon was fastened
into a clamp, and the other was touched to it, and then drawn away
about three-eighths of an inch. A very brilliant light was produced.
Probably about 1800 candle-power. The ammeter _A_ showed 10 amperes,
and the volt meter _V_ showed 45 volts. 45 volts × 10 amperes = 450
watts, 1800 candle-power, 25 watts per candle-power.
The arc light is the cheapest of all lights but is too dazzlingly
bright for household purposes. It is used for outdoor lighting
chiefly, and particularly for large search-lights. The temperature is
over 6000 degrees, which boils the carbon and fills the gap between the
two pencils with a stream of carbon vapour. This conducts the current
like the filament in an incandescent lamp. The air gap between the
carbon pencils would have a resistance of many thousand ohms if it
were not for the presence of the carbon vapour. The hot carbon vapour
reduces the resistance of this space to 4.5 ohms.
(45 volts)/(4.5 ohms) = 10 amperes.
or
(110 volts)/(6.5 + 4.5 ohms) = 10 amperes.
The carbon pencils account for part of this resistance--not more than a
third of an ohm however.
It is evident that arc lamps in use must have an automatic mechanism
which shall permit the carbons to touch whenever the current is not
passing, but which shall draw them apart to the proper distance after
the carbon vapour has been formed, or, as we say, after the arc has
been established. This mechanism is nothing else than electro-magnets
which are operated by the lighting circuit itself. It may require
thoughtful examination to recognize these as electro-magnets, in every
case, but that is what they are. Sometimes they are coils of wire,
which do not have iron cores and armatures separate to be sure--but
nevertheless they have both of these united in one movable rod, and
they produce magnetic fields.
Suppose I pass an electric current around this coil _A_ (Fig. 100).
The region about the coil becomes a magnetic field with its north pole
situated at a point in space, say _N_. The influence of this field
causes the iron rod to become a magnet with its south pole uppermost,
and if the current is strong enough, and the field which it produces is
strong enough, it will lift the iron rod up into the coil. By varying
the strength of the current you see I may make this rod dance up and
down in space touching nothing--a veritable ghost dance.
[Illustration: Fig. 100]
It may be pettifogging to say that the upper portion of this iron
rod is the core of the magnetic field, and its lower portion is the
armature. Yet this is right, and pettifogging may be right when it is
the only way to bring out the fact.
Our great study now is to produce light without heat, or at least to
come as near to it as the firefly does. The firefly gives 98 per cent.
light and two per cent. heat. The arc lamp gives 12 per cent. light
and 88 per cent. heat. The carbon filament gives 4 per cent. light and
96 per cent. heat. When we have made considerable progress in that
direction we shall take electric lamps out of the chapter on electric
heating and form a new chapter on electric lighting.
One might expect that a rod made of carbon would quickly burn up,
particularly when raised to the exceeding high temperature of the
electric arc. While it is true that carbon in the form of charcoal
burns so readily that it is used instead of kindlings for lighting a
fire, carbon in the form of graphite in our so-called "lead" pencils
and carbon as it is prepared for electric light pencils burns only very
slowly even at exceedingly high temperatures. The carbon rods used in
arc lamps endure a temperature of over 6000 degrees, without losing
more than one inch an hour, and half of that is simply volatilized--not
burned.
One of the most interesting improvements ever made in the arc light
is that of enclosing the arc in an inner glass globe. This globe
is closed airtight below with a small opening above. When the arc
is formed the oxygen of the air in the inner globe is soon consumed
and then combustion is no longer possible. We illustrated this by
an experiment. An ordinary cork was chosen to fit the large end of
an argand lamp chimney and through a hole in this was passed one of
the carbon rods (Fig. 101). A metal clamp made connections between
this carbon and the negative wire from the dynamo. The other carbon,
attached by a clamp to the positive wire, was thrust down into the
upper end of the chimney until it touched the negative carbon, and then
drawn upward a short distance, drawing an arc, as we say. This soon
makes an atmosphere within the chimney where combustion cannot go on
for want of oxygen. The arc, however, continues to glow as in the open
air, and the carbons may be drawn further apart than in the open air
without breaking the arc, hence more of the external resistance may be
cut out and a higher voltage put upon the lamp.
[Illustration: Fig. 101]
Carbons which burn out in a single night if used in open arc lamps last
two weeks in enclosed arc lamps.
The lower carbon, when removed from the lamp chimney of the last
experiment, served as a lead pencil to write on paper. The positive
carbon would not make a mark on paper. In all arc lamps carbon is
distilled from the positive pencil, condensing upon the negative pencil
as graphite, which is the material used in making "lead" pencils. They
are called "lead" pencils because they were originally made of lead,
but now they are made of graphite which is mined from the earth.
As soon as the arc is broken it becomes evident that the positive
carbon has been heated much the hotter of the two, a fact that could
not be detected while it was lighted because of the dazzling brightness
of the arc. The negative carbon turns black almost immediately, while
the positive carbon remains at a bright red heat for some time.
This fact needs to be borne in mind when adjusting arc light carbons in
search-lights, stereopticons, and all like apparatus in which the light
must be placed at the focus of a lens. That is, it is necessary to know
from what point the light really comes and it is necessary to have some
adjusting device to keep this point continually at the focus of the
lens.
24. _Search-Light._--(Fig. 102). This is simply an arc lamp with
reflectors behind it and lenses in front of it. The whole apparatus is
pivoted so as to be easily made to shine in any direction. The function
of the lenses and the reflectors is to collect stray rays of light and
send them all out in the same direction. This is shown in Fig. 103
where for simplicity the lens is represented as a single piece. _L_
represents a point of light which will naturally send its rays out in
all directions as the radii of a sphere; _m_, _m_, _m_ represents a
bright reflecting surface which is given that peculiar curve called
a parabola. It has the unique faculty of reflecting in a parallel
direction all the rays which may fall upon it from _L_, so long as _L_
is kept at that particular point called the focus, _a b_ is a lens of
glass which has that peculiar curve that enables it to bend all rays
which fall upon it from _L_, so that they may pass out parallel.
[Illustration: Fig. 102]
25. _Stereopticon._--This also has the necessary devices to gather the
rays of the arc lamp and send them forth parallel, and in addition it
has a series of lenses which produce upon a distant screen an enlarged
picture of any transparent object held in these parallel rays.
[Illustration: Fig. 103]
[Illustration: Fig. 104]
26. _Burglar's Flash-Light._--There are many forms of this. The one we
examined is represented in Fig. 104. We unscrewed a metal ring at the
left-hand end and found, first a glass lens and behind that a miniature
electric light, requiring three volts and half an ampere. We knew,
therefore, that it must be supplied with two cells, since one cell
may give not more than 1.5 volts. We also knew that it would only be
used to _flash_ a light, since if dry cells are required to furnish
half an ampere continuously they soon run down. Behind the lamp there
was a bright metal reflector--the lens and reflector are fairly well
represented in Fig. 103. The filament of the lamp is connected with two
small battery cells in the handle. These may be removed and replaced
by new ones by unscrewing a cap at the right-hand end. The circuit is
closed by a metal spring on the side of the tube, which acts as a push
button. It is situated where it may be conveniently pressed by the
thumb. The small batteries necessarily have a short life and must be
replaced quite frequently. Being a special thing they cost nearly twice
what the regular dry cell does.
[Illustration: Fig. 105]
27. _Mercury Vapour Lamp._--This is an interesting variety of arc
light in which the vapour of mercury takes the place of the vapour of
carbon. _G_, in Fig. 105, represents a glass tube from which the air
has been exhausted. The wires of the lighting circuit are fused into
the ends of the tube. At one end, and in contact with one of these
wires, is a small pool of mercury. By pulling the cord _c_ the tube is
tilted on the pivot _p_, so that a stream of mercury flows along the
whole length of the tube and closes the electric circuit. When the tube
falls back into its normal position, as represented in the figure, the
electric arc persists upon the mercury vapour. Incandescent mercury
vapour gives light strong in green, blue, and violet, but deficient in
red and yellow. It, therefore, gives nothing its natural appearance but
casts a ghastly hue over everything.
This lamp was invented in 1902, by Peter Cooper-Hewitt, grandson of the
founder of Cooper Union in New York City.
It gives a very suitable light for making photographic prints, and is
much used for that. This lamp operates upon the 110-volt circuit. It
is the longest step yet taken toward getting light without heat, but
perhaps shows what we must expect when we reach that goal, namely,
unsatisfactory colour values in the light. Probably such is the case
with the firefly.
28. _The Moore Light._--In 1896 Prof. D. McFarland Moore brought out
his vacuum tube light (Fig. 106). We visited an ordinary dry goods
store which had been equipped with this. Glass tubing is put together
very much as one would put up a stove pipe or a job of plumbing. The
joints are fused and made air-tight by playing a flame upon them after
the pipe is up in place. This pipe is led around into all nooks and
corners where there would be dark places. The air is pumped out of
this tube and a trifling amount of some vapour is introduced, the kind
varying according to the tint of colour which is desired.
[Illustration: Fig. 106]
Metal terminals are fused into the ends of this tube. The tube we saw
was seventy-five feet long. A 1000-volt alternating current is applied
to the terminals and the vapour becomes incandescent, filling the whole
tube full of light. The first thing that the boys remarked was that
although the room was brilliantly lighted no object cast a shadow. It
seemed as though light was everywhere and there was no chance to screen
it off.
29. _The Nernst Lamp._--In 1897 the Nernst lamp appeared in Germany. It
is a good illustration of an insulating substance becoming a conductor
when heated to a high temperature. The "glower," as it is called, is
composed of one or several short rods of clay-like material. This is
first heated by sending the electric current through resistance wire
placed directly underneath it and connected in shunt with it. When it
gets hot, current begins to pass through it, and is automatically cut
off from the resistance coil. The glower produces an intensely bright
and white light although it does not itself exceed the temperature of
1742 degrees.
Electric installations are now so carefully constructed that fires from
poor insulation are very rare. Less than one fire in three hundred
appears to be traceable to that cause.
30. _Electric Welding._--Nothing is more common in electrical matters
than heat produced by poor contacts. In this laboratory are two
chandeliers, each controlled by a wall switch. After the current has
been on the chandeliers for half an hour you will always find one of
those wall switches warm, while the other is not perceptibly warmer
than other objects in the room. The explanation is that there is poor
contact in one of them. When two metal conductors touch one another
at a mere point the electric current, in passing from one of these
conductors to the other across such a narrow bridge, meets resistance
and develops heat--sometimes heat enough to fuse the point, and either
break the contact, or, what is more likely, start a minute arc at that
point. In some cases this makes the apparatus dangerously hot, and in
other cases it bridges the gap with a broader and better contact--a
true electric weld. Electric welding is applied to everything, from
chicken fence to railway rails. Enormously large currents are used for
the purpose, in some cases as high as 50,000 amperes being employed.
The rails of railroads are welded end to end by a current of several
thousand amperes sent through the joint by perhaps two or three volts.
The joint heats and fuses together merely because the poor contact
offers resistance to this enormous current.
IX
LIGHTING A SUMMER CAMP BY ELECTRICITY
Summer had arrived. The Science Club had held its last meeting for the
season. Harold had engaged three other boys to spend the summer at the
farm. I had the roof of an old mill reshingled and gave it to them
for a camp. They were to make it over inside. I sent the boys to the
country as early as it was possible for them to get away. It would be
six weeks later before I could follow them.
[Illustration: Fig. 107]
When I did arrive I found they had elaborate schemes indeed. The first
floor of the mill had been partitioned off into rooms, as shown in
diagram (Fig. 107), _a_, _b_, _c_ and _d_ being bedrooms; _e_ was a
wash room, the like of which has never been seen before. It had not
occurred to me that the mill pond _m_, which came to the very corner
of the building, would furnish the boys a complete system of city
water-works. At _g_, in the corner of this room, they had cut a hole
in the floor and nailed slats across upon the under side of the
timbers, making a depressed floor for a shower bath. This was directly
over a stream of water which issued from the mill pond. Hanging from
the ceiling over this spot was the nozzle of a garden hose. The other
end of this hose ran into the mill pond. The nozzle was capable of
delivering either a stream or a shower, according to which way it was
twisted in its socket. It was also capable of shutting off entirely the
flow of water. The boys asked me to hold my hand in the shower, and to
my astonishment it was warm. "What, pray, is your heating system?" I
inquired. They invited me to go and see. Moored outside in the mill
pond at the corner of the building was our motor boat, which the boys
were allowed to use freely and which they understood as well as any one.
[Illustration: Fig. 108]
They said that ordinarily they used for the shower the cool water of
the lake, which they much preferred, and which ran of its own accord,
the lake being a trifle higher than the nozzle of the shower, but
knowing my antipathy for the cold bath they had slipped the end of the
rubber hose over the outlet pipe of the pump which served to cool the
gasolene engine in the boat. The engine uncoupled from the propeller
was heating and pumping water for my shower bath, and I immediately
accepted the invitation to enjoy it.
Certainly no bath was ever more delightful than that one, coming, as it
did, at the close of a hot, dirty ride from the city.
I had hastened the bath, because it was already dusk and I had no
candle at the mill, but suddenly the room lighted up as if by magic.
I saw then what had before escaped my notice, a miniature electric
lamp, six-volt, two-candle-power, tungsten, such as are used for tail
lights on automobiles. Since tungsten requires about 1.25 watts per
candle-power it was a 2.5-watts lamp, and since it was adapted to six
volts it would take about four tenths of an ampere.
6 volts × .4 ampere = 2.4 watts. The little wire filament looked to be
about 1.5 inches long. Its resistance must have been 15 ohms.
6 volts/15 ohms = .4 ampere.
A battery of five cells was used to furnish electric current for the
lamp. Lamps were installed in the bedrooms also and were not intended
to be used more than half an hour at a time. Dry battery cells are
excellent for this purpose, and for so small a current the cheapest
dry cells are as good as the more expensive ones. These cost fifteen
cents a cell. They were connected by short pieces of bare copper wire;
No. 18 "in series," as shown in Fig. 109. A wire ran from the central
(carbon) binding post of one cell to the marginal (zinc) binding post
of the next cell. This battery was placed on a shelf in a convenient
place. A bare copper wire, No. 18, was attached to the carbon post at
one end of the battery and another to the zinc post at the other end
of the battery, and these two wires ran to all the rooms where lamps
were placed. The wires were fastened up on the walls by staples, taking
care that they should nowhere come in contact with each other and
"short circuit" the battery. Whenever it was necessary for one wire to
cross another, small pieces of pasteboard were tacked up to prevent
their touching each other. The lamps _L_ (Fig. 109) were connected
to these wires "in parallel." They cost forty cents apiece, and the
miniature sockets, into which they were screwed, cost five cents each.
One of these sockets was screwed to the side of the door casing in each
bedroom. Wires were attached to the line wires, simply by twisting them
together. One of these came down to one side of the socket and the
other came to the other side of the socket through a switch, _s_, made
of a strip of sheet zinc. The cost of the entire installation was as
follows:
5 dry cells at 15c .75
5.2 cp., 6-volt tungsten lamps at 40c 2.00
5 miniature wall sockets at 5c .25
Wire, etc. .20
----
$3.20
[Illustration: Fig. 109]
Suppose each lamp is used thirty minutes a day for 100 days, making a
total of fifty hours. There are five lamps, making a total of 250 lamp
hours. Each lamp takes .4 of an ampere, making a total of 100 ampere
hours. The lamps are operated at six volts, making a total of 600 watt
hours.
100 days
.5 an hour each day
---
50 hours
5 lamps
---
250 lamp hours
.4 ampere for each lamp
---
100 ampere hours
6 volts
---
600 watt hours
This amount of electrical energy would cost six cents if generated
by a dynamo. It is generally stated that electricity costs fifty
times as much if generated by battery as by dynamo. In this case the
battery actually did serve for the whole season of 100 days and was not
exhausted at the end of the season.
Indeed, since that season, the boys have found that battery cells
which had been too much exhausted for use on the engine served very
well on the lamps. By use the cells lose, not much in voltage, but in
the ability to furnish sufficient quantity in amperes to make the hot
spark required for igniting the mixture of gasolene and air in an
engine cylinder. When they have been discarded for use with the engine
they may still furnish the small amount of current required for the
lamps--provided not too many lamps are used at one time.
The dynamo current is always surprisingly cheap when compared with
that produced by a battery, but, on the other hand, we are never as
economical in the use of the dynamo current as we are with that of the
battery.
If all five of the lamps in the above equipment were lighted at the
same time and kept burning for half an hour, the battery would run down
rather badly and would not fully recover. But if one only is used at a
time and for not more than thirty minutes, or if more than one is used
at a time and for a proportionately shorter period, the battery will
receive no damage.