later, are closely related to this. They all create magnetic fields in
the same way and are all electro-magnets.
[Illustration: Fig. 64. Transformers]
VII
ELECTRIC HEATING
It was Washington's birthday. The schools were to have a holiday and
the Science Club was to hold a special, open meeting at which I had
been asked to present the subject of electricity in the household. I
replied to the programme committee that that was too large a subject,
but that I would talk upon electric heating. I warned them, however,
that it would be a dry study, and not an entertainment. They replied
that the father of his country had been born at a time of the year
when the weather was unfavourable to outdoor sports, and that February
usually found them acclimated to vigorous study. Neither they nor their
friends objected to study if it seemed to have a motive.
I found an audience composed of old and young, men and women, girls and
boys. Most of them had left school--many of them because their teachers
thought they were incompetent to continue.
[Illustration: Fig. 65]
Not far from here is "a wheel in the middle of a wheel ... as for their
rings they are so high that they are dreadful ... and the spirit of the
living creature is in the wheels." Those wheels are now sending the
electric current to this room for our experiments. I propose to show
that we convert electricity into heat by offering resistance to its
flow. Experience teaches us that resistance to motion always produces
heat. At Niagara Falls thousands of tons of water descend at the rate
of one hundred and sixty feet in three seconds. When the water reaches
the bottom of the falls, it is moving a little faster than a mile a
minute. The resistance which this mass meets after its fall retards its
motion and generates heat.
Hundreds of meteors fall into our atmosphere daily, travelling a
thousand times as fast as the waters of Niagara Falls. The resistance
to their motion, which our atmosphere offers, heats them white hot,
melts them, vaporizes them, burns them up, so that very few of them
reach the solid earth in a solid condition.
An iron spile driver, measuring two cubic feet, weighs about half a
ton. When it falls sixteen feet upon the end of a spile it is moving
at the rate of twenty miles an hour. The energy of this moving mass
depends upon both its weight and its velocity, and when its motion is
arrested by the spile that energy of motion is largely converted into
heat energy, from which both the spile and the spile driver get hot.
A piece of iron may be made red hot by pounding it with a trip hammer.
Count Rumford found, in 1798, while boring cannon in the arsenal at
Munich, that the resistance which the iron offered to the motion of the
boring tool furnished heat enough to boil water.
Seven hundred and seventy-eight foot pounds of mechanical energy when
converted into heat would raise one pound of water (one pint) one
degree. This is called the British thermal unit. The spile driver,
weighing 1000 pounds, falling 16 feet upon a spile, produces heat
enough to raise 1 pint of water 20 degrees.
[Illustration: Fig. 66]
Here are two binding posts, _a_ and _b_, 8 feet apart (Fig. 66),
connected by copper wires with the dynamo circuit. The volt meter
indicates 112 volts of pressure. I will close the circuit by stretching
between _a_ and _b_ 8 feet of No. 24 iron wire. (This wire is about the
thickness of a common pin.) The iron wire offers resistance to the flow
of the electric current, thereby producing heat--heat enough as you see
to make the wire white hot, indeed heat enough to raise it to something
over two thousand degrees Fahr., for now you see it has melted.
We will put in a fresh piece of wire and connect also the ammeter
in the circuit (Fig. 67). As I close the circuit the needle of the
ammeter at first indicates 20 or 30 amperes, but in a second drops to
8 amperes, and remains there a second until the wire melts and falls
apart. One hundred and twelve volts of electric pressure are able to
push 8 amperes of electricity through this wire when hot.
[Illustration: Fig. 67]
(112 volts)/(14 ohms) = 8 amperes
112 volts × 8 amperes = 896 watts
746 watts = one horse-power
Hence it required about one and one fifth horse-power to melt the wire
in a second, and the heat produced was a little less than one British
thermal unit, a unit much used by engineers.
1 pound raised 1 foot = 1 foot pound
550 foot pounds per second = 1 horse-power
778 foot pounds (1.4 H.-P.) = 1 B. T. U. (British thermal unit) =
heat required to raise 1 pound of water 1° Fahrenheit
1 volt × 1 ampere = 1 watt
746 watts = 1 horse-power
In order to hold back 112 volts of electric pressure so that not more
than eight amperes of electricity should pass, the iron wire must have
offered about 14 ohms of resistance.
The behaviour of the ammeter needle showed that the wire offered very
much less resistance when cold than when hot. Indeed eight feet of No.
24 iron wire offers about one and one third ohms resistance when cold,
hence heat had increased its resistance to the passage of the electric
current tenfold.
This piece of iron wire offered resistance to the flow of the electric
current. It offered resistance to the motion of the dynamo. This
offered resistance to the steam-engine which drives the dynamo. This
caused the governor of the engine to open and pass more steam from
the boiler. This reduced the pressure at the steam gauge. This caused
the fireman to shovel more coal into the furnace. The heat of the
burning coal melts the wire, but it does it only after several changes.
First, it is converted into mechanical energy in the steam-engine with
great loss--about nine tenths being lost. Second, it is converted
into electrical energy by the dynamo, with some loss, and, third, it
is conducted to the iron wire and converted back to heat with still
further loss. It is evident that the most economical way to heat the
wire would be to take it to the furnace. Yet all electric cooking is
done by sending electric current through wires embedded in the walls of
the cooking utensils, and it is the most wasteful method of using the
energy stored in coal that has yet been devised.
[Illustration: Fig. 68]
That merely connecting the binding posts _a_ and _b_ (Fig. 67) by a
small piece of wire should throw a load upon the dynamo miles away;
should offer resistance to its motion, and make it require 1.18
horse-power more of energy to keep up its speed of revolution, is,
indeed, uncanny. I will attempt to make it seem more real. At one end
of the lecture table I have a rotary pump _P_ (Fig. 68). The end of
the rubber tube _a_, which leads to the pump is lying upon the table
outside of the tank of water, _T_. While things are in this condition
I move the crank which operates the pump with perfect ease. Now while
still turning the crank I pick up the tube _a_ and drop its free end
into the water tank. I cannot now conceal the fact, even if I were
disposed to do so, that I must work hard to keep the pump going. The
pump itself tells you by its laboured sound that it is working hard,
and the stream of water which issues from the pipe _b_ tells how much
work I am performing. The pump is discharging five and a half pints of
water per second, that is 5.5 pounds, and it raises this water 10 feet.
Hence I am doing 55 foot pounds of work per second, which requires
one tenth of a horse-power. Here is a lad who consents to try the
experiment for us. He turns the crank easily while I am holding the
tube _a_ out of the water, but when I lower it into the water he finds
the resistance so great that, tug however much he may, he is unable to
keep the pump going.
At the other end of the table I have a small hand dynamo, _D_ (Fig.
68), _M_ is an ammeter, _V_ is a volt meter, _S_ is a switch. All the
wires are good-sized copper, and offer little resistance, except that
stretched between the binding posts _a_ and _b_. This is a piece of
fine German silver wire. While the switch is open I turn the crank
of the dynamo with perfect ease. A small amount of current is going
through the volt meter, but this is too slight to offer any perceptible
resistance to the motion of the machine.
Notice that the volt meter needle moves according to the speed of
revolution. If I turn the crank once a second the needle stands at
25 volts. The electric pressure increases or decreases according to
whether I rotate the armature faster or slower. Now I will attempt
to keep the machine revolving at a constant rate while I close the
switch _S_, and surely you must see that I have hard work to do so. The
wire _a b_ has now become red hot. The volt meter shows 25 volts of
pressure, and the ammeter shows 3 amperes of current.
Twenty-five volts × 3 amperes = 75 watts, which require one tenth of
a horse-power (746 watts = 1 horse-power). The lad now takes my place
at turning the machine and finds it easy when the switch is open, but
I actually overload him by merely closing the switch. Heating the wire
red hot requires more energy than he is able to put forth.
I proposed to the president that my lecture close at this point, and
that each one in the room have a chance to _feel_ the load which was
thrown upon the dynamo each time it was required to heat the wire.
I suggested that each person should get a realizing sense of this
fact, first by doing the work himself, and second by going home and
reflecting upon this hint. When the switch is closed three amperes of
electricity pass around the circuit. This increases the magnetism in
both the field and the armature of the dynamo, and it requires one
tenth of a horse-power more to keep the armature moving within the
field against this magnetic pull.
I further desired to announce that during this hour I had delivered to
them the second key to the Electrical Show which I had promised a few
days ago. The second key is:
Heat (and light) is produced by offering resistance to the flow of the
electric current. The first key is the electro-magnet. These two unlock
all the mysteries of the show.
The president closed the formal exercises with the facetious remark
that I had warned them before the lecture that they must work, so now
each would be expected to take a turn at the cranks of the pump and
dynamo.
VIII
APPLICATIONS OF ELECTRIC HEATING
The programme committee decided that each member of the Science Club
should busy himself looking for _applications of electric heating_ and
should consult me freely about the matter. My telephone was kept busy,
my laboratory was in great demand, and we were all getting a good deal
more education than the school was giving us credit for.
The boys generally came to me in pairs, and each pair having worked up
some illustration of heat produced by electricity reported it to the
club. These were spread by the secretary in due form upon the minutes
of the club and constituted "The Proceedings of the Science Club."
[Illustration: Fig. 69]
1. _The Electric Sad Iron_ (Fig. 69).--Removing three screws the iron
comes apart, revealing a lot of No. 24 German silver wire wound upon a
sheet of mica. This is put between other sheets of mica (Fig. 70) and
tucked away within the body of the iron. German silver offers about
twice the resistance of iron when it is cold, but, at the temperature
of the sad iron when in use, there is not much difference between the
resistance of the two metals. German silver wire, however, does not
rust as iron wire would, and hence it is chosen. German silver is an
alloy of copper, zinc, and nickel.
[Illustration: Fig. 70]
We put the 112-volt current upon this wire of the iron, and according
to the ammeter it passed 4 amperes. Its resistance must therefore have
been 28 ohms.
(112 volts)/(28 ohms) = 4 amperes
Electricity costs us about 10 cents per kilowatt hour. That is 10
cents for 1000 watts for an hour, or 1 cent for a hundred watts for an
hour, or, on a 100-volt current, 1 cent for an ampere for an hour. It,
therefore, costs about 4 cents or, more accurately, 4-1/2 cents an hour
to heat this iron.
Persons sometimes carry electric irons with them, when they travel, to
iron pocket handkerchiefs and other small articles while stopping at
a hotel. Before connecting an iron in a chandelier one must know the
voltage used in the building. If the voltage in use in the building is
not the same as that stamped upon the iron, it is not safe to connect
it. Not knowing this, many persons have had the embarrassment of
"blowing a fuse" and extinguishing their own lights, and perhaps those
of others in the same building, and very likely also ruining the iron.
Suppose we take for example this iron stamped _110 V; 400 Watts_. (A
slight variation of 5 or 10 volts will not injure an iron.) The wire in
this iron we found to offer about 28 ohms resistance when hot, and it
lets pass 4 amperes. This is about all the current which it is able to
carry without melting. Now suppose a 220-volt current is used in the
building where it is proposed to connect the iron. This would force
through the wire enough current to melt it. The wire was seen to be
at a very dull-red heat when examined in a dark room. Its temperature
was about nine hundred degrees. At this temperature its resistance is
about three times what it is when cold. We estimated by measurements
that the iron contained about twenty-five feet of the wire. The boys
then took twenty-five feet of No. 24 German silver wire and stretched
it between two nails driven up in the laboratory (Fig. 71, _a b_). The
dynamo current was then sent through this. The end, _c_, of the wire
from the dynamo was provided with a metal clip which could be slid
along on the German silver wire. Sliding this to the left, and thus
shortening the distance on the German silver wire through which the
current must pass, increased the amount of current and heated the wire
hotter. The resistance decreases as the wire is shortened.
[Illustration: Fig. 71]
The boys wound this wire upon a piece of asbestos board (Fig. 72),
about nine inches square and one eighth of an inch thick, taking
care to keep the successive turns half an inch apart. Asbestos paper
was wrapped around this. The two ends of the wire were left free for
connections. This they called a "hot plate."
[Illustration: Fig. 72]
2. _Electric Hot Plate_ (Fig. 73).--This when opened was found to have
wire coiled up inside in the same manner as the sad iron. Indeed the
sad iron supported bottom side up makes a perfectly good hot plate.
The particular hot plate which we examined had a three-point switch
which gave three different heats for the plate. (See Fig. 74.) When the
switch _S_ is upon the first point the current goes through 112 ohms of
resistance and 1 ampere passes:
(112 volts)/(112 ohms) = 1 ampere
[Illustration: Fig. 73]
[Illustration: Fig. 74]
This warms the plate slightly--enough to keep food warm which has been
already cooked. This costs about one cent an hour.
When the switch is placed upon the second point the current goes
through 56 ohms of resistance and 2 amperes pass.
(112 volts)/(56 ohms) = 2 amperes.
This makes the plate warmer and is adapted to certain cooking
processes. It costs about two cents an hour.
When the switch is placed upon the third point the current goes through
28 ohms of resistance and 4 amperes pass.
(112 volts)/(28 ohms) = 4 amperes.
[Illustration: Fig. 75]
We placed upon this hot plate a basin containing 1 pint of water
(equals 1 pound) and heated it from the temperature of the room (68
degrees) to boiling (212 degrees) in 7 minutes and then put an egg in
and boiled it 3 minutes. Using 4 amperes for 10 minutes cost two thirds
of a cent. If it takes 7 minutes to boil a pint of water it would
require 1 hour to boil a gallon upon this hot plate using 4 amperes, or
448 watts. That is, it costs us about 4.5 cents a gallon to boil water
by electricity. The cost is usually put at three and a half cents per
gallon, but much depends upon conditions.
3. _Traveller's Cooker_ (Fig. 75).--This consists of a hot plate with a
covered basin permanently attached to it.
4. _Electric Coffee Percolator_ (Fig. 76) consists of a hot plate
with a coffee percolator to sit upon it. The coffee percolator might
sit upon any other hot plate or this hot plate might serve any other
purpose, but people do not seem to think of that.
[Illustration: Fig. 76]
5. _Electric Chafing-Dish_ (Fig. 77) consists merely of an electric hot
plate with a chafing-dish attached. The electric coffee percolators and
chafing dishes require from 300 to 600 watts according to size. If used
on the 110-volt current they take about 3 to 6 amperes, and if adapted
to the 220-volt current they take from 1-1/2 to 3 amperes, but cost the
same to operate in either case. They have connected with them flexible
cords and plugs to screw into the lamp sockets.
[Illustration: Fig. 77]
6. _Electric Broilers_ are merely hot plates, generally corrugated to
conduct off the melted fat. One that we examined had a switch for three
heats: low, requiring 360 watts--costs 3.6 cents per hour; medium,
requiring 600 watts--cost 6 cents per hour; high, requiring 1280
watts--cost 12.8 cents per hour.
7. _Electric Oven._--This one has double walls to retain the heat and
has two large hot plates, one on the bottom and one on the top. It is
large enough to hold four loaves of bread. It required 1520 watts for
40 minutes to heat it to the baking temperature and one hour to bake
the bread. Hence the cost of the electricity is about 25 cents, about
what the bread would cost in the market.
8. _Electric Incubator._--This is simply a well-ventilated oven warmed
by an electric hot plate and automatically controlled so that it keeps
a constant temperature of 103 degrees. Under these conditions chickens
hatch from hens' eggs in three weeks. An incubator for 5 dozen eggs was
found to take 25 cents' worth of electricity for the whole process of
incubation.
9. _Electric Toaster._--The wire coiled up in sad irons and hot plates
becomes hot enough to scorch cloth and paper, and even set fire to them
if they come in direct contact. We proved this by opening the iron and
touching paper to the wire while it was carrying the current. We also
lighted a cigar by touching it to the wire. Electric toasters have the
hot German silver wire simply covered by a screen.
10. _Electric Cigar Lighters_ (Fig. 78).--The one we examined hung by
a flexible cord from the chandelier. It had a small disk on the side
which contained a lot of fine wire covered by perforated mica. The wire
became red hot when the push button in the handle was pressed. It took
half an ampere of 110-volt current, and operated only while the button
was pushed. As near as we could calculate it cost .0003 of a cent to
light a cigar.
[Illustration: Fig. 78]
[Illustration: Fig. 79]
11. _Electric Curling Iron_ (Fig. 79).--One who has flat hair needs no
curling iron, but those who have round hair may curl it temporarily, if
they will unscrew an electric light bulb and screw into its socket the
plug of an electric curling iron. The flexible cord contains two wires
insulated from each other. One of these wires is attached to the outer
shell of the plug, the other wire is attached to the central button of
the plug. These make connections with the two separate dynamo wires in
the socket. The current comes down one of the wires in the flexible
cord, passes through a coil of fine German silver wire inside of the
curling iron, and returns by the other wire in the flexible cord. The
small wire in the curling iron offers 220 ohms of resistance when hot
and passes half an ampere of the 110-volt current.
(110 volts)/(220 ohms) = .5 ampere.
12. _Electric Soldering Irons_ (Fig. 80).--Or coppers, as they should
be called, are ideal implements for soldering. They remain continually
at the proper temperature and are free from corrosion. They require
from 55 to 220 watts. On the 110-volt current they take from one half
to two amperes.
[Illustration: Fig. 80]
13. _Electric Heating Pad_ (Fig. 81).--This consists of resistance
wire inside of a pad of soft material. It maintains a temperature of
180 degrees, and is an excellent substitute for a hot water bag. It
contains about two hundred and twenty ohms of resistance and requires
the same current as a 16-candle-power lamp.
[Illustration: Fig. 81]
14. _Electric Fuses_ (Fig. 82).--Fuses are made of short pieces of wire
or thin sheet metal. The metal is an alloy of lead and tin which melts
at a low temperature. They derive their name from the fact that they
readily fuse or melt. A building is wired in various separate circuits.
The size of the copper wires used in each circuit is determined by the
amount of current which the circuit is expected to carry. Each circuit
is protected by one or more fuses. These melt and cut off the current
whenever too much passes for the copper conductor to carry without
getting hot. The fuse wire melts at about six hundred degrees, while
the copper will not melt until it reaches nearly two thousand degrees.
This temperature is sufficient to set fire to wood, paper, and cloth.
When any fuse melts, the current is cut off from all chandeliers,
etc., in the particular circuit controlled by the fuse. This produces
consternation among people who do not understand the function of a
fuse. They become panic-stricken and begin to trample their neighbours
to death in the theatre or on the electric train when they hear that a
fuse is "blown" (which is the electrician's way of saying that it has
melted). Everyone should know that a fuse is a safety device. It is
always enclosed in a box lined with sheet iron or asbestos, so that it
is impossible for the flash, which occurs when the circuit is broken,
to set fire to anything.
[Illustration: Fig. 82]
[Illustration: Fig. 83]
15. _Electric Gas Lighter_ (Fig. 83).--These usually have two or three
small, dry battery cells in the handle. By pushing a button in the
handle connection is made between this battery and a short piece of
resistance wire in the tip. This wire gets red hot and lights the gas.
It is a surprise to many that we can light illuminating gas without
bringing a flame to it, and it is equally surprising that some flames,
or at least sparks, may not be able to light the gas. The fact is that
it is wholly a matter of _temperature and kind of gas_. Iron heated to
dull red will not light the illuminating gas now being furnished in
New York City, while iron at a bright red heat will do so. Iron may be
hot enough to light illuminating gas but too cool to light gasolene
vapour, which requires a dazzling white heat. Iron which is just under
the temperature at which it gives any light may set fire to wood and
paper. After it has cooled a good deal below that, it will set fire to
sulphur, and when it has cooled so that one may hold it in the hand,
it is still hot enough to set fire to phosphorus. The glowing end of
a lighted cigar, the spark made by striking flint, or the spark from
a spark coil with a feeble battery, all fail to set fire to gasolene
vapour, simply because they are not hot enough.
Fresh battery cells must occasionally be put in the handle of the
electric gas lighter.
Four facts regarding the resistance of wires it is well to remember:
1. The longer the wire the more resistance it offers to the electric
current.
2. The smaller the diameter of the wire the more resistance it offers.
3. Some materials offer more resistance than others, for example, iron
about six times as much as copper and German silver about twelve times
as much as copper.
4. The common metals offer more resistance when hot than when cold,
about double the resistance when heated to five hundred degrees. It is
the reverse with carbon, which offers more resistance when cold than
when hot. The carbon filament lamp offers about double the resistance
when cold as when lighted to full brilliancy.