lights. The one which we examined was constructed according to the
plan shown in Fig. 85. The lighting circuit is brought to the binding
posts _b_ and _c_. A small insulated wire of high resistance connects
_b_ and _c_, being wound around the metal bar _a b_. The resistance of
this wire, when added to that of lamps, permits not more than one fifth
of an ampere to pass, and this warms the wire slightly. The bar _a b_
is composed of two strips of metal, brass above and iron below. Heat
expands brass more than iron. The result is that when the current is
turned on, the bar begins to curve downward until presently it touches
the metal base of _c_. Then the full current required to light the
lamps which are in circuit passes. While the circuit is closed through
the large metal strips not enough passes through the fine wire to warm
it. On cooling, _a b_ curves upward and breaks the connection with _c_,
and now the current begins again to warm up the small wire.
[Illustration: Fig. 84]
[Illustration: Fig. 85]
The flasher that we examined was adapted to operate: one
32-candle-power lamp; or two 16-candle-power lamps; or four
8-candle-power lamps, on a one ampere circuit of 110-volt pressure.
Let us see what would happen if it were connected either with a
current of higher voltage or a circuit of more lamps. Suppose we have
a 32-candle-power carbon filament lamp in circuit. This requires one
ampere to light it. Its resistance when hot is 110 ohms.
(110 volts)/(110 ohms) = 1 ampere.
When cold its resistance is about double or 220 ohms. The German
silver wire of the electric flasher offers 330 ohms of resistance, and
together they make 550 ohms. Thus the current is cut down to .2 ampere.
(110 volts)/(330 + 220 ohms) = .2 ampere
Suppose now we should undertake to use the same flasher and the same
lamp on a 220-volt current. This might push more current through than
the small wire could carry. It might melt, or its insulation might burn
off before _a_ made contact with _b_; if not the lamp would certainly
burn out after the contact. If we undertook to operate with this
flasher several 32-candle-power lamps instead of one upon the 110-volt
circuit, the result would be the same, for in that case the resistance
would be reduced and, therefore, a greater current would pass than the
wire could carry without undue heating.
[Illustration: Fig. 86]
The boys were at first troubled to see how increasing the number of
lamps in a circuit would decrease the resistance in that circuit. Fig.
86 was drawn to explain the matter. The lamps _l_, _l_, _l_, etc., are
connected _in parallel_. Each lamp makes an independent connection from
one feed wire to the other. The flasher _a_ acts as a switch to close
the circuit for the whole.
Now if we think of these wires as pipes to conduct water we would say
that water flows from _D_ to _E_ through ten pipes more readily than
through one. It would meet with only one tenth as much resistance. The
result would be the same, if we should substitute for the ten pipes one
pipe ten times as large in cross section. So it is with wires which are
conducting electricity. Introduce two in parallel, and you allow twice
as much current to pass by reducing the resistance to one half. Ten
parallel conductors reduce the resistance to one tenth and allow ten
times as much current to pass.
[Illustration: Fig. 87]
It is to be noticed that this flasher is an automatic switch which is
opened or closed according to temperature. Remove the fine wire from
_a_ and we have precisely the device which regulated the temperature in
our electric incubator. Suppose the "thermostat" (as it is called in
that case) is placed within the egg chamber which is to be kept at 103
degrees. A screw in the metal strip _c_ underneath the end of _a_ may
be set so that it will normally touch _a_. Suppose now the brass strip
is underneath the strip of iron in _a_. As the hot plate warms up the
egg chamber, the brass will expand more than the iron, and the bar will
curve upward and break the connection with _c_. As soon as the current
stops the temperature of the chamber begins to fall, and the bar curves
downward again until connection is made. This device is capable of
adjustment so as to keep the temperature constantly at 103 degrees or
any other desired degree. The device is in use for scores of different
purposes, including the regulation of temperature in school rooms.
17. _Electric Car Heaters._--Ten or fifteen years ago there were
no heated street cars in New York City. Now they are all heated by
electricity and their maximum and minimum temperatures are regulated by
law. The resistance wire may be seen in coils underneath the car seats.
Electric street cars usually operate on a 500 or 600-volt current. The
amount of current used for heating varies from 2 to 12 amperes. Perhaps
3 amperes may be taken as an average.
500 V × 3 _a_ = 1500 _w_ = 1-1/2 kilowatts.
It costs the large electric railway companies about 1.5 cents per
kilowatt hour to generate their supply of current. Eighteen hours is
considered a car day.
1-1/2 kilowatts × 18 hours = 27 kilowatt hours.
27 kilowatt hours at 1.5 cents = 40 cents per car day.
18. _Heating Apartments by Electricity._--For heating apartments
by electricity the same sort of apparatus is used as that already
described for heating cars. A family of four adults, living in an
eight-room apartment with at least 120 cubic feet of fresh air
admitted per minute, will use on an average ten amperes of the 110-volt
current. The cost will be about two dollars and fifty cents per day
or seventy-five dollars per month. Although this is as much as the
entire rental of a perfectly comfortable apartment, the novelty and the
convenience attract tenants and the extra cost of rent does not deter
them.
[Illustration: Fig. 88]
19. _Electric Bedroom Heater._--One of the boys constructed a heater
for his own room as follows: He procured a box eight inches deep by
eighteen inches square on the bottom. This he lined with asbestos
paper. He then stood it upon its side and arranged four incandescent
light sockets as shown in Fig. 88. These were connected by a flexible
cord to a plug which he could insert in place of a lamp in the
chandelier. He placed this heater on the floor underneath the window
and usually had 16-candle-power lamps in the sockets. He claimed that
it was a jolly foot warmer and kept the room comfortable without other
heat. He turned on from one to four lamps according to his need and
replaced the 16-candle-power lamps by 32-candle-power lamps when
the weather was extremely cold. I remarked that he must have light
along with heat by this arrangement, and I should think that might be
objectionable when he desired to sleep at night. He said that he always
turned it off, and opened the window at night, always preferring a cold
room to sleep in.
[Illustration: Fig. 89]
20. _Cooking with Incandescent Lamps._--This piece of apparatus was
devised by the boys and used in my laboratory. A sheet iron basin _a_,
was inverted over four 16-candle-power incandescent lamps, shown in
elevation by Fig. 89, and shown in plan by Fig. 90. The sides of the
basin were cut so as to admit the glass globes of the lamps, but the
sockets and keys were outside, so that it was convenient to turn on
and off the lamps separately, thus using one half to two amperes of
current, as desired. This rested upon another basin, _b_. Basin _b_
was covered with asbestos for the lamps to lie on and the whole was
attached to a board base, _c_. A flexible cord and plug allowed us to
attach this to the chandelier. A pint of water was boiled upon this
stove in fifteen minutes, and refreshments have been served hot from it
repeatedly.
[Illustration: Fig. 90]
21. _Electric Fireless Cooker._--There are five indictments against
ordinary cooking processes.
1. They heat the house in summer.
2. They convert what would be pleasant flavours in the food into
noxious odours about the house.
3. They cannot be controlled with regard to time and temperature as
scientific experiments should be.
4. They confine the cook too closely and are not sufficiently automatic.
5. They are wasteful of fuel.
It would seem that electricity might enable us to cure most of these
evils. To be sure the production of heat by electricity is wasteful
of fuel, and it seems doubtful how the account will balance regarding
the fifth item. But the remaining four items furnish a very hopeful
field for research. I use the last word advisedly, and think it is just
as applicable to high school boys as to university students. After
experimenting awhile the boys and I concluded to give a dinner party
in the laboratory and invite a few friends to test the results of our
cooking.
We procured a cylinder of magnesia such as is used for covering large
steam-pipes. This was inverted over our electric stove which was
illustrated in Fig. 89. The magnesia was cut at the bottom, so as to
give access to the key sockets of the lamps, (Fig. 91). First upon the
electric stove was placed a covered dish containing a roast of lamb.
Above this was another dish containing a vegetable, and upon the top of
that was a pudding. A flat piece of magnesia was used as a cover to the
whole. Through a hole in this was suspended a thermometer.
[Illustration: Fig. 91]
This "fireless cooker" was sitting in the centre of the dinner
table when the guests gathered around it. We had these problems for
investigation:
1. Will this cooker heat the house in summer?
All testified that they did not know that there was any heat about it
until they laid their hands upon it, and then they found it only very
slightly warm.
2. Is there any smell of cooking here? The process has been carried on
from start to finish right on this table.
All agreed that no smell could be detected.
I then turned off the electric current which had been running until now
and served the meat and vegetable, leaving the pudding inside to be
kept warm by the hot walls of the cooker.
3. Regarding the control of the process: we were using 32-candle-power
lamps, which gave us a variable current, from 0 to 4 amperes, and a
watch and a thermometer. We had control, but as yet lacked knowledge of
how it should be used. In the present case we had arbitrarily decided
to begin with temperature of 400 degrees, continue it for 20 minutes,
then turn off all the electric current, and let the temperature fall
gradually. This had been done at our convenience in the morning before
school. At a quarter before twelve we had found the temperature at
200 degrees, and turned on all the current, and now, at five minutes
past twelve o'clock, all testified that the lamb was particularly
good--neither too well done nor undercooked, and that its flavour was
better than usual.
As for economy of fuel, we find at least that we get better results
from incandescent lamps than from hot plates used in the same
apparatus, and the electric equipment enables us to put the heat
exactly where it is needed and nowhere else.
22. _Incandescent Lamp._--We feel quite justified in putting the
incandescent lamp under the heading, _Applications of Electric
Heating_, since the electric lamps in general use convert 96 per cent.
of the electric energy into heat and only 4 per cent. into light.
They were originally made by introducing a short piece of fine wire
into the circuit, choosing the kind of wire, its diameter, and its
length so as to make the proper relation between resistance and
voltage, in order that enough current might pass to make it white hot,
but not quite melt it. Platinum wire was first chosen because it would
stand the highest heat without melting and without rusting.
We will pass our 112-volt current through 9 feet of the No. 24 iron
wire. The wire is heated to bright red, but does not melt as it did
when we used 8 feet in a former experiment. The increased length has
added resistance, and, as you see by the ammeter, cut the current down
from 8 to 7.5 amperes. I will now darken the room and you find that
it is giving light enough to read by. But you notice that the light
is growing dimmer, its colour is growing redder, and the ammeter
indicates that less current is passing. I will cut off the current
and let you examine the wire and you notice that a crust has formed
upon it. This is due to the oxygen of the air which unites with the
iron, forming iron rust. Iron rust does not conduct electricity. We
have converted No. 24 iron wire into a wire of smaller diameter with a
sheath of iron rust around it. We might prevent the rusting by putting
the wire in a glass globe and exhausting the air from it.
I have here a piece of No. 24 platinum wire which has about the same
resistance as iron wire when cold, but you notice that I may use a
very much shorter length than I did of the iron wire because it will
endure a very much higher heat without melting. Reducing the length
would reduce the resistance, but reducing the resistance would allow
more current to pass. If more current should pass it would make the
wire hotter, and raising the temperature would increase the resistance,
which would cut down the current, etc. By sliding the clip _c_ (Fig.
92), along, I finally reach a point where conditions balance so that I
get a very brilliant light, dangerously near the fusing point of the
platinum which is three thousand degrees above the boiling point of
water.
In 1879 Mr. Thomas A. Edison literally searched the whole world for
something better than platinum for the filament of an incandescent
lamp. He finally decided upon charred threads of a bamboo which he
found in Japan. No research was ever more timely than this. Whereas
there was practically no electric lighting before 1880, soon after
that there began a phenomenal demand for carbon filament lamps. In
1890, 800,000 of these lamps were manufactured in the United States. In
1900 the number had risen to 25,000,000. In 1909 central stations were
supplying electric current to 41,807,944 incandescent electric lights.
By far the greatest number are still made with carbon filaments.
[Illustration: Fig. 92]
[Illustration: Fig. 93]
We examined an ordinary 110-volt 16-candle-power carbon filament lamp,
(Fig. 93). As near as we could estimate, its filament measured about
eight inches in length. We broke open the bulb of this lamp by laying
it upon the table and tapping it with a board. The bulb broke with
rather a loud noise and the brittle carbon filament broke into many
pieces. We found one of these pieces and measured its diameter with a
wire gauge, (Fig. 94). It was the same size as No. 33 wire, which we
also found by the wire gauge was the size of No. 90 sewing cotton. The
diameter of No. 33 wire was given upon the wire gauge as .007 inch.
When lighted, the filament of this lamp had looked to be about the size
of No. 18 wire, which has a diameter of .04. That is, the filament
when lighted looked six times as thick as it really was. Those who
use sewing cotton learn quickly to know the size of the thread by its
number. So those who have much to do with wire easily learn the system
of designating sizes by numbers. Here are some selected figures easy
to remember. A trolley wire is about one third of an inch in diameter.
It is designated as No. 0. Notice in the following table that as the
numbers rise by six the diameters are divided by two. Notice also that
as the diameters diminish by two the resistance increases by four.
[Illustration: Fig. 94]
TABLE OF RESISTANCE OF COPPER WIRES
_Nos._ _Diameter_ _Resistance_
0 .32 inch 10560 feet to the ohm
6 .16 " 2640 " " " "
12 .08 " 660 " " " "
18 .04 " 165 " " " "
24 .02 " 40 " " " "
30 .01 " 10 " " " "
36 .005 " 2.5 " " " "
42 .003 " 1 " " " "
10,560 feet equal two miles.
Number 36 is the wire used upon the spools of telegraph receivers. They
offer 75 ohms of resistance and therefore contain 30 feet of wire (30 ×
2.5 = 75). These resistances are for ordinary school room temperatures.
Since iron has six times, and German silver twelve times the resistance
of copper, divide the figures of the third column by six, and the table
will answer for iron wire, or divide those figures by twelve and the
table may be used for German silver wire, thus:
_Number Feet to the Ohm_
_Nos._ _Diameter_ _Copper_ _Iron_ _German Silver_
0 .32 inch 10560 1760 880
6 .16 " 2640 440 220
12 .08 " 660 110 55
18 .04 " 165 27 14
24 .02 " 40 6 32 inch
30 .01 " 10 1.5 8 "
36 .005 " 2.5 .45 2 "
42 .003 " 1 2 inch 1 "
These figures are not exact, but useful.
We procured a string of eight small lamps (Fig. 95), such as are used
in lighting Christmas trees. Each was marked 14 volt, 2-candle-power.
The carbon filament of each was about one inch long and apparently the
same diameter as that of the 16-candle-power lamp. When the 110-volt
current was sent through the group of eight connected in series they
seemed to give about the same light as the single 16-candle-power lamp.
It is as though the filament of the 16-candle-power lamp had been
cut into eight pieces, and distributed through eight small lamps. We
introduced an ammeter into the circuit and found that half an ampere of
electricity passed through the single 16-candle-power lamp--and half an
ampere likewise passed through the group of eight 2-candle-power lamps.
[Illustration: Fig. 95]
The 110-volt current can push an ampere of electricity through eight
inches of carbon thread seven thousandths of an inch in diameter, and
when this happens the filament gets hot enough to give out as much
light as sixteen standard candles. In the place of the 16-candle-power
lamp, we put a 32-candle-power 110-volt lamp. The ammeter indicated
one ampere. The carbon filament was larger (No. 30, diameter =
.01 inch), so as to allow more current to pass. An 8-candle-power
110-volt lamp was substituted; one quarter of an ampere passed. A
4-candle-power 110-volt lamp was used; one eighth of an ampere passed.
A 100-candle-power 110-volt lamp was substituted; three amperes of
current passed through it. In all these cases the lamps which passed
the larger current had the larger filaments. A little practice would
enable one to distinguish between these lamps without labels by
examining their filaments. Among these 110-volt lamps, it is to be
noted that the amount of light which they give is proportional to the
amount of current which they pass. And it is convenient to remember
that one ampere of electricity for one hour costs about one cent.
We introduced into the socket a "Hylo" lamp (Fig. 96). The filament,
_A_, took half an ampere of electricity, gave 16-candle-power of
light, and cost half a cent an hour. When the lamp was turned in its
socket the current was switched off of the filament _A_, and on to the
filament _a_. This took .03 of an ampere, gave one candle-power of
light, and cost .03 of a cent an hour, or at the rate of about $3.00 a
year, burning continuously day and night.
[Illustration: Fig. 96]
The uses of such a lamp are apparent in rooms which have no daylight.
However, a wall switch at the entrance of such a room, making it easy
to throw on and off the light entirely, seems to be a more satisfactory
arrangement. One of the boys connected a wattmeter in the circuit with
a hylo lamp and found that the small filament did not pass current
enough to move the armature of the wattmeter. Hence that may be burned
alone without affecting the consumer's bills.
We took a 16-candle-power 220-volt lamp, and lighted it by a 220-volt
current. The meter showed that it allowed only one quarter of an ampere
to pass. The filament was very much smaller than that in the 110-volt,
16-candle-power lamp. The pressure was twice as great as before, but
the resistance was four times as great, and hence only half as much
current passed. We find that it costs just as much to generate one
quarter of an ampere at 220-volt pressure as it does to generate half
an ampere at 110-volt pressure.
We must, of course, pay for electricity according to the cost of
producing it. To produce .5 ampere at 110-volt pressure costs the same
as one ampere at 55-volt pressure, or .25 amperes at 220 volts. It will
be noticed that the products of the two factors in each case are the
same. The product of an ampere multiplied by a volt is a watt. In each
of the above three cases the amount of electrical energy is 55 watts.
This will produce a definite quantity of light--about 16 candle-power
when the carbon filament is used, and this quantity does not vary as
either volts or amperes, but as the product of these, namely, watts.
Each of these lamps is called a 55-watt lamp, and, since they each give
16 candle-power of light, a carbon filament lamp gives one candle-power
of light for three and a half watts of electricity. Electricity for
lighting purposes usually costs _10 cents per kilowatt hour_, that is,
10 cents for 1000 watts for one hour, or one cent for 100 watts for
one hour. Hence a 55-watt lamp costs a trifle more than half a cent for
one hour, or exactly .55 cents, and a 32-candle-power lamp costs 1.1
cents per hour.
We introduced into the socket a 48-candle-power 110-volt tungsten lamp
(Fig. 97), and turned on the 110-volt current. The ammeter showed
55 ampere. Hence the lamp is a 60-watt lamp, and requires one and a
quarter watts per candle-power. That is, the metal tungsten is nearly
three times as efficient as carbon for producing light from electricity.
[Illustration: Fig. 97]
With pincers we broke off the tip of a 32-candle-power carbon filament
lamp, making a small hole in the large end of the bulb. The air rushed
in. We then put the lamp in the socket and turned on the current.
The carbon filament glowed as usual, and slowly burned up, growing
smaller as it did so. The ammeter which was in circuit showed that the
current, which was one ampere at the beginning, grew steadily less as
the filament grew smaller, until finally when it was about one quarter
of an ampere, the circuit was broken by the filament burning in two.
We removed the lamp from the socket and with a dropper tube introduced
a little lime water, and shook it to absorb any gas which might have
been formed in there. It became milky white, as it always does when
introduced where carbon has been burned. This would be a sufficient
proof that the filament was made of carbon, if we did not already
know it. The air is exhausted from these bulbs to prevent the carbon
filament from burning up.
[Illustration: Fig. 98]
The carbon filament lamps were, as has been said, the invention of
Mr. Thomas A. Edison in 1879. Such a statement must, however, be
qualified by the assertion that this, like nearly all invention, was
but the consummation of a long line of researches made by many men for
many years. The early filaments were made of bamboo thread, charred,
but now they are drawn like spider's web out of a sticky liquid and
carbonized at a high temperature. They are attached in the lamp to
short pieces of platinum wire which are sealed through the glass walls
of the bulb. One wire connects with the brass collar of the bulb, and
the other with the central piece of brass at the base of the bulb.
We dissected a socket and found that when the lamp is placed in the
socket, the collar of the lamp is screwed into the collar of the
socket, and the base of the lamp comes in contact with a brass spring
in the bottom of the socket (Fig. 98). The spring is connected with
one copper wire bringing electricity from the dynamo. The collar is
connected with the other wire from the dynamo. This connection is made
and broken by turning the key of the socket. The wires are made of
copper since copper is a particularly good conductor of electricity.
No electricity can flow unless this circuit is complete. Socket keys
and wall switches make or close gaps in this circuit. No copper wires
for carrying electric-lighting current are smaller than No. 12, which
has a diameter of .08 or about one twelfth of an inch. The intention is
to have as little resistance to the current as possible, except in the
filament of the lamp itself. There resistance is purposely introduced
in order to convert electricity into light, light without heat if that
were possible, but since that has not yet been found possible, heat for
the sake of the accompanying light. Unhappily only 4 per cent. of the
electrical energy goes into light and 96 per cent. goes into useless,
or even harmful, heat. The tungsten lamps, which are now coming into
use, are nearly three times as efficient in the production of light as
are the carbon filament lamps. The dynamo exerts its entire pressure
upon the lamp and furnishes current as follows:
A dynamo of 110-volt pressure gives:
1 ampere = 110 watts, through a 32-candle-power lamp, cost one cent an
hour, or
.5 ampere = 55 watts, through a 16-candle-power lamp, cost half a cent
an hour, or
.25 ampere = 27-1/2 watts, through an 8-candle-power lamp, cost a
quarter of a cent an hour.