the South Pole of the needle attract the North or South Pole of the
compass? Try this with the second magnetized needle. See if you can
prove the rule that like poles repel (drive away) and unlike poles
attract.
[Illustration: Figure 5]
Connect one end of a wire loop to the battery and run the wire directly
over the compass. Touch the other end of the wire to the battery. Which
way does the compass point now? If you get some motion out of the
compass needle, this proves there is a magnetic field around the wire
when current is flowing. This relation between electricity and magnetism
is the thing that makes electric motors and generators work.
[Illustration: Figure 6]
Make Many From One
Lay the third needle (unmagnetized) on a table and stroke it with one of
the magnetized needles. (See diagram) Always stroke it in the same
direction. Raise the magnetized needle at least two inches on each
return stroke. Thus you can magnetize the needle by using the other
needle.
[Illustration: Figure 7]
Use the wire cutters to cut the first magnetized needle in short
lengths. (Cover the needle with a cloth to keep the pieces from flying.)
Can you show by using the compass that each piece is a complete magnet?
Hold one end, then the other, of each piece to a compass. Does each
piece have both a North Pole and a South Pole?
Magnetism and Animals
The things you have done show that electricity and magnetism are related
in many ways. Magnetism is mysterious, and there are still things to
discover about it. It is thought that animals and birds are aided in
their sense of direction by magnetism. It is commonly known that when a
person gets lost in the woods, he tends to go around in circles.
Possibly this is caused by the earth's magnetic field.
What Did You Learn?
1. Where are natural magnets obtained?
2. How can artificial magnets be made?
3. What material is needed for a permanent magnet? For a temporary
magnet?
4. How can you find out which is the North Pole of an unmarked magnet?
5. How many poles does a magnet have?
6. Which magnetic poles attract each other?
7. Why couldn't you make a compass out of a strip of plastic?
8. What causes the compass to change direction when a wire carrying
battery current is held over the needle?
9. List the materials you would need and tell how you would build a
homemade compass.
10. Tell what you enjoyed most about becoming acquainted with mysterious
magnetism.
LESSON NO. B-14
Credit Points 2
Give your appliances and lights a square meal
Would you say that having enough to eat was pretty important in the home
that you know?
The "food" for your appliances and lights is electricity, and like you
they must be "fed" enough.
What to Do
1. List the appliances and lights in your home.
2. See if any of them are "starving" for the electricity they need.
3. Learn how the electricity gets to where it's used.
4. Make a chart of the electrical circuits in your home.
5. Make sure that each circuit is protected with the right fuse or
circuit breaker.
Count Your Electrical Blessings
Many people in much of the rest of the world wish that they could trade
places with us, because we have so many electrical appliances in our
homes.
Of course, we have not always had as many appliances as there are today.
When electricity first came along, people used it only for lights. Then,
they began to add flatirons, washing machines, refrigerators, coffee
percolators, and radios.
Then more and more electrical things were made for people to use and
enjoy. Now we have dozens and dozens of uses for electricity in our
homes.
How many different uses for electricity are there in your home today?
Ask your parents how many there were when your home was built or first
wired. How many were _common_ when your parents began to keep house?
Some Homes Are Behind Times
Many older homes were built before electricity was available, and were
wired later. And like them, some older homes that were wired as they
were built had only enough wiring for lights and a few other appliances,
because those were the only uses that were known at that time.
But people kept on living in these homes, and kept adding to the uses
they made of electricity without adding to their wiring.
What has this meant? Well, if electricity were like cars and trucks, you
could say that some people are trying to put turnpike traffic through a
back-country dirt road!
Watch for Signs of Starvation
Of course, as your state has done with its highways, some people have
expanded and modernized their wiring. But many others have not yet seen
this need, or if they have, they may have to do it again.
Here's why:
Your power supplier delivers current to you at the right voltage or
electrical pressure. If the wires in your house are large enough, they
will pass this full voltage on to the appliances.
But if your wiring is too small, the electricity arrives at the
appliances so weak that they can't work properly, and much of what you
pay for is wasted.
Here are some things you can watch for in your own home. They will tell
you whether your appliances are getting enough electrical "food" or not.
1. _A shrinking TV picture_--If it draws in from the sides of the
screen, fades, loses contrast, or if the sound becomes distorted, you
may have low voltage.
2. _Too much fuse blowing or circuit breaker tripping._
3. _Heating appliances are slow to do their jobs._
4. _Lights dimming_, when motors or other appliances are turned on.
There Should Be Enough Ways to Get "Appliance-Food" Around
If appliances in your home show these starvation signs, then you may not
have enough ways for the electricity to get to where it's used.
There are three kinds of these electrical highways or circuits, and your
home should have enough of each:
1. _General purpose circuits_--These serve lights all over the house,
and convenience outlets everywhere except in the kitchen, laundry, and
dining areas.
A rule-of-thumb is: There should be at least one general purpose circuit
for each 500 sq. ft. of floor space.
2. _Small appliance circuits_--These are not used for lights, but
instead they supply convenience outlets in the kitchen, laundry, and
dining areas where portable appliances are most used.
Every home should have at least two small-appliance circuits.
3. _Individual or special-purpose circuits_--One of these is needed for
each: electric range, dishwasher, water heater, freezer, automatic
washer, clothes dryer, air conditioner, pump, and house heating
equipment.
+----------+------+------+------+------+-------+
| | | | | | |
| Actual | | | | | |
| Size | | | | | |
+----------+------+------+------+------+-------+
| Gauge | | | | | |
| Size | 14 | 12 | 10 | 8 | 6 |
+----------+------+------+------+------+-------+
| Fuse or | | | | | |
| Breaker | 15 | 20 | 30 | 40 | 55 |
+----------+------+------+------+------+-------+
|Max. Watts| | | | | |
|at 115 V. | 1725 | 2300 | 3450 | 4600 | 6325 |
+----------+------+------+------+------+-------+
|Max. Watts| | | | | |
|at 230 V. | 3450 | 4600 | 6900 | 9200 | 12750 |
+----------+------+------+------+------+-------+
[Illustration: Wire sizes commonly used in homes]
Each Circuit Big Enough
The capacity of each circuit is limited by the size of its wires. The
chart above shows you the actual sizes of wires commonly used in
permanent home wiring, and what each will carry. Notice that each size
is given a number, and the smaller the number, the bigger the wire.
Also notice that a given size of wire will carry twice as many watts at
230 volts as it will at 115 volts. (Watts are figured by multiplying
amps times volts.)
General purpose circuits usually are either Number 14 or Number 12 wire,
at 115 volts. What is the capacity of each, in watts? (Number 12 wire is
recommended for all new general purpose circuits.)
Small appliance circuits are required to be at least Number 12 wire.
Individual circuits are always sized according to the appliance they
serve. Find the size wire that should be used for a 10, 000-watt,
230-volt range; a 1500-watt, 115-volt dishwasher; a 4500-watt, 230-volt
clothes dryer. ________ ________ ________
Only One Fuse Size Right
A fuse in an electrical circuit is like an alert traffic
policeman--stopping everything if there's danger. A circuit breaker
serves the same purpose, and the right size is installed when the wiring
is done.
A policeman uses his brain to tell him when to blow his whistle, but a
fuse depends on the size of the little fusible (meltable) metal link
that you see under the glass.
If too great an electrical load is added to a circuit, this link will
melt and prevent a dangerous overload. If you put in a fuse with too
heavy a link, it will not melt in time, and the wiring and equipment may
be damaged.
Therefore the right size of fuse is very important, and is something
that you should check in your own home.
See the chart above for the right fuse for each size wire.
Make a Circuit Chart
At one or more places in your home there is a box or panel containing
the fuses or breakers for the various circuits. Attached to the inside
of the door of each such panel should be a chart something like this:
[Illustration]
+-----+---------------------+-----------+
| No. | Description | Fuse size |
+-----+---------------------+-----------+
| 1 | Range | 40 |
+-----+---------------------+-----------+
| 2 | Kitchen Outlets | 20 |
+-----+---------------------+-----------+
| 3 | Dining Room Outlets | 20 |
+-----+---------------------+-----------+
| 4 | Living Room Outlets | 15 |
+-----+---------------------+-----------+
| | | |
+-----+---------------------+-----------+
| | | |
+-----+---------------------+-----------+
Notice that in our chart we have made columns for a description of what
each circuit serves, its number or position in the panel, and the proper
size fuse for it.
Because most such charts leave out this last very important bit of
information, you should make a complete new chart, like the one shown.
Provide as many lines as there are fuse positions. Paste or tape it to
the inside of the panel door.
Then, ask permission of your parents to disconnect all the circuits by
unscrewing the fuses or flipping the circuit breakers. _Do not touch
anything but the fuse rim._ Then reconnect them, one at a time, to find
out what each circuit serves. Turn on as many lights as you can, to help
you in your detective work. Use a test lamp at those outlets that do not
have a light connected to them. Write two or three words describing each
circuit on the proper line on your chart.
On a separate sheet, keep track of the appliances and lights that are on
each circuit, and add up the watts. (If the name-plate of any appliance
gives "amperes", "amps", or "A" instead of watts, just remember that
amps times volts equals watts.) This will tell you if any of them are
overloaded. Show this sheet to your parents.
Check the Wire Sizes
_Disconnect the main switch_, and determine the size of the wires in
each circuit. Don't include the insulation in your measurement.
_BE CAREFUL!
Even though you have disconnected the main switch, the wires coming into
it are still "live". So, do not touch any wires. Instead hold the wire
size chart near them so that you can tell which gauge each one is._
Write in the proper size fuse for each circuit on your chart.
Replace Any Wrong-Size Fuses
Do the fuse sizes you have written on your chart agree with the ones
that are in place in the panel?
Get the right size fuses and replace any that are wrong. Make sure that
you have a reserve supply of the right sizes, and that they are handy
for future use.
Talk it Over With Your Parents
Do you think that your home has enough of the proper size circuits? If
not, talk it over with your parents. They may want to ask an electrician
to go over the wiring and make the necessary changes.
What Did You Learn?
(Underline the right answer.)
1. A (television set, radio) is very sensitive to changes in voltage.
2. Dimming lights mean (static in the wires, an electrical overload).
3. Wires that become warm from overload make it (more expensive,
cheaper) to operate the equipment.
4. A home of 2,000 sq. ft. should have at least (three, four) general
purpose circuits.
5. One solution to low voltage symptoms is (heavier fuses, more
circuits).
6. Full capacity for a Number 14 wire circuit at 115 volts is (1725
watts, 3000 watts).
7. A room air conditioner should be on (a general purpose, an
individual) circuit.
8. The purpose of a fuse is to (let you disconnect the circuit,
automatically prevent overloading the circuit).
9. The right size fuse is determined by (wire size, the store where you
buy it).
10. A circuit chart should give (circuit description and fuse size, the
maker's name).
Demonstrations You Can Give
Ask your leader to help you plan a demonstration. You can show how
lights dim when too many other appliances are connected, how a fuse
protects against overloading, and the danger of using too large a fuse.
For More Information
Ask your Extension agent, power supplier, or electrician for additional
help.
LESSON NO. B-15
Credit Points 4
YOU CAN MEASURE ELECTRICITY
Instruments that can detect or measure the flow of electricity have
helped to make possible the wonders of electricity as we know them
today.
Scientists in laboratories must have measuring devices for experiments
leading to new uses of electricity. Power suppliers must have
instruments that tell what the generating equipment is doing and to
measure the amount of electricity being sold to users. Factories need
instruments that keep tab on electrical equipment to make sure
electricity is being used efficiently.
In fact, almost anywhere you find electric power at work you'll find
electrical instruments--even in your home. The one you know best
measures the amount of electricity used. Another, in the family car,
shows whether the generator is charging the battery or if the battery is
discharging.
What to Do
1. Make a simple kind of direct-current meter that will show you that
there's a magnetic field around a wire carrying an electric current and
that will detect a very tiny current.
2. Make a more refined D.C. instrument (galvanoscope) and measure the
voltage of different sizes of dry batteries, and show how an electric
current can be induced.
Tools and Materials You'll Need:
Pair of pliers, knife, small hammer
30 feet of No. 24 bell or magnet wire
Compass
Two coins--a penny and a dime
Fine sandpaper
Blotting paper
Plastic or cellophane tape
Wooden blocks (See Figure 4)
Glue
2 small nails
One #905 dry cell, a penlight battery, and
two regular flashlight batteries
Table salt
Drinking glass
2 paper clips
Two machine bolts
How They Work
Like many electrical things, most electrical instruments depend on the
action of magnetism created by an electric current. There is a magnetic
_field_ or lines of force around any wire carrying an electric current.
If this field is controlled and made to react on a sensitive device,
like an easily moved pointer, we have an electrical instrument.
Detect a Magnetic Field
First, let's prove that there is a magnetic field around any wire
carrying an electric current. Take a piece of wire about two feet long
and scrape off about an inch of insulation from each end. Connect one
end to a battery terminal. Make a loop of wire that crosses the face of
your compass, north to south. Now touch the other end of the wire to the
other battery terminal.
(DO NOT attempt to substitute alternating current, as from a model
railroad transformer because its alternating current will cause the
compass needle to swing rapidly from one side to the other.)
[Illustration: Figure 1.
Put your right hand beneath the wire so that your fingers point the way
the needle deflects, and your thumb will point in the direction that the
current is flowing.]
What happens? Your compass needle should move to one side because it is
very sensitive to magnetic influences. This proved that the wire created
a magnetic field or lines of force when we passed electricity through
it. (Figure 1)
Detect a Tiny Current
How sensitive is your simple electric meter? Take about five feet of
wire and wrap it around your compass as in Figure 2, keeping the turns
bunched together as much as you can. Leave about six inches at both ends
of the wire extended for leads. Scrape the insulation off the last inch
of both. Rotate the coil and compass until the needle and coil are
parallel, both pointing north and south.
[Illustration: Figure 2]
Take a copper penny and a dime, and clean off any corrosion or film on
the coin faces with a bit of fine sandpaper. Now take a piece of
blotting paper about the size of the penny and dip it into strong salt
water. Place the damp blotting paper between the penny and the dime.
Place one of your compass coil leads against the dime, and the other
against the penny as shown in Figure 3. Be sure you have good
metal-to-metal contact between the wires and the coins.
[Illustration: Figure 3]
At the instant that you squeeze the leads against the coins, watch what
is happening to the compass needle. It should move for an instant from
the north position each time you press the leads against the two coins.
Obviously, the little coin battery you have just made produces a very
weak electrical current. Even so, your instrument should be able to
detect it.
Make a Simple Galvanoscope
Now let's make a meter that is a little more practical to use. Broadly
speaking, a galvanoscope is an instrument that detects the presence of
electric currents. It sounds complicated but it is really quite simple.
It is named in honor of an Italian professor named Galvani who made
important early experiments with electricity.
A refinement of the galvanoscope is today's galvanometer. Other related
instruments are the voltmeter and ammeter. These are very important
instruments to the electrical engineer.
Using a glass or anything three to four inches in diameter, wind about
20 turns of wire in a "bunched" coil as in Figure 4. Wrap the coil at
several points with cellophane or plastic tape to keep it from
unwinding.
[Illustration: Figure 4]
Make a wood base for your coil as shown in Figure 4. The compass support
blocks can be thin wood slats. Do not attach them with steel nails or
tacks. Use glue instead. Hold the coil in the slot between the blocks
with glue or melted wax or use copper staples. Place the compass on the
supports and rotate the base so that the compass needle and coil are
parallel, pointing north and south.
Measure the Voltage of Batteries
Do you know what difference the size of dry cell battery makes in the
voltage it supplies? Your meter can tell you.
To test the voltage of batteries we must be able to control our
galvanoscope. To do this, connect a glass of strong salt water in series
with the battery as shown in Figure 5. Make sure the wire ends immersed
in the salt water are scraped free of enamel.
[Illustration: Figure 5]
With one of the batteries connected, move the wires in the salt water
first closer, then farther apart (keeping them parallel to each other)
while watching your compass needle. When the needle stays 15 to 20
degrees off north, lock the wires in the salt solution in place with
paper clips.
Now disconnect the battery you have been using and connect a smaller
battery. If both batteries are fresh, the compass needle should return
to almost the same spot. This proves that both batteries regardless of
size put out the very same voltage. The larger ones, however, are
designed to last longer.
Measure the Difference between Series and Parallel
Using the salt solution as in the previous experiment, connect two
flashlight batteries in series as shown in Figure 6. The compass needle
should move about twice as far as it did with one battery connected.
This shows that when you connect batteries this way you double their
voltage.
[Illustration: Figure 6]
Now place your batteries side by side and connect the two top terminals
and the two bases as shown in Figure 7. The compass needle should move
only as much as it did for one battery. This is called a parallel
connection. You can see that this arrangement does not double the
voltage, even though you used two batteries.
[Illustration: Figure 7]
While you have this hookup, try reversing the position of the leads
connected to your batteries. Notice that reversing the direction of
current flow in the coil causes the compass needle to swing in the
opposite direction.
Test for Induced Current
Make a simple coil by winding about 50 turns of wire around a machine
bolt core. The bolt should be 1/4 to 1/2" in diameter and about two
inches long. Connect the coil to your galvanoscope as shown in Figure 8.
Pass the coil back and forth close to the end of a permanent magnet.
[Illustration: Figure 8]
Notice a slight deflection of the compass needle with each pass. You
have shown that electricity can be induced in a wire coil by moving it
through a magnetic field. Currents generated in this way are called
induced currents.
[Illustration: Figure 9]
Now make another coil and core just like the first one and arrange them
and a connection as shown in Figure 9. If you make and break the current
to the second coil, you will build up and collapse a magnetic field
around the first coil and again induce a current in it. You will see the
compass needle swing back and forth again.
These last two experiments give you a crude idea of how an electric
generator works, producing electric current by induction as a coil-wound
rotor revolves within a magnetic field.
What Did You Learn?
What does every current-carrying wire have around it? How does this help
us to measure electricity? How sensitive are electrical instruments?
What is the difference in voltage between (a) a large and a small dry
cell? (b) batteries connected in series and in parallel? (c) your
original connection and the reverse of it? What similarity does the test
for induced current show between movement through a magnetic field and
the making and breaking of a direct current?
Demonstrations You Can Give
Show others how your galvanoscope can detect: whether a battery is
producing current, which way the current is flowing, and whether a
current is strong or weak. Demonstrate how a current can be generated
using magnetism.
For More Information
Ask your power supplier representative to show you some of the
instruments used by his organization, and to give you a brief
explanation of how they work. Ask him or an electrician to give you a
demonstration of a split-core ammeter.