I hope you have enjoyed Part I and Part II, and here is what I will write about in Part III.

Wheatstone Bridge Circuits, Switching, Rotary Switches, Capacitors, Magnetism;

Let's begin.

This is how the Wheatstone Bridge Circuit looks like.  It is used for measuring unknown resistances.  A circuit like this using resistors or as well as other components finds a wide range of usages in electronic circuits.

The above illustration is an example of a basic Wheatstone Circuit.  The two input terminals are connected to a voltage source, Vs.  The meter between points A and B is a sensitive galvanometer which is used to measure magnitude and direction of current.  R1 and R2 represent the ratio arm of the bridge.  Rs is a variable standard resistor and Rx is the unknown.

The bridge is in balance when the voltage drops across Rs and R2 are equal and there is a proportional division of voltage across the bridge.

In this "null" condition, the voltage at point A is equal to voltage at point B.

The meter reads ZERO since there is no difference in potential.  A slight change in resistance Rx would cause an unbalance condition, and the proportional voltage division would be upset.

The differences in voltage between the points A and B would cause the meter to deflect.  A small adjustment of Rs can bring the bridge back to balance.

As you can clearly see this stuff is getting much more involved then previous chapters I and II.  So if you havn't read them, I would recommend that you do or you will be lost.  Also do a lot of research and reading on your own and together you will come out just fine.

So, what is a Wheatstone Bridge the easy way ?  Here is the easiest way that I know of how to explain it.  Have you ever saw or maybe even used a VOLT meter ?  you know, one of those meters that you can buy at Radio Shack, they have these probes that you touch with, BLACK for negative and RED for positive ?

Well, this is something similar to it.  The input terminals can be compared to those probes.  With this circuit you can also find E, I AND R.

SWITCHING -

Switches in electronics, you guessed it are used to OPEN and CLOSE circuits, also to switch between circuits, but mainly to turn ON or OFF it's power supply.

There are all kinds of switches for different type of purposes, here are some of them.

This type of switch is called an SPST, which stands for SINGLE POLE, SINGLE THROW.  The pole is the little piece of plastic that you move with your fingers to switch the switch ON or OFF.  This switch disconnects one side of a line in a single wire circuit.

This is a SINGLE POLE, DOUBLE THROW switch used in switching from one circuit to another.  The OFF position is in the center and the ON's are on the left and right sides.

This is a DOUBLE POLE, SINGLE THROW used to switch both sides of a two wire circuit at the same time, similar to the SPST above.

This is a DOUBLE POLE, DOUBLE THROW switch used to switch between circuits and two of them at that.  Similar to SPDT, but you can switch between 2 circuits at a single throw meaning that if the center is OFF and the sides are ON, each side can turn ON or OFF two circuits, vs. only one in the SPDT.

This is a momentary switch, a push button switch to be more exact used in switching circuits momentarily.  Like your bell door, alarms or to momentarily close a circuit.

Rotary switches are devices that use a rotary switch.  Above you can see a SPRS, SINGLE POLE, ROTARY SWITCH.  There are different types of rotary switches and I am not about to cover every single one of them, so you will have to do some research on that.  This is to show you only the basic diagram and how they look like.  Some of the older television or radio sets still use them to this very day.
 

MAGNETISM

Magnetism dates back a long time, before we even knew how and why it worked.  It is said that a Chinese emperor used magnets as a direction finding device over five, that's 5,000 thousand years ago.

I even read somewhere that early ship navigators used this strange stone as a compass by attaching it to a small piece of floating wood and placing it into water.  Since there was nothing holding the floating wood, the magnet would cause it to move towards the northerly direction, now that's really neat for their time.

But anyway, I am not going to talk about all the folklore and superstition that is associated with the magnet but instead will explain to you how and why it works the way it does, because it is very important in electronics.

Please again understand that if I didn't think this was important, I would not be writing about it.  If you find this type of reading boring and this does not interest you enough to go out and do research on your own, then maybe electronics are not for you.

Scientists have discovered that our planet "the earth" is one big magnet.  It's core, deep deep into the ground, in the middle is an enormous magnet.  It's ends, like, imagine a football that is not round but more oval shaped, it's ends are pointing close to the earth's surface which happen to be in the north and south poles.

These magnetic forces can not be seen with the naked eye, but trust me they are there.  That's why a compass is such a great direction finding device, because no matter where you stand on the surface of the planet it will always point towards NORTH.

As a matter of fact if you want to be more exact and for your information, here is the exact definition of magnetic North and South.  The magnetic North that we know as North is in fact the south end of the earth's magnetic field and force.  We in the early days were not yet that advanced about the knowledge of how this worked, so we called North because the compass always pointed in the Northern direction.  But I do not think scientists quite understood how it worked back then.

Today, South would be North and North would be South.  Cool or what ?

Further more, the magnetic north is not the true exact north.  Why ? because of angular differences.  The angular difference between magnetic north and the true NORTH POLE is called the ANGLE of DECLINATION or VARIATION.

Today magnets can and are artificially made using ferrous materials.  The magnetic field has a name technically and that is called FLUX.  If you take a magnet and place it under a piece of paper while you sprinkle some iron filings you can observe the particles aligning with the forces around the magnet.  You will see that the bulk of the iron filings will concentrate at each ends of the magnetic poles.  These are called the POLES, and each end is it's own POLE, either NORTH or SOUTH.

You can also observe that the filings align in the form of lines, and that the lines never cross each other.  They also expand and separate more in the middle of the magnet.

These lines and their separation will depend on the power and strength of the magnet.

Here is the most important LAW for you to memorize about magnetism.  They are as follows:

This law is similar to the LAW of ELECTROSTATICS where, like forces repel and unlike attract.

If you want to learn about the MAGNETIC FIELDS in ATTRACTIVE or REPULSIVE POSITIONS, read a book :- ).

Anyway, since you know that like magnetic poles repel and unlike attract, you should also be aware that the distance between the magnets has a lot to do with that.  The attractive or repulsive forces of poles varies inversely with the distance that the magnets are set apart.  This difference is measured in distance squared or like this:

M = magnet, strength of magnet

d = distance between magnets

The force is also directly proportional to the force of each acting magnet.

and that funny symbol that's u shaped is (mu) = permeability: is the ability of a material to conduct or carry magnetic lines of forces.  It is the property of a material which permits easy distribution of magnetic force lines within the material.  In magnetic circuits we use a greek letter of u, the funny shaped one above, (look at the picture) pronounced (mu) to permeability.

Permeability of different materials is assigned a number, this number has no unit of measurement and expresses the permeability of the material in respect to air, or more accurately, to vacuum.  Vacuum has a permeability of 1.

That is why (mu) is included in the equation for magnetic force.

retentivity: is the ability of a material to hold it's magnetism after the material has been removed from a magnetic force or magnetism that remains after a magnetic force has been removed from is called residual magnetism.

Anyway, the most important magnetic materials used to date in electronics are called a FERROMAGNETIC materials because of their ease to magnetize and keep it's magnetism.  These include iron, steel, cobalt, permalloy and alnico.  If you think these are strange names then you are right.  Alnico is a trade name for an alloy of iron, nickel and aluminum.

PARAMAGNETIC materials are the opposite, where they only hold a small fraction of the magnetism after passing through a strong magnetic force.  These include aluminum, chromium and platinum, you know that very expensive metal.

A DIAMAGNETIC material is one that can also be slightly magnetized but it will assume a polarity opposite to the polarity of its magnetic force or magnet.  These include copper, silver, gold and mercury.  Some of the best conductors!, and which one is the best, do you remember ? Silver, that's right.  Most people still think it's gold, haha.

Ok, what else is there to say.  Magnetism plays a very important role in electronics, did I mention that yet ?  Motors work because of it and many other components and things in electronics.

Now, if you want to find out how exactly magnetism works, then look in more common textbooks under WEBER'S MOLECULAR THEORY or DOMAIN THEORY.  There is more then one.

What is MAGNETIC INDUCTION you ask ?

Very simple, when you bring a magnet close to any other material that has a high permeability like iron, steel, the force from the magnet will pass into the material and it will attach to the magnet.

So that material is in fact attracted to the magnet.  This magnetization of that material whatever that might be while in the FLUX or magnetic force of the magnet is called MAGNETIZATION BY INDUCTION.

Never heat a magnet and try not to drop them on the floor.  What is MAGNETIC SHIELDING? : There exist no 100% effective known shield against magnetism.  Magnetism will pass through any material, and I mean anything, glass, plastic, water, insulation, even you.

Some materials basically conduct magnetism better then others, just like electricity.  If you want to shield something from magnetism, like your speakers, you would use a high permeability material and surround the item to be shielded to move the magnetism around it instead through the item.

Delicate instruments like meters, the ones with those dials have exactly this installed around the sensitive meter, because magnetism would interfere with a correct reading.

Anyway, a Scientist of the name Hans Christian Oersted, was the first to emphasize the relationship between electricity and magnetism in 1819.  He discovered that a compass would act very strange while very close to a flowing circuit and then it would go back to it's normal reading after the circuit was turned off.

From this we learned that you can also make a magnet by using electric current in a conductor.  One easy way is to take a nail and wind some wire around it and hook it up to a electric source.  You will in fact create an electromagnetic field and the reason why it is called an electromagnetic field and not a magnetic field is because the field was created using electricity and not a regular magnet.

When electric current flows through the wire, the current creates a clockwise circular magnetic field.  When you reverse it, it will produce a counterclockwise field.  As a matter of fact if you know how magnetism works you can quickly find out which way the electric current is flowing, up or down.

And you thought this was all boring :- )

When electric current flows through a wire that is wound around a nail for example, the magnetic field around the wire joins and reinforces each other.  The coil or nail will assume a magnetic Polarity just like a magnet.  One end becomes North and the Other South.  If you reverse the electric current, the poles will switch in force.

If you wind wire around an empty piece of coil, that is called a SOLENOID.  If the coil that you wound the wire around is made out of magnetic material, then you just made an electromagnet.

Here are a couple of points you should remember about electromagnets if you plan to make one and you should just for fun.

The number of turns of wire wound around the coil, more coil is better

The current flowing through the coil, stronger is better

The kind of core material

The ratio of coil length to its diameter

If you want to make the electromagnet more powerful there are only two ways to do it.  One, either wind more wire around the coil around the material that you are using to make the electromagnet or increase the current.

Another one can be by changing the type of wire you are using, silver will work much better then copper, because it resists less and is a better conductor.  Of course it is more expensive.  You do not need to use gold for even better conduction, because silver is a better conductor.

The magnetic field intensity of a coil will remain uniform, that means the same throughout the cross section of the coil if the length of the coil is ten times or more greater then its diameter or twenty times the coil radius.

Anyways, it's time to get familiar with the electromagnetic terms and definitions.

There is no one system in use when it comes to measuring electromagnetism.  The most accepted measurement systems are: the CGS (Centimetre-Gram-Second) system, the MKS system, and the English System units.

I will write about the English system and the CGS systems only.  As you already know FLUX is the measurement of the total number of lines of magnetic force.  The greater the FLUX reading, the more lines, the more powerful magnetic force, not that hard.

We use a Greek symbol of (phi)

to represent FLUX.  In the English system, FLUX is measured in Kilolines or thousands of lines, as Kilo means thousand.  In the other system (CGS), a single line is called a MAXWELL.

Another term called, FLUX DENSITY is used to measure the number of flux of field lines in the cross sectional area of the flux field.  Density is expressed in KILOLINES per SQUARE INCH in the English system and MAXWELL PER SQUARE CENTIMETRE in the CGS system is called a GAUSS.

FLUX DENSITY has a symbol of B assigned in Electronics and the formula goes like this:

MAGNETOMOTIVE FORCE or (mmf) is the total force producing a magnetic field or FLUX.  It's like this:

(flux) = mmf/R

above is the formula for mmf.

Ok, a new term that I am going to teach you now is called RELUCTANCE: is the opposition to the establishment of flux lines in a material.  RELUCTANCE is like RESISTANCE in electronics.  Every time electricity flows through a conductor, it will be met with opposition or like friction, which is called RESISTANCE.  Well, every time you create an electromagnet, you will be also creating opposition to the FLUX lines in that material that you used to create the magnet.

Also, FIELD DENSITY is the magnetic force that we use to describe unit length of the flux path.  It is assigned a symbol of H.  In the English System this is expressed with AMPERE TURNS PER INCH.  In the CGS System it is expressed in GILBERTS PER CENTIMETRE.

Here is the formula for field density.  U means what ? look above, anyways, u = B divided by H.  Look above for those as well, I explained all of this already.

GILBERT is the unit of magnetomotive force in the CGS System.  You can convert it to the English System by this following formula:

ONE GILBERT = 1.256 AMPERE-TURNS

Here is a to the point and easy to understand formula for calculating Magnetomotive force.

First you have to find out how many turns are around the coil that you use to produce your electromagnet.  For example, if you gave it 250 turns, then you take that number and multiply it by the AMPS reading that is flowing into the circuit.

If you are feeding the electromagnet at 4 AMPS for example, then you multiply 4 x 250 = and you get 1,000 IN, the IN stands for AMPERE-TURNS.

Any good Electrician must understand this before he can work on motors, generators, relays and other electrical components.

Many times the electromagnets that you will work with are store bought, you will not be actually making them by hand and they have a fixed turns of wire around the coil.  So the magnetomotive force can only vary by AMPS.  You will therefore multiply AMPS x the fixed number of turns for each electromagnet to get a reading.

Field INTENSITY is also a very important term to understand more deeply.  Field intensity is distributed more over the overall total length of the magnetic circuit or the electromagnet and it's core material.

So if the electromagnet is 5 inches long then here is what you do to find field intensity:

H = IN/inches

Or in our previous example,   H = 1,000/5 = 200 IN/inch

Field density works closely with Field Intensity or H.  Try not to confuse them, as they are not the same term.

According to ROWLAND'S LAW that states   (flux) = mmf/R, see formula above, if mmf is increased by an increase in AMPS will increase the magnetic flux.

Well, there is much, much more to magnetism then just what I have covered here.  You know what to do if you want to learn more, right ?

BUT KNOW this, if you are planning a career in electronics or electronic engineering, MAGNETISM is VERY! important.  You will need to understand it very well, what terms like SATURATION means and how the characteristics of a magnetic material function within a coil.  You will need to understand this theory in order to work with chokes, transformers, memory cores of computers and many other electrical components.

Read a Book.

Next I will cover AC/DC and RELAYS to end chapter 3, and will begin chapter 4..

What is a RELAY ? ha, ha

Now you will appreciate all the magnetism talk, and you will find out first hand why we learn it :- )

The key word here is: SOLENOID.  I covered what that is above, but to refresh your memory, here it is again.

Take a piece of pipe, and I do not mean pipe like in your plumbing system, but piece of coil, a round thing, you know ?

Make sure that it is made out of high permeability material, and I covered what metals are and are not.  Wind some wire around it to make a ELECTROMAGNET or SOLENOID.  Just keep in mind the major difference that of a SOLENOID and a ELECTROMAGNET.  The only minor difference is that a ELECTROMAGNET must be made out of magnetic material for it's core and a SOLENOID does not have to.

I believe that if I place this picture you will understand it much faster.

When you apply a voltage source, and ES stands for that, E stands for Volts and S stands for Source, you activate the coil or the electromagnet.  That rod that is connected to the spring first of all must be made out of a metal that falls within the good permeability metals, plastic won't work.  Once the electromagnet energizes and creates a magnetic field, the rod will become a part of it, like one big magnet and the FLUX lines will transfer into it as well.

Since the magnet lines are seeking the shortest path between the poles and since there is only a spring holding it, the electromagnet will cause the rod to move into the magnet to complete this shortest path as far as the magnetic field can pull it in.  This depends on the power of the electromagnet.

The electromagnet exerts a SUCKING IN FORCE and the core or rod moves in.  When you turn off the voltage source from the electromagnet, the coil returns to it's previous place because the spring will exert or pull it back.

All that relays are, are MAGNETIC SWITCHES or to be more exact, ELECTROMAGNETIC SWITCHES.  Relays are used in high power circuits where smaller conventional circuitry would be out of the question to use due to all kinds of heavy hazards.

Your washing machines uses relays to control the HOT and COLD water amongst other things and starters in cars also use a different type of relay.  Your bell door alarm, uses a relay to hit the bell.  The advantage of relays is that you can switch it ON and OFF from remote location via remote control or automatic programming and they are durable compared to TRANSISTORS which are very sensitive components and are not suitable for these types of jobs.

Well, now you know what a RELAY is, isn't that nice ? :- )  Relays can be connected in SERIES or in PARALLEL just like other components in electronics.  PARALLEL connection are called SHUNT or SHUNTS.

Relays are made to be either voltage or current sensitive, depending on how you are planning to use them in a circuit.  You must understand this difference.  And relays that are used in parallel will differ from those made in series.

Basically what I mean is that they are made out of different materials and are not the same.  Relays are also used as circuit brakers, you can compare them to a FUSE.  And there exist very small relay FUSE like components where many people confuse them with FUSES, but in fact, they are relays.  These are called REED relays.

Picture of a Reed Relay.

Well, that's it for relays, if you want to learn more about them, please read a book. Please keep in mind that these days relays come in all kinds of shapes and packages.  Many look like SOLID STATE Circuits and are designed to work with printed circuits, others are not.  There are heavy duty relays and light duty.  It all depends.  If you take a trip to Radio Shack and look at some of the electronic components, you will quickly have the opportunity to see what I am talking about.

Before I go I would also like to tell you to do some investigating on your own about FUSES and what they are, the different types and so on.  Also read about BUZZERS and CHOPPERS, and BI-METAL THERMOSTATS.

What is A/C ?

A big question, deserving a BIG, and LONG Answer and some more.

It doesn't take a genius to figure out what A/C stands for, it stands for Alternating Current.  Now everything in electronics, most of the time has a reason for a certain name, just like A/C does.

From these two words, the key words here are, Alternating and Current :)

Let's begin.  Another term that you will quickly get to know today is FREQUENCY.  It is very important in Electronics :)

Ok, A/C Electricity or it's Current that flows through the wire is an alternating current and it alternates at 60Hz per second.

First some basics.  One hertz or (Hz) same thing is one cycle per second of something of repetitive nature, and that is why it is called FREQUENCY.  Like, how many times something is happening in how many cycles per second or time.  For example, your car Engine cycles in RPM's, that is Revolutions per Minute.  That is too FREQUENCY, it is just a different type of.

Before 1967 we did not use the term Hertz or (Hz) for FREQUENCY, we simply said, (cps) cycles per second.    But then in 1967 in order to honor the memory of HEINRICH HERTZ, the German scientist who first discovered radiating waves, we switched from (cps) to (HERTZ) or (Hz).

Again:

One (Hz) = one cycle per second, not minute!

KILO stands for thousand and MEGA stands for million, GIGA stands for billion and so on.

One (KHz) = one thousand cycles per second

One (MKz) = one million cycles per second

One (GHz) = one billion cycles per second

By LAW, yes it is true, A/C in America flows at 60Hz per second, that is, 60 cycles per second and the way it works is like this.  Imagine a line, mark it 0 value, everything above it is + (plus) and everything below is - (minus).

The A/C current flows in one direction for a while and then flows in the opposite direction for a while, very different then D/C (Direct Current) which flows in the same direction all the time.

The VOLTAGE across this A/C line Goes from 0 to it's maximum value on the positive side, back to 0 and then to it's maximum value on the negative side and thus resulting in an Alternating current.  Why this happens, is because of the way that this electricity is Generated and most of the time it is generated via. BIG GENERATORS, this is a new story in it's self.  Basically you use Magnetism to produce electricity and because as the Generator turns around, it changes in polarity as it passes the North and South POLES.  This Of course is the easy explanation, and right about now I am wondering, if maybe I should have covered GENERATORS first ? :) But because so many people have been begging for A/C here it is.

So anyways, this 0 to +, back to 0 and to -, is happening 60 times per second at 115VOLTS Fixed for normal A/C Values.  If you ever look at some GENERATORS or other equipment that is fed off of A/C Lines it will say 60Hz, 115VOLTS A/C be written right on the part.

The major different between A/C and D/C is that D/C while it can change in MAGNITUDE, it can not like it's Friend A/C change in direction.

Magnitude is the term we use for the value of current.  Like how strong, the bigger the Magnitude, the bigger the Value or MORE VOLTS.  But to make it easy, a generator can produce electricity if the conductor is moved through a magnetic field, this will produce a voltage.  Either the conductor or the magnetic field can be moved, it will result in the same effect.

If you hold a magnet in a fixed position and then move it up or down the coil, this will produce a current, or voltage.  First in one direction and then in the other.  That is why it alternates and thus why it is called Alternating Current.;

In Europe A/C is more then a mere 115 VOLTS, I think it is 230 VOLTS.  How do they produce more volts ?  Simple.  If you move the GENERATORS faster then 60cps or 60 Hz, the induced force on the electrons within the conductor will also be greater and a greater voltage will develop.  In Europe, they simply move the generators fasterrrrrr.

Also keep in mind that GENERATORS only convert Mechanical energy into Electrical Energy and not the other way around.  Later I will teach you about this as well.  You will know what a SIN WAVE is, and how a simple Alternator works.

I will also introduce you to some new formulas and laws, you will learn about such terms as PEAK VALUE, PEAK TO PEAK VALUE, WAVELENGTH.

To end this short introduction into A/C, know this too.  An A/C Wave, is made up on positive and negative ALTERNATIONS.  The average value of the wave is considered the average value of ONE ALTERNATION.

This value is considered at 63.7% percent of the peek value which I will teach you about later.  A/C is a complex subject, because it involves the time to learn many other subjects that are A/C related, Like Generators, which is a Subject all in it's own.

To confuse you further there also exist D/C Generators, wana learn more ? come back a later date and I will explain.  I will also explain to you about a very neato type of Tool called an Oscilloscope that is used to measure DC and AC Waveforms among other things.

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