Why Wire Diameter Is Important

Here Size Does Matter 

A metal consists of a lattice of atoms, each with a shell of electrons. The outer electrons are free to dissociate from their parent atoms and travel through the lattice, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (a voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the influence of the electric field.

The larger the cross-sectional area of the conductor, the more electrons are available to carry the current, so the lower the resistance. The longer the conductor, the more scattering events occur in each electron's path through the material, so the higher the resistance. Different materials also affect the resistance.

Simply stated… as electrons move across a wire, they constantly collide with atoms making up a wire. These collisions impede the flow of electrons and are what cause the wire to have resistance. Thus, if the diameter of the wire were larger, it would only make sense that the electrons don't collide as much, therefore creating less resistance due to a larger wire. This is all in accordance to Ohm's law.

The resistance is the ratio of the voltage difference across an object to the current that passes through the object due to the existence of the voltage difference (Resistance = Voltage /  Current). If the object is made of a material that obeys Ohm's Law, then this ratio is constant no matter what the voltage difference is. 

Consider a copper wire that passes some amount of current, say 1 Ampere (A), when a voltage difference of 1 Volt (V) is applied between the ends of the wire. Now consider an identical but separate wire connected across that same 1V source. You would expect that it would also conduct 1A  (R=V/A   R=1/1   therefore R = 1 Ohm).

Now think of joining those two wires together side by side into one, thicker wire. Much like using a thicker pipe to increase the supply of water, it is reasonable to expect that this wire should carry 2 A of current if the potential difference across the wires is still 1 V. Thus, the new, thicker wire will have a reduced resistance of ½ Ohm compared to the original wire with its resistance of 1 Ohm. (R=V/A   R=1/2   therefore R = .5 Ohm).
Why? Basically, a thicker wire creates additional paths for current to flow through the wire. This reduces resistance which results in less generated heat. This is one of the reasons you should use heavy gauge wires when, for example, running a voltage supply to your 12 volt mobile radio. If you don’t, you could find your expected 12-13 volts DC is actually significantly lower. 

American Wire Gauge 

American wire gauge (AWG), also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire. The cross-sectional area of each gauge is an important factor for determining its current-carrying capacity.

The steel industry does not use AWG and prefers a number of other wire gauges. These include W&M Wire Gauge, US Steel Wire Gauge, and Music Wire Gauge.
Increasing gauge numbers give decreasing wire diameters, which is similar to many other non-metric gauging systems. This gauge system originated in the number of drawing operations used to produce a given gauge of wire. Very fine wire (for example, 30 gauge) required more passes through the drawing dies than did 0 gauge wire. Manufacturers of wire formerly had proprietary wire gauge systems; the development of standardized wire gauges rationalized selection of wire for a particular purpose.
The AWG tables are for a single, solid, round conductor. The AWG of a stranded wire is determined by the total cross-sectional area of the conductor, which determines its current-carrying capacity and electrical resistance. Because there are also small gaps between the strands, a stranded wire will always have a slightly larger overall diameter than a solid wire with the same AWG.

Stranded wires are specified with three numbers, the overall AWG size, the number of strands, and the AWG size of a strand. The number of strands and the AWG of a strand are separated by a slash. For example, a 22 AWG 7/30 stranded wire is a 22 AWG wire made from seven strands of 30 AWG wire.

AWG 18 has a solid diameter of about 1mm. Adding 6 halves the diameter, Subtracting 6 doubles the diameter. Adding 20 divides the diameter by 10, and subtracting 20 multiplies the diameter by 10. The following table lists the minimum recommended wire gauge for the length of supply cable in high power radio systems.

Recommended wire gauge for a given amperage and length

Conductivity of Common Metals

The 15 most common metals are listed at right, in order of their conductivity. But, you may say, gold is always touted as best for contacts! It’s true… while gold is not the best conductor, it does not corrode like some other metals and therefore provides more reliable contact over a longer period of time.  

Another surprise is that lead and tin, two of the most common elements in solder are relatively low on the conductivity list. The reason for using them is the fact that lead and tin are used for solder because not only do they have low melting points, but more importantly they form a “eutectic” alloy which has a considerably lower melting point than either one individually (many metals form eutectics), at the disadvantage of higher resistance.

Because its conductivity is the second highest of any metal and its cost is relatively low, copper sees use in most wire, connectors, printed circuit foils and related electrical parts. The resistance of a 24-gauge copper wire 1,000 feet long at room temperature will be about 26 ohms.
Silver's higher conductivity and cost make it a niche product. It's used as wire and solder in specialty electronics. By comparison, a silver 24-gauge, 1,000-foot-long wire would measure about 24 ohms.

Field Day and Special Events: Why ICS?

The Incident Command System 

ICS or the ‘Incident Command System’ is a management structure for planning significant events, both large and small, be they emergencies or not.  It was developed in California to manage large fire situations, but was quickly adopted by other organizations as an effective method to scale up—or scale down a response to a significant event. It is now used universally by emergency services for planning and responding to significant events. Its modular, meaning it can be scaled up, or down, to suit an event.

Amateur Radio Field Day is a significant event, requiring a multitude of people and resources to proceed efficiently. Most clubs, SARC included have a committee structure for their Field Day planning. ICS was first introduced in 2011 and helped to identify duties and responsibilities. In 2012 we hope to build on that experience and further develop our planning and execution so that the process in documented for future years.

Surrey Emergency Program Amateur Radio (SEPAR) provides ICS level 100 training. While we cannot hope to cover that material in a short article, here is a brief overview of ICS positions as they relate to Field Day.


Incident Commander—The Field Day Chair


Safety Officer Has the absolute final say on safety issues in and around the site. Also ensures the site has adequate security.


Information Officer Public Relations or Public Information Officer (PIO)
Media releases before
Hospitality / Public relations at event
Media releases after the event


Planning Section [Section Chief & Committee]
Situation Unit: Ensures FD rules and class are followed in planning the event.
Resources Unit (recruiting): Help other committees schedule people before FD. May utilize Talk-In (Logistics/Services/Comms) to recruit club members in real-time on Field Day.
Demobilization Unit (takedown planning): Planning who/how to take it all down, and (via Logistics) get it all, and everyone, back to where they belong.
Documentation: Preserve guest book, contest logs with Bonus Unit. May help plan logging process, process/submit logs afterwards.

Logistics Section

Service Branch
Communications Unit (this is where we, as Hams, usually fall within someone else's ICS operation). Provides talk-in and any other non-contest comms operations, whether on radio/cell/Internet, whether on ham, commercial or PS licensed, or FRS radios. Might include scoring the NTS points with/instead of Operations/Bonus Unit. Monitor the repeater, 146.52, and club simplex frequency for queries (and a couple of extra points), provide communications liaison with the Safety Officer, relay information for any supply needs. Also, set up wireless or wired internet access.
Food Unit Can break down into Planning & Shopping, Drinks, Snacks, Cooking, Lunch, Breakfast, Dinner and Dessert. Can leverage the Support Branch to find & move the stuff once a list & budget are prepared.

Support Branch
Supply Unit (truck-type transport; leg-work on purchasing for food and supplies; transporting borrowed equipment for other sections' unfilled requirements, e.g. antenna tuner, and return!)
Facilities Unit (arrangements & interface with the City; renting outhouses and other equipment; arranging borrowing of equipment based other sections' unfilled requirements, e.g. generator.)
General Ground Support General hands/labour details (in other words, everyone!) Antenna and tent raising, lugging.
Power Group runs the generators, charges batteries, manages available power. Once you're operational, it's the primary component of Support. 

Finance & Admin Section (Typically the organization's Treasurer)

Operations Section 
Deputy Operations Chiefs who lead while the Chief sleeps and take on the role of Station Manager. License trustee; ensures the operators are working the right band with the right equipment and antenna; ensures FD rules and class are followed. Also ensures the operator schedule is followed so everyone has an opportunity to participate.
Operations is divided into functional Branches as needed; with further geographic or functional Divisions or Groups, as needed. Most of our functional Divisions can naturally be called "Stations".

Contesting Branch: Each Station counted in our 3-Alpha Field Day class would be one "Division" or "Group" for ICS purposes, both for planning and operations. 
HF SSB Division Station (Station leader; shift operators, shift loggers)
HF CW Division (ditto)
VHF+ Division(s) (ditto)
Digital Division(s) (ditto)
Satellite Division (ditto)

Bonus Branch: The bonus points may cross into other ICS Divisions, but may be included in Bonus Branch
Handling NTS traffic (for points)
Any other special points available (satellite or digital)
Making Natural Power contacts (for points)
Demo points

It may sound like a lot of positions, but the advantage is that everyone in the structure knows who their team leader is and what their responsibilities are. Obviously there are more positions than people so, in an organization an individual may have more than one responsibility. In the end you should have a structured approach to Field Day with a written manual that others can follow.

For an on-line ICS-100 course, visit the FEMA website: https://emilms.fema.gov/IS100c/curriculum/1.html

Using Fuses Effectively

A Communicator Reprise: May 2012

In today’s electronics, fuses play a very important role that is frequently misunderstood, with the result that expensive equipment is often not fully protected, and this can result in expensive repair bills. The history of fuses is as old as the use of electricity and probably goes back to the time of the first short circuit! At first, fuses were simple open-wire affairs, but around 1890 Edison enclosed the wire into a lamp base to make the first enclosed fuse. By 1904 the Underwriters Laboratories had introduced specifications covering fuse size and ratings to meet the safety standards of the period. In 1927 Littelfuse started making the first of their range of low amperage fuses for the budding electronics industry. Since that time many new types of fuses have appeared on the market, some with very special characteristics for particular types of protection. Today the choice is extremely large and protection can be provided inexpensively and the risk of expensive repair bills reduced.

Purpose of a Fuse

A fuse is a device which is wired into an electrical circuit to prevent excessive current flowing when a fault occurs. On overload the wire forming the fuse element will heat up and melt (‘blow’) thereby breaking the circuit, interrupting current flow, preventing damage from excessive current to the remaining circuits. It is the electrical equivalent of a "safety valve".
The most important characteristic to the user is the current rating which, unfortunately, is often misunderstood. The current rating of a fuse is established by the manufacturer after a series of tests under controlled conditions. This enables the manufacturer to publish a set of specifications for its product which design engineers can use to decide which is the correct type of fuse for a particular circuit. In order to understand the current rating of a given fuse it is important to know the conditions under which this rating was achieved. There are three main groups of fuses:

1. slow-blow (or anti-surge),
2. normal quick acting, and
3. very fast acting

There is also a fourth type known as time delay fuses. 
Each of these types will protect a circuit from excessive continuous current, but act very differently under surge or short time conditions. The fitting of the wrong type could mean no protection at all is being provided, or fuses that keep "blowing" for no apparent reason.
Let’s take a detailed look at each type. The blowing time in seconds plotted against percentage overload for the three main types of fuse mentioned above is shown.


It can be seen that up to 100 per cent overload there is very little difference between the three types. But if we take a current overload of say 500 per cent we can see that the fast acting fuse blows in 0.001 seconds (a millisecond) and the slow-blow in about 2 seconds with the normal acting fuse at about 0.01 seconds. Quite a considerable difference between the three types. In fact the ratios (taking our normal acting fuse as the reference) work out at one tenth of the time for our fast acting fuse and 200 times longer for our slow-blow type! A very big difference indeed and more than enough to ‘release the smoke’ in expensive semiconductors under fault conditions.
Temperature also has an effect on the current rating. As the ambient temperature becomes lower the amount of current required to "blow" a fuse becomes higher and this can make a considerable difference to the blowing times under surge conditions.
Ah, you may be thinking, let's use a fast acting fuse all the time and be safe. Regretfully this is not practical as many circuits have a high surge current when first switching on, or switching to change operating conditions.

Construction

Let us now consider the construction of a typical cartridge fuse. First it has to have a body or barrel and this is normally made of glass or ceramic material. The barrel will have some form of termination at each end, usually brass or copper, which has been plated to prevent corrosion. The fuse element will be connected between the two end terminations and enclosed within the barrel. 
It will consist of a single wire in the case of a quick acting fuse, or may be one or more wires arranged in a specific way for delay and anti-surge types. Sometimes a filler is used to modify the action of the fuse and this may be sand or quartz powder. This filler will absorb the energy of the arc when the current is interrupted.
Fuses are of course marked in some way as to type and ratings, normally on one or both of the end caps and in addition there may be an indication of one of the many standards that the particular fuse complies with, e.g. BS, SEMKO, etc. The size of fuse may vary but there are a number of standard sizes and the most common are the  Standard metric which is 20mm long by 5mm diameter and the US standard of 1 x .25 inch diameter.  Many other sizes are available ranging from 5mm long to over 200mm. 
Particularly in automotive applications there are also ‘blade’ type fuses (see graphic below), and of course there are a number of other styles available for special purposes.

Fuse Characteristics

There are two main characteristics which will concern the Amateur and these are maximum continuous current rating and the surge rating of slow-blow types. The rupturing capacity of a fuse may also be important and for completeness is mentioned here. A high rupturing capacity fuse is capable of interrupting currents in the order of thousands of amperes. It would have a ceramic body and also contain an arc-quenching medium. Non-HRC fuses (more common in Amateur Radio equipment) do not have an arc-quenching medium and are only suitable for surge currents up to about 50 amps. With higher currents than this they would be very likely to explode when they blow.

Fusing Speed

As we have seen, a quick-acting fuse is designed to react both to short and long term overload conditions. They are very robust in construction and will withstand shocks and vibration. But they do tend to have a higher resistance and the voltage drop caused by this may be a problem in some applications. This higher resistance also means that more heat is produced and this must be effectively dissipated.
Time delay fuses will react to long term overload currents but will withstand transient surges without harm; several types are available. For example, one type has what looks like a spring inside the barrel and these will stand up to surges of around ten times the normal rating for 75 milliseconds. Another type has a "blob" in the middle of the fuse element and this type has a reduced surge capacity, typically ten times rated current but only for 25 milliseconds. Time delay types have a very low resistance and can be used in enclosed places as there is little self-generated heat but they are only available in the lower current ratings. Both the "spring" and the "blob" type are time delay fuses.
So far we have mentioned only the current rating of fuses, but they also have a maximum voltage rating. This voltage rating has no effect on the current rating but is important nevertheless. When a fuse "blows" an arc is developed between the two ends of the broken fuse element and, if the voltage across these ends is high enough, the arc will be maintained and the current will not be interrupted. This condition could result in considerable damage to the equipment. Arcs are readily produced in high voltage circuits or where inductive loads are being used and, in these conditions, the voltage rating of a fuse must not be exceeded. Fuses can be used at their current rating at all voltage levels up to their maximum. When it is known for certain that, although the circuit has a high voltage present, the power available is limited, it is possible to use a fuse at a higher voltage than that for which it is rated. This is common practice in domestic electronic equipment and quite safe. But. if in doubt, keep within the voltage ratings given by the manufacturers.

Mobile Fuse Applications

In one sentence: Place the fuse as close to the battery as possible in both the positive and negative line.

That’s good advice because if for any reason the positive wire’s insulation is damaged and the wire touches the chassis or engine (a hot manifold is a frequent cause of this problem) then it will blow the fuse if it is close to the battery but not if it is between the radio and the short.

It’s also good practice to place a fuse in the negative line.  Many hams think that is unnecessary because you will not cause a short if the negative wire touches the metal of the vehicle.  So why place a fuse in this line? In many installations the negative wire goes straight back to the negative terminal of the battery.  If the battery cable develops resistance between the cable and the body of the vehicle by rust or corrosion or the wire itself corrodes to the point that it is not a good conductor this type of installation can cause problems. When the engine is being started a lot of current is being drawn from the battery and the wiring to the mobile radio is not designed to handle any where that much current.  Simple Ohm’s law will tell you that the maximum current will flow through the path of least resistance and if that path happens to be through the negative wire of the radio to the negative terminal of the battery then that is the where the most current will flow. Frequently the unit is not grounded well at the mounting bracket but the shield side of the coax makes a good ground by the antenna mount.  In that case the current for the starter will attempt to flow through the coax shield to the coax connector on the radio then on to the negative wire to the battery. If that wire is not fused the coax shield will smoke. If the radio is grounded at the mount, the negative wire to the battery is not big enough to handle the load and it will smoke.  Either way there is a fire danger.

This type of hookup is not recommended.  If you do this and the fuse blows you may not know it is blown because the radio finds sufficient contact between the mounting bracket and/or the antenna ground to continue to operate.  The resistance between the battery and the vehicle chassis does not have to be high enough that the vehicle will not start to cause this phenomenon. The antenna ground and the mounting bracket are not designed to be the negative source for the DC power of the radio and it will cause more problems than you can imagine. Tracing the source of these problems can drive you crazy. The suggestion is therefore to run the ground wire to the chassis of the vehicle. Use an eye terminal with one outer locking lock washer between the head of the screw and the lug and another between the lug and the chassis.  Scrape the paint off the place where the lug will come in contact with the metal. Run the screw down tight but do not strip it out.

A Short Course On Baluns

A Communicator Reprise: April 2012

Before getting into this, it is necessary to describe a somewhat puzzling phenomenon (called "skin effect") associated with radio frequency currents.

SKIN EFFECT

High frequency AC currents in a conductor flow only in that portion of the conductor which is very close to the surface.  Very close being 0.1 mm (the "skin depth") or less.  While a wire has only one surface, the shield on a coax cable, being a cylinder, has two surfaces - an inner surface and an outer surface.  Consequently, you can have a high frequency AC current flowing on the inner surface and an entirely different one flowing on the outer surface, with neither of them affecting each other. It's like there's an insulator keeping them separated.
Weird but true.

BALUNS

Unless precautions are taken to minimize it, all coax fed antennas will have significant current flowing on the outside of the coax shield while transmitting.
If the antenna is a horizontal dipole with the coax hanging straight down then the coax acts as a vertical antenna.  This is because this  current flowing on the outside of the shield will radiate energy exactly as a vertical antenna would.  
For casual ham HF operation, this can be a good thing as radiation will take place over a wider range of elevation angles which in turn makes it possible to make contacts over a larger range of distances.  However, for a communications antenna system designed to have a specific radiation pattern, this unwanted radiation from the coax changes the pattern and reduces the effectiveness of the system.
When receiving, the coax still acts as a vertical antenna and couples whatever it picks up into the actual antenna and from there into the receiver.
An example of these effects on a Near Vertical Incidence Skywave (NVIS) antenna system follows:

TRANSMITTING

For NVIS the desired radiation elevation angles (the range in angles with respect to the horizon over which the antenna radiates most of the transmitted energy) is about 60 to 90 degrees, 90 degrees being straight up.  A horizontal dipole 1/4 wavelength or less above ground will provide this.

If this antenna is fed with coax and no precautions are taken to minimize current flowing on the outside of the shield, the shield will act as a vertical antenna with a range of radiation elevation angles between about 5 to 40 degrees.  The amount of power radiated at these low angles can be 50% of the power radiated at the desired elevation angles.  This means that less power is available to be transmitted at the desired high angles.  In addition, this power radiated at low angles can cause interference to other stations thousands of miles away.

RECEIVING

This is where the coax shield acting as an antenna really sucks.
The shield, acting as a vertical antenna, will pick up those previously mentioned stations several thousand miles away and feed them into the dipole which will then happily send those signals to the receiver where they will interfere with the signals you want to hear.  
In addition, most noise on the lower HF bands is caused by lightning in the tropics.  If your antenna responds only to high angle signals (NVIS) it won't respond to these lightning induced static crashes because they arrive at low angles.  Consequently, you won't hear them.  However, if the coax shield is acting as an antenna you will hear them and they may make it impossible for you to copy the signals you want to hear.
Because the coax comes right into your station, it runs close to all kinds of noise generating equipment such as computer monitors, plasma TVs, switch mode power supplies, etc.  Because the coax is acting as an antenna, it will pick up all kinds of garbage that the dipole won't hear because it's much further away from these noise sources.

BALUNS TO THE RESCUE

Well, not just any old balun. "Balun" is a contraction of the phrase "balanced to unbalanced". An ideal horizontal dipole is said to be "balanced" with respect to ground.  What this means is that:
The voltage between ground and the point where one wire of the feedline is connected to the antenna is exactly equal (but of opposite polarity) to the voltage between ground and the point where the other wire of the feedline is connected to the antenna.
The current flowing out of one wire of the feedline into the antenna is exactly equal to the current flowing into the other wire of the feedline out of the antenna.

This is an ideal which is never achieved in practice.  Things like one end of the antenna being lower than the other, closer to a tree or building, etc., will all cause the currents in the two legs to be different.  Consequently, no antenna is truly balanced.  Some come close, though.
Feedlines such as the old TV twinlead, window line and ladder ("open wire") line are all considered to be balanced as they are symmetrical with respect to ground (as long as one side of the feedline isn't closer than the other to ground, metallic objects, etc).  
Our transmitter outputs are "unbalanced" with respect to ground.  This is because the outer shell of the output coax connector is connected directly to ground through the metal case of the radio. Similarly, the coax cable we connect to the transmitter is unbalanced as the shield is connected directly to ground via the coax connector on the radio.
So now we have a balanced antenna to which we want to connect an unbalanced coaxial cable. Physically, this is easy to do, we just connect them.  But, as soon as we do, we destroy the balance of the antenna and we get current flowing on the outside of the coax shield with all the resulting bad effects.

So, what we need is a box between the antenna and the coax to make the transition between the balanced antenna and the unbalanced coax feed line, and, yes, these boxes exist and they are called baluns.

There are two types of baluns:

Voltage Baluns

These force the voltage between ground and the point where one wire of the feedline is connected to the antenna to be exactly equal (but of opposite polarity) to the voltage between ground and the point where the other wire of the feedline is connected to the antenna.  The idea here is that, if the voltages are equal, the currents will be too.

However, as most supposedly balanced antennas are at least somewhat unbalanced, the currents in the two legs will not be the same.  This causes a current equal to the difference between them to flow on the outside of the coax shield.  i.e. the coax shield now acts as an antenna.  Just what we DON'T WANT.

Current Baluns (also known as "Current Chokes")

These force the currents in the two legs (halves) of the dipole to be equal.  As there is now no difference in these current values, no current flows on the outside of the coax shield.  This is just what we want, so these are the ones to use.

SUMMARY

Every coax fed antenna system designed to have specific characteristics needs a current balun located at the transition between balanced and unbalanced.  These are often built into VHF and UHF antennas (skirts, sleeves, etc.) but are generally separate items in HF systems.

Without a current balun the antenna is strongly coupled to the outside of the coax shield. 

This means that energy transmitted by the antenna will be coupled into the coax shield which will reradiate it and thereby affect the radiation pattern of the antenna system.

It also means that signals picked up by the coax shield acting as an antenna will be coupled into the antenna and passed to the receiver.

It is important to note that, like any other conductor, the coax shield will pick up signals and re-radiate them.  The current balun simply ensures that these re-radiated signals don't get coupled into the antenna and so don't reach the receiver.

APPENDIX

SKIN EFFECT DETAILS

If you pass a DC current down a wire, the current flows uniformly down the wire.  i.e. the amount of current flowing down the centre of the wire is the same as the amount of current flowing near the outer edges of the wire.

This is not true for AC currents.  For a current with a frequency of 1 MHz in a copper wire, the current will be concentrated in a region with a depth of less than about 0.1 mm from the perimeter of the wire.  This 0.1 mm is called the "skin depth".  The skin depth gets smaller the higher the frequency.  It doesn't matter if the wire is 1 cm or even 1 metre in diameter, only this 0.1 mm thick "skin" has significant current flowing in it.

See http://en.wikipedia.org/wiki/Skin_depth but skip the eye-glazing math and check out the chart at the end.

As mentioned earlier, the shield of a coax cable isn't a wire with one surface - it is a cylinder with two surfaces, an inner and an outer.  This means that, provided its thickness is more than a few skin depths at the frequency of interest, you can have a current flowing on the inside of the shield and an entirely different current flowing on the outside!  This is because neither current penetrates far enough into the shield material to affect the other current.  It's just as if there were an insulator keeping them separated.

The point to take away from all this is that, counter-intuitive as it may be, you can have two distinct AC currents flowing in a coax shield, one on the inner surface of the shield and one on the outer.

~Jim Smith VE7FO

Using Multi-Meters

A Communicator Reprise: March 2012

Using Multimeters—Part 2

Last month we looked at multi-meter types. Multi-meters have the ability to measure voltage, current and resistance and more expensive models may add other functions such as temperature. A basic addition to your household tool kit, there are low cost multi-meters available, frequently on sale for less than $10. I’d suggest purchasing a digital rather than analog model as a digital meter is easier to use and will suffice for basic measurements.

Digital models are generally "auto-ranging", a useful feature because you don't need to change the dial to measure different levels. If you think you might be using it in low light, consider getting one with a "back-light."
The first rule for getting the most out of your multi-meter is to read the manual. The manual will have instructions for basic operation of the instrument and safety information about potential dangers. Once you have read the manual, added the batteries, and attached the probes (the wires, which are usually red and black), try some of the example measurements below.

Basic Multi-meter Tests

Resistance Test

Set the multi-meter to read "resistance." Check that the two probes are inserted in the right holes.

What does the readout say when the probes are not touching anything? When the two probes are separated, there is an infinite resistance separating them, since air does not conduct electricity. Make a mental note of your multi-meter's readout for infinite resistance, because it varies with the manufacturer.
Touch the two probes together. Now what does the readout say? When you touch the two probes together, the resistance is close to zero. The metal tips are excellent conductors and the wires offer little resistance to current flow.

Try this…  I first did this as a science fair project. Set the knob to the highest Resistance scale on the meter Dampen two fingers and press one probe to each fingertip. Do you get a reading? With dry fingers you probably won’t get a reading. Dry skin has a resistance of about 1 million ohms, whereas the resistance of moist skin is reduced by a factor of ten or more.
Try it with different liquids including salt water. Did the resistance change? What you are seeing is a Polygraph (lie detector) in its simplest form. As the subject is stressed from telling an untruth, the body produces perspiration which changes the skin’s resistance [scientifically known as Galvanic Skin Response]. Also measured in a professional instrument are blood pressure, pulse and respiration. Once calibrated, a polygraph and trained operator can record and interpret the readings to determine when the subject is truthful or not. For more experimentation check Google or have a look here for a basic kit.

Measure the resistance of some resistors that are not attached to a circuit. For example, test resistors of 100 Ω (ohms), 10,000 Ω, and 1 MΩ (mega-ohm, or 1 million ohms). You can buy these online or at a local supplier. Touch the probes to the wires on either side of the central cylinder. Watch the units: a "k" means kilo-ohms (thousands of ohms), and an "M" means mega-ohms. 
Never measure resistance in a circuit when power is applied. You must also discharge capacitors in a circuit before measuring resistance, because if there is any source of current other than the multi-meter itself, you will get erroneous readings.

Voltage Test

Touch the probes to the terminal ends of a 9-Volt battery [see photo]. You should get a reading of approximately 9 Volts. This one reads 7.57 volts so is obviously spent. The battery has a positive [red] and a negative [black] pole. Note that your multi-meter also has a positive and a negative probe. If you attach the positive probe to the negative side of the battery, it will still read voltage, but it will have a negative sign in front of it.

Current Test

This is probably the trickiest reading to make and one that can damage the meter should you pass too much current through it. Set the multimeter to read "direct current (DC)." Important: Check your multimeter to see where the probe should be plugged in so it reads "current."

Don’t be shy to experiment with your multi-meter.
You will likely find a multitude of uses for it around the house.

In order to measure current, you have to open up the circuit and attach the leads from the multimeter so that the current flows through the multimeter in series. To do this, use jumper wires and wires with alligator clips to add the multimeter to the circuit, as shown.
The current now flows from the 12 volt source through the switch, resistor and the multi-meter because it is part of the circuit.  Because it is connected in series, the meter can be inserted at any point in the circuit shown and show the same reading.