Characteristics and capabilities of printers.
Printers available today are usually either laser printers using electrophotographic technology or inkjet printers using electrostatic spray technology. Dot matrix printers using impact technology are used in applications that require carbon copies.  The list of printer selection criteria.

Capacity and Speed Printer capacity and speed are factors to consider when selecting a printer. Inkjet printers are usually slower, but they might be adequate for a home or small office. The speed of a printer is measured in pages per minute (ppm). The speed of an inkjet printer is 2–6 ppm. The speed of a laser printer is 8–200 ppm.

Color or Black and White A computer monitor produces colors through the additive mixing of dots that are displayed on the screen. The eye picks up the colors directly. The dots produce the color range using red, green, and blue (RGB) dots.
A printer produces colors using subtractive mixing. The eye sees a color that reflects from the combination of colors on the paper. Figure 2 shows a CMYK color wheel.
The choice between a black-and-white printer and a color printer depends on the needs of your customer. If your customer is primarily printing letters and does not need color capability, a black-and-white printer is sufficient. However, an elementary school teacher might need a color printer to add excitement to lessons.

Quality The quality of printing is measured in dots per inch (dpi). The more dpi, the higher the resolution. When the resolution is higher, text and images are usually clearer. To produce the best high-resolution images, you should use both high-quality ink or toner and high-quality paper.

Reliability A printer should be reliable. Because there are so many types of printers on the market, research the specifications of several printers before selecting one. Here are some of the options available from the manufacturer:
            .Warranty – Identify what is covered within the warranty.

             .Scheduled servicing – Servicing is based on expected usage. Information is  found in   the manual     or on the manufacturer's website.

            .Mean time between failures (MTBF) – There is an average length of time that the printer works without failing. Information is found in the manual or on the manufacturer's website.

              .Total Cost of Ownership Consider the cost when selecting hardware. When buying a printer, there is more than just the initial cost of the printer to consider. The total cost of ownership (TCO) includes a number of factors:
Initial purchase price
Cost of supplies, such as paper and ink
Price per page
Maintenance costs
Warranty costs
When calculating the TCO, you should also consider the amount of material printed and the expected lifetime of the printer.

Reset bios password hp compaq 6820.

Ok, as long as the high security level has not been set in the bios then the following will work to reset / disable the bios password.
1. Remove the battery and mains adapter.
2. Remove the memory cover on the back of the laptop and remove the 2 keyboard screws.
3. Open the laptop and at the top of the keyboard you will see 4 sliding tabs. They are located between the 'Esc & F1 keys' 'F4 & F5 keys' 'F8 & F9 keys' and 'F12 & delete keys'. Slide these down and lift the keyboard up and pull forward slightly so as to disengauge the bottom lugs. Be carefull as there is a ribbon cable at the bottom that does not need to be tugged or removed.
4. You will now see at the bottom behind the keyboard a small battery connected to the motherboard. Gently pull the small white plug upwards untill it is removed from the board.
5. Wait 5 minutes and re-connect the small battery plug.
6. Re-install the keyboard and push back the clips. You do not need to replace the screws yet.
7. Apply the DC jack and power on - DO NOT INSTALL THE MAIN BATTERY.
8. Turn the laptop on while pressing f10 repeatedly but slowly - Do not hold it.
9. If high security is disabled you should now be in the setup screen, select 'Restore Defaults' and press F10 to save.
10. The password is now removed.

If this does not work then HP say that there is no other way to reset it. Im not sure about that, there probably is by some method of de-soldering the bios chip. However the above method worked for me so I hope it does for you. Please rate if you have any luck.

Electrical principles - Measurement
Meters

There is a range of meters used for measuring various electrical quantities. These meters are often expensive and delicate pieces of equipment, so they should always be handled with care and used according to the manufacturer's instructions.

Meters are available with two different forms of readout:
analogue – readout on a graduated scale with a pointer operated by a magnetic field
digital – readout on a digital screen.

Analogue display

An analogue meter must be positioned correctly. If placed on its side or an angle, the pointer could be affected by gravity and give an incorrect reading.

When viewing the scale, position yourself directly in front of the scale. If viewed from an angle, you will get an incorrect reading – the greater the angle the more inaccurate it will be. This is because the pointer is separated from the graduation scale by a short distance. This is called the parallax effect.

An analogue meter must be checked for zero before being used. There is usually a zero set screw adjustment to ensure that the pointer starts on the zero calibration before measurement is taken. Any time you change scales when measuring resistance, you must always set the zero again.

Always connect meters with the correct polarity – black to negative and red to positive. Failure to do this could damage the meter, particularly an analogue meter.

Digital meters are difficult to read in direct sunlight. Most have a battery to supply power. Make sure that the battery is not flat and that you have a spare available.

The four most common meters are:
voltmeter
ammeter
ohmmeter
multimeter.
Voltmeter

voltmeter
A voltmeter is used to determine the voltage (electrical pressure) present at any point in a circuit.
Connecting the voltmeter across the battery measures the voltage present in the battery.

Measuring battery voltage

Connecting one lead of the voltmeter to the input side of a globe and the other lead to earth results in a reading of the voltage present at the globe. Voltmeters are mostly used to measure voltages across components. They are connected in parallel with the component.

Measuring voltage at the globe

Ammeter
An ammeter measures the amount of current flowing through a circuit.
An ammeter must be connected in series with the circuit. An ammedter has a very low internal resistance and must never be connected directly to a power supply without being in series with a load. The ammeter used must be capable of carrying the current that you expect to read.
To connect an ammeter, the circuit needs to be disconnected (broken) and the ammeter placed in series with the circuit.

Measuring current flow

Ohmmeter
An ohmmeter is used to measure the amount of resistance in a component or section of circuit.
An ohmmeter has its own power supply. The circuit being tested must always be de-energised (turned off) when using the ohmmeter.

Measuring resistance of a globe

When using an ohmmeter:
isolate the component or circuit section that requires measurement to ensure a correct reading occurs
avoid touching the metal probes of the ohmmeter as this may lead to a false reading.
Warning
Failure to disconnect or isolate the component being measured from the power supply will cause damage to the ohmmeter.
Multimeter
A multimeter, as its name suggests, has multiple uses. Some of the quantities it can measure include voltage, current and resistance. Multimeters are available with analogue or digital readouts. Digital multimeters are commonly used as they give accurate results and are easy to read, except in direct sunlight.

Multimeters

The two different multimeters shown can measure a range of electrical quantities including voltage (AC or DC), ohms, continuity, capacitance, frequency and current (AC or DC).
The rotary switch is used to select the quantity (and range) you wish to measure. The left side meter is auto ranging and the user needs only to select the type of quantity to be measured. For the right hand side meter, the user needs to select the electrical quantity as well as an appropriate range based on the value expected.  It is best to start with a high range and then progressively rotate the switch to a lower range.

The test leads of the multimeter need to be connected into the meter in the appropriate positions. The black lead connects into the COM (common) position and the red lead into either V, A or mA position, depending on whether you are measuring voltage (V), resistance (Ω) or current (A or mA).

When testing a circuit with a multimeter, you need to connect the leads to the circuit appropriately depending on whether you are measuring voltage (V), resistance (Ω) or, current (A or mA). The connections are the same as for the voltmeter, ammeter and ohmmeter described above.

Connection of camera and scanner.

Storage device

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