Ammeters measure current, voltmeters measure voltage, and ohmmeters measure resistance.

These devices are often combined into one instrument called a multimeter or VOM (volt-ohm-milliemmeter).
Analog versions of ammeters and voltmeters typically utilize a galvanometer, which relies on magnetic effect.
A needle is deflected by an amount proportional to the current passes through the device (coil).
A good ammeter should have a resistance that is small compared to other resistance in the circuit.
A good voltmeter should have a large resistance.
This java applet shows how to utilize a galvanometer into a multimeter.
[b]The full scale current for the galvanometer is 50uA with internal resistance R=2k ohm.[/b]

If galvanometer was connected in serial with a 198kohm resistor, it becomes a 10V voltage meter.
Because total resisitor become 200k, it need 10 V between two ends to produce 50 uA current.
And the current flow will be proportional to voltage across the device.
If a smaller resistor were connected in parallel to galvanometer, it can be used to detect more current than 50uA.
For example: if R/9 were connected in parallel to galvanometer, then the total current flow through galvanometer is 1/10 total cureent flow through both elements (the whole device).

The following applet shows the simplified version circuit diagram for a real multimeter.

You can operate at different mode:
1. select the Mode (Voltage/Current/Resistance) :
2. select the appropriate range for that mode.
3. You can change values of the DC voltage source V and resistor R:
You can also modify V/R by click the mouse button at the device.
Right/Left click mouse button once: to increase/decrease its value.
Right/Left click mouse button twice: to change the increment value.
If you press down the mouse for more than 1 second, it will change V's (or R's) value automatically.(every 0.5 sec.)
When you have the value you like , release the mouse button.
The unit for resistor is killo-ohm.
The Text Field shown can also be used to enter value for R, R2 or V,
depends on where you click the mouse button. ( "," input will be treated as "." e.g."2,3" will be read as "2.3" )
if you click the voltage source, then the value in TextField is new V.

4. Hit the OFF/ON toggle button to connect/disconnect the meter to the test circuit.
If the meter were overloaded, the fuse (green line in white background) will break.
Please turn off connection and change range.  (It will turn off connection automatically in 8 sec.)

Change the range to a better one and click the button again.
The effective (simplified version) circuit diagram is shown at the lower right edge.
The internal resistance of the meter is also there.

1. Mode: Voltage VR to measure voltage across an element (placed in parallel with that element)
There are two resistors R and R2 connect in series. (R2 can only be modified by mouse click)
The meter will measure the voltage across resistor R when it is connected.

2. Mode: Current IR to measure current flows in a circuit. (placed in series with the circuit).
It will measure the current in the test circuit when connected.

3. Mode: Resistor R to measure the resistance for a unknown resistor.
The meter provides the DC voltage source (3V+9V battery) needed for the measurement.
The galvanometer first forms a voltmeter (Vo=3V or 12V), Unknown resistor is in series with a known resistor and then connected to the voltage source.
From the voltage measurement, we will know the value for the unknown resistor
V= R_known/(R_known + R_unknown) *Vo
This is not a linear equation for R_unknown,  so the scale in the meter is not linear.
% shown in the scale represents infinity.

4. The Rm checkbox is checked as default.
If unchecked, the meter has infinite internal resistor (which is not real).
This mode is useful for teaching the student when they first learn the meter. (make it easier).
It should be checked in order to simulate a real multimeter.
(when the internal impedance of the multimeter can not be ignored).
Try it and find out the difference.

**** When operated in resistance mode:
The red (+) connector has a lower voltage related to the black(-) connector. Do you know why?

5. Learn / Exercise Mode :
Cilck the mouse at the center of the meter  (within the white circle area)
will toggle between exercise and learn mode,

Learn mode: ( default )
When the meter is connected to the test circuit, the needle of the meter will be shown.
Exercise mode:
The needle will not be shown, and you will have 3 chances to enter the correct value into the textField.