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.
The full scale current for the galvanometer is 50uA
with internal resistance 2k ohm.
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:
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.
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)
The meter will measure the voltage across resistor R when it is connected.
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.
Learn mode: ( default )
When the meter is connected to the test circuit,
the needle of the meter will be shown.
The needle will not be shown, and you will have 3 chances
to enter the correct value into the textField.
Your suggestions are highly appreciated!
Please click email@example.com for your feedback!
Author¡GFu-Kwun Hwang, Dept. of physics, National Taiwan Normal UniversityLast modified : More physics related java applets