Ammeters and Voltmeters – Electrical measuring devices EN
AMMETERS AND VOLTMETERS – ELECTRICAL MEASURING DEVICES
Electrical measuring devices – Ammeters and Voltmeters
Ammeters
An ammeter is an instrument that measures the intensity of an electric current. Some ammeters
They also indicate , in addition to the intensity of the current, its direction, which, when the indication is positive, flows in a clockwise direction and negative, in an anticlockwise direction.
As the ammeter indicates the current that passes through it in the section of the circuit where it is inserted, its internal resistance must be zero , otherwise it would indicate a current of lower intensity than that which actually passes through the section.
So it must behave like an ideal wire, with zero internal resistance , that is, it must behave as if it were in a short circuit.
An ideal ammeter should have zero internal resistance.
Voltmeter
Voltmeter is an instrument that measures potential difference (ddp), voltage or voltage.
So that the current passing through the device whose ddp you want to measure does not deviate to the voltmeter, an ideal voltmeter must have an extremely high internal resistance , tending to infinity .
An ideal voltmeter should have infinite internal resistance.
Suppose you want to measure the current passing through point B and the potential difference between points C and D in the figure, having a voltmeter and an ammeter, both ideal.
To do this, you must open the circuit at B and insert the ammeter there, as it must be in series with the section traveled by i B , so that i B passes through it.
The voltmeter terminals must be connected to points C and D so that the voltmeter is in parallel with the section between C and D, where you want to measure the ddp.
Note that the internal resistance of the ideal ammeter must be zero so that all i B passes through it and that the internal resistance of the voltmeter must be infinite so that i CD does not deviate into it.
What you should know, information and tips
Ammeter instrument that measures electric current intensity. 
The ammeter must be connected in series in the section where you want to measure the electric current.
An ideal ammeter should have zero internal resistance.
Voltmeter instrument that measures potential difference, voltage or voltage.
The voltmeter must be connected in parallel with the device or section whose potential difference or voltage you want to measure.
An ideal voltmeter should have infinite internal resistance.
Method for solving circuits with ideal ammeters and voltmeters.
Example that should be analyzed carefully:
In the circuit below, determine the readings of the supposedly ideal ammeters and voltmeters.
Steps:
1 a Check that the devices are connected correctly, that is, the ammeters connected in series and the voltmeters in parallel.
Yes, they are.
2. Since the devices are ideal, they do not affect the circuit , so you must remove the ammeters, placing ideal wires in their place , and remove the voltmeters, re-schematicing the circuit. And solving according to the sequence below:
Coming back:
Look carefully at the figure below and see where the ammeters and voltmeters are inserted, whose indications will be:
If you want to master the subject, you need to solve many exercises, but I have separated some that can help you and check carefully the commented resolutions . They are: 06, 10, 14, 24, 27 and 35.