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# ideal meters problem. what internal resistance is ideal for a voltmeter? what internal resistance is ideal for an ammeter?

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### James

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get ideal meters problem. what internal resistance is ideal for a voltmeter? what internal resistance is ideal for an ammeter? from EN Bilgi.

## Ammeter Impact on Measured Circuit

Read about Ammeter Impact on Measured Circuit (DC Metering Circuits) in our free Electronics Textbook

# Ammeter Impact on Measured Circuit

## Chapter 8 - DC Metering Circuits

Just like voltmeters, ammeters tend to influence the amount of current in the circuits they’re connected to. However, unlike the ideal voltmeter, the ideal ammeter has zero internal resistance, so as to drop as little voltage as possible as current flows through it.

Note that this ideal resistance value is exactly opposite as that of a voltmeter. With voltmeters, we want as little current to be drawn as possible from the circuit under test. With ammeters, we want as little voltage to be dropped as possible while conducting current.

Here is an extreme example of an ammeter’s affect upon a circuit: With the ammeter disconnected from this circuit, the current through the 3 Ω resistor would be 666.7 mA, and the current through the 1.5 Ω resistor would be 1.33 amps. If the ammeter had an internal resistance of 1/2 Ω, and it were inserted into one of the branches of this circuit, though, its resistance would seriously affect the measured branch current: Having effectively increased the left branch resistance from 3 Ω to 3.5 Ω, the ammeter will read 571.43 mA instead of 666.7 mA. Placing the same ammeter in the right branch would affect the current to an even greater extent: Now the right branch current is 1 amp instead of 1.333 amps, due to the increase in resistance created by the addition of the ammeter into the current path.

When using standard ammeters that connect in series with the circuit being measured, it might not be practical or possible to redesign the meter for a lower input (lead-to-lead) resistance. However, if we were selecting a value of shunt resistor to place in the circuit for a current measurement based on voltage drop, and we had our choice of a wide range of resistances, it would be best to choose the lowest practical resistance for the application. Any more resistance than necessary and the shunt may impact the circuit adversely by adding excessive resistance in the current path.

One ingenious way to reduce the impact that a current-measuring device has on a circuit is to use the circuit wire as part of the ammeter movement itself. All current-carrying wires produce a magnetic field, the strength of which is in direct proportion to the strength of the current. By building an instrument that measures the strength of that magnetic field, a no-contact ammeter can be produced. Such a meter is able to measure the current through a conductor without even having to make physical contact with the circuit, much less break continuity or insert additional resistance. ### Clamp-on Ammeters

Ammeters of this design are called “clamp-on” meters because they have “jaws” which can be opened and then secured around a circuit wire. Clamp-on ammeters make for quick and safe current measurements, especially on high-power industrial circuits. Because the circuit under test has had no additional resistance inserted into it by a clamp-on meter, there is no error induced in taking a current measurement. The actual movement mechanism of a clamp-on ammeter is much the same as for an iron-vane instrument, except that there is no internal wire coil to generate the magnetic field. More modern designs of clamp-on ammeters utilize a small magnetic field detector device called a Hall-effect sensor to accurately determine field strength.

Some clamp-on meters contain electronic amplifier circuitry to generate a small voltage proportional to the current in the wire between the jaws, that small voltage connected to a voltmeter for convenient readout by a technician. Thus, a clamp-on unit can be an accessory device to a voltmeter, for current measurement.

### Magnetic-field-sensing Ammeter

A less accurate type of magnetic-field-sensing ammeter than the clamp-on style is shown in the following photograph: The operating principle for this ammeter is identical to the clamp-on style of meter: the circular magnetic field surrounding a current-carrying conductor deflects the meter’s needle, producing an indication on the scale. Note how there are two current scales on this particular meter: +/- 75 amps and +/- 400 amps.

These two measurement scales correspond to the two sets of notches on the back of the meter. Depending on which set of notches the current-carrying conductor is laid in, a given strength of magnetic field will have a different amount of effect on the needle. In effect, the two different positions of the conductor relative to the movement act as two different range resistors in a direct-connection style of ammeter.

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Answer to Solved 3.) Ideal Meters Problem. What internal resistance is

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## Why is the resistance of an ideal ammeter zero and that of an ideal voltmeter infinity?

Answer (1 of 13): A current measuring device should be connected in series with the circuit. As the resistance of the current measuring device will add to the resistance of the circuit, the current measured will be less than the actual current. Thus, an ideal current measuring device should have ... A current measuring device should be connected in series with the circuit. As the resistance of the current measuring device will add to the resistance of the circuit, the current measured will be less than the actual current. Thus, an ideal current measuring device should have zero or negligible resistance.

A voltage measuring device is connected in parallel to the circuit. If the resistance of the voltage measuring device is small, a large amount of current will be diverted from the circuit, to flow through the voltage measuring device. Thus the potential difference measured will be small. Th

A current measuring device should be connected in series with the circuit. As the resistance of the current measuring device will add to the resistance of the circuit, the current measured will be less than the actual current. Thus, an ideal current measuring device should have zero or negligible resistance.

A voltage measuring device is connected in parallel to the circuit. If the resistance of the voltage measuring device is small, a large amount of current will be diverted from the circuit, to flow through the voltage measuring device. Thus the potential difference measured will be small. Thus, a voltage measuring device should have an infinite resistance so that the current through the circuit does not get deflected to pass through the voltage measuring device. This would mean that the potential difference measured by the voltmeter or, the voltage measuring device will be the same as the actual potential difference, because the current through the circuit element across which the voltage measuring device has been connected, does not change or , decrease. Because you don't want to change the system you are testing. A non zero resistance ammeter has a voltage drop across it, and that adds another resistive element to your circuit you have to take into account. And it may fundamentally change the behavior of active parts of it. For instance it can change the gain of a transistor circuit, which itself may cascade to other circuits downstream. The same with a voltmetet. A non infinity input means it draws current from your circuit, with possibly unforeseen effect or, again, circuit changes you have to take into consideration. The simple answer is that a zero-resistance ammeter and a infinite-resistance voltmeters would have no loading effect on the circuit they are connected to (assuming you’re connecting either meter correctly, that is). In other words you want it so you can add and remove them from an idealized test circuit and nothing will change either way.

An ammeter is connected in series with the circuit/component you are testing, so basically you want it’s electrical characteristics to be equivalent to the wires connecting the circuit together. A series circuit with a component that has zero resistance might

The simple answer is that a zero-resistance ammeter and a infinite-resistance voltmeters would have no loading effect on the circuit they are connected to (assuming you’re connecting either meter correctly, that is). In other words you want it so you can add and remove them from an idealized test circuit and nothing will change either way.

An ammeter is connected in series with the circuit/component you are testing, so basically you want it’s electrical characteristics to be equivalent to the wires connecting the circuit together. A series circuit with a component that has zero resistance might as well have a wire in the place of that component instead.

A voltmeter is connected in parallel the circuit/component that you’re testing, meaning that an ideal voltmeter would be equivalent to an open branch since that means there’s no leakage current. Electrically, an infinite resistance branch on a parallel circuit might as well not be there.

Both of these principles can be mathematically proven using Kirchoff’s circuit laws and Ohm’s law, with the Kirchoff’s Voltage Law (KVL) applying to the ammeter’s case and the Kirchoff’s Current Law (KCL) applying to the voltmeter’s case. I can’t be bothered to write that all out right now, though, so I hope my simple answer will do. In both cases, it is because the meter would change the behaviour of the circuit and this give a false reading.

An ammeter with non-zero resistance reduces the current in the circuit.

A voltmeter with finite resistance allows a finite amount of current to bypass the the circuit.

Of course, these are ideals. Real ammeters and voltmeters do alter the circuits they are connected to. You are being taught about this to make you aware that it is happening. Resistance of an ideal ammeter is zero because ammeter is connected in series to measure the current to ensure that there must not be any volatage drop because of the internal resistance of ammeter it is ideally taken as zero.

While voltmeter is connected in parallel with the circuit to ensure that no current flows through the voltmeter its internal resistance is taken as infinite. If the Ideal Ammeter resistance is not nearly zero when it is connected in series to measure the current in the circuit/load it would result in drop of voltage in Ammeter which would alter the current in the circuit which we intended to measure. Similarly if voltmeter which is connected to measure voltage across an element/ branch has less resistance it would draw current again it would alter circuit current and there by resulting changes in voltages across the elements. In such cases it would result in erroneous results. In an ideal ammeter having zero resistance because of if having high resistance load current will not flow so internal resistance is low.

In an ideal voltmeter having infinity because of current will not effect instrument. The resistance of a voltmeter must be infinity as voltmeter is connected in parallel in electric circuit so a minimum amount of current must flow through it. while in case of ammeter, its resistance must be zero because it is connected in series in electric circuit. if a ammeter will have higher resistance than it will dissipate some current. so as to ensure that it will not dissipate current, it is having high resistance. while in case of voltmeter it must have infinity resistance.

iideal voltmeter has high resistance almost equivalent to infinity so that there is no flow of current through the voltmeter causing voltage drop otherwise the reading will get wrong.

iideal ameter are having almost 1ohm resistance so that there is no obstruction to flow of current takes place when current flow through ameter otherwise again the reading will get wrong. While measuring the voltage or current in an electrical circuit, it is ideal that power loss in measuring instrument should zero. when ammeter is connected in series in circuit so that load current passes through it, power loss is given by i^2 x Resistance of ammeter, so power loss will be zero if resistance of ammeter is zero.

Similarly voltage measurement is carried by applying the load voltage across the voltmeter and power loss in voltmeter is given by V^2/Resistance of voltmeter, Resistan...

The dream of an ammeter or voltmeter is to connect it to a circuit without changing or disrupting any of the variables from what they were before the introduction of the meters. Since a voltmeter is connected in parallel to the load, if its resistance is anything less than infinite, it will add a path of conduction, thus indicating a higher total current than if its resistance were infinite. Similarly, since the ammeter is connected in series with some or all of the load, if it has any resistance, that will reduce the current in the circuit to a level lower than were it not there. So, ideally,

The dream of an ammeter or voltmeter is to connect it to a circuit without changing or disrupting any of the variables from what they were before the introduction of the meters. Since a voltmeter is connected in parallel to the load, if its resistance is anything less than infinite, it will add a path of conduction, thus indicating a higher total current than if its resistance were infinite. Similarly, since the ammeter is connected in series with some or all of the load, if it has any resistance, that will reduce the current in the circuit to a level lower than were it not there. So, ideally, the only way that introducing either meter will allow us to know the correct values of voltage and current, the voltmeter needs to be of infinite resistance and the ammeter of zero resistance. Because a voltmeter should not draw any operating current, yet should be very accurate.

Because an ammeter should draw full current while showing the same. Current is the flow of electrons from one point to another point. So if you have any type of resistance in between those two points, the electrons face obstruction and thus lose energy in the form of heat, leading to a drop in the current. So, an ideal ammeter has zero resistance as it is the ideal condition for current to flow with loss and measurement will be accurate.

Voltage on the other hand is merely the potential difference between two points. If there is less than infinite resistance, there is always a flow of current from the higher potential to the lower potential. So if you have less

Current is the flow of electrons from one point to another point. So if you have any type of resistance in between those two points, the electrons face obstruction and thus lose energy in the form of heat, leading to a drop in the current. So, an ideal ammeter has zero resistance as it is the ideal condition for current to flow with loss and measurement will be accurate.

Voltage on the other hand is merely the potential difference between two points. If there is less than infinite resistance, there is always a flow of current from the higher potential to the lower potential. So if you have less than infinite resistance in a voltmeter, you risk a reduction in the measured voltage due to the flow of current through the reduced resistance. A theoretically “ideal” ammeter would be able to get a true reading yet have zero resistance, so it would not affect the current it was trying to measure. An ideal voltmeter would have infinite resistance, so it would not lower the voltage it was trying to measure, yet be able to get a reading.

Neither is possible in practice. As we know that we have to connect voltmeter in parallel to any element to know the voltage across it

So while connecting voltmeter to any circuit we are indirectly providing another path for current to flow from,which in turn change the actual current flowing from desired path and this creates error in measuring of voltage

But if we connect the voltmeter with infinite resistance (ideally),no current can flow from the voltmeter and there will be zero error in the measured voltage.

The aim of voltmeter is to measure voltage and to do so, we have to connect it in parallel.

So, if we connect a voltmeter across a load (let say resistance 100 ohms) to measure it accurately we need all the current (say 10A) to pass through it so we can get (1000 V).

Hence in an ideal voltmeter , internal resistance is infinite so that current chooses the least resistive path and give accurate voltage.

Similarly in an ideal current source , to measure current we have to connect it in series so that all current passes through it. So the internal resistance of ideal current source should be zero.

A typical automotive ammeter has a solid copper bar with a moving magnet next to it. It would have less resistance than the wiring connected to it. There are high current ammeters for alternator checking that are just placed next to the wire, and measure the magnetic field. Another type of DC ammeter would use a clamp-on magnetic core with a Hall element. This would not introduce any resistance to the wire. A clamp-on AC-only ammeter has a magnetic core and a transformer coil. This also would not introduce any resistance, and only a tiny amount of inductance to the wire.

An ammeter with 0 resistance because then it would give the exact reading of the current without any decrease in its amount…… although 0 resistance is a theoritical concept for an ammeter (don't think of superconductors here)  The internal resistance of an ideal voltmeter is infinity and the internal resistance of an ideal ammeter is zero. Ammeter is connected in series and voltmeter is connected in parallel with the electric appliance.

Ideal voltmeter is a theoretical concept.

Yes, practical ammeters do have a resistance.

Traditional ammeters measure the voltage drop on a shunt resistor. The maximum voltage drop in each range corresponds to the full scale voltage of the voltmeter. Considering a cheap multimeter, the voltage drop is around 0.2 V, and the ranges are 20 uA, 200 uA, 2 mA, 20 mA, 200 mA, 2 A, 20 A. The shunt resistors are 10 kohm, 1 kohm, 100 ohm, 10 ohm, 1 ohm, 100 mohm, 10 mohm, respectively. In the low ranges the resistance is quite significant! If your device has a high inrush current at power-on, it might cease to start with the ammeter in series. On

Yes, practical ammeters do have a resistance.

Traditional ammeters measure the voltage drop on a shunt resistor. The maximum voltage drop in each range corresponds to the full scale voltage of the voltmeter. Considering a cheap multimeter, the voltage drop is around 0.2 V, and the ranges are 20 uA, 200 uA, 2 mA, 20 mA, 200 mA, 2 A, 20 A. The shunt resistors are 10 kohm, 1 kohm, 100 ohm, 10 ohm, 1 ohm, 100 mohm, 10 mohm, respectively. In the low ranges the resistance is quite significant! If your device has a high inrush current at power-on, it might cease to start with the ammeter in series. On the other hand, there is high dissipation when measuring high currents, like 4 W with the above multimeter. That limits the maximum time for the measurement, as overheating can be an issue.

It’s possible to build ammeters with very low internal resistance. For alternating current, a Current transformer can be used to transform between the primary and secondary currents and resistances. The magnetic core and the secondary often just surrounds a piece of wire which is there anyway, nothing has to be inserted into the current path. A device suitable for ad hoc measurements is the Rogowski coil.

For DC measurements similar Current clamp devices can be used. A Hall effect sensor is placed into the magnetic circuit, and current in the secondary winding of the current transformer is regulated to counteract the field of the primary current, resulting in zero total field. Primary current is proportional to secondary current.

Why is zero the ideal ammeter resistance?

Zero is the ideal resistance of an ammeter because all the measured current and adding additional resistance would affect the amperage of the measured amperage. It cannot actually be zero resistance, because the ammeter needs to have some small amount of voltage drop to drive the galvanometer to provide the reading. The ideal resistance for the ammeter is that resistance which provides the galvanometer full current when connected in parallel with the low resistance shunt. Theoretically an ideal ammeter has zero resistance, so that if you were to connect a voltage source directly to it, the only thing that would limit the current through the ammeter is the internal resistance of the source. Since most ammeter movements are not designed to handle much current, connecting a significantly large, low-impedance voltage source across the ammeter will likely instantly damage the ammeter movement, or (hopefully) blow the protective fuse, if one is provided.

In general, an ammeter movement is typically much more sensitive than the current it is designed to measure accurat

Theoretically an ideal ammeter has zero resistance, so that if you were to connect a voltage source directly to it, the only thing that would limit the current through the ammeter is the internal resistance of the source. Since most ammeter movements are not designed to handle much current, connecting a significantly large, low-impedance voltage source across the ammeter will likely instantly damage the ammeter movement, or (hopefully) blow the protective fuse, if one is provided.

In general, an ammeter movement is typically much more sensitive than the current it is designed to measure accurately. For example, an ammeter designed to measure 10 amperes full-scale could utilize a movement with a sensitivity of 100 mA. In order for the ammeter to correctly measure higher currents, the manufacturer installs what’s known as a “shunt” resistance in parallel with the ammeter movement, forming a current divider that is sized so that when the movement is carrying 100 mA (causing full-scale deflection of the meter), the shunt is carrying 9.9 A, so that the total current through the meter + shunt is 9.9 + 0.1 = 10A. The meter is calibrated so that its reading takes the shut resistance into account as it measures the current flowing through the movement alone.

As others have written, an ideal ammeter has zero resistance and it is true that we often make a compromise and allow a negligibly small resistance so the flowing current produces a measurable voltage we then use to read the current flowing. However, one can build ammeters which demand no extra resistance added in the current path. Current flowing through a wire produces a magnetic field with strength directly proportional to the current. Measure the field and you have a way to measure the current. The company Hewlett Packard sold such a meter already back in the days where vacuum tubes were s

As others have written, an ideal ammeter has zero resistance and it is true that we often make a compromise and allow a negligibly small resistance so the flowing current produces a measurable voltage we then use to read the current flowing. However, one can build ammeters which demand no extra resistance added in the current path. Current flowing through a wire produces a magnetic field with strength directly proportional to the current. Measure the field and you have a way to measure the current. The company Hewlett Packard sold such a meter already back in the days where vacuum tubes were still common and it was able to measure down into the range of a milliampere. There is another virtue here besides no extra resistance; the user does not have to interrupt the flowing current to insert an ammeter. The measuring head can be just clamped around the wire and there is essentially total electrical isolation between the measuring circuit and the circuit being measured. The HP instrument uses the flux gate principle but the are devices using Hall effect sensors.

The above solutions can measure DC. For AC currents the are simpler methods working basically like transformers. Many have a construction so they can be simply clamped around the wire. These can be relatively inexpensive and when working with high power wiring carrying rather high (dangerous!) it is a really nice method. Source : www.quora.com

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James 9 month ago

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