The difference between the external connection and the internal connection of the ammeter, how to di

First of all, whether the ammeter is connected internally or externally, it will cause error, the first error of this error is that the voltmeter will also have current flowing, and the ammeter will also divide the voltage, so from the actual situation, it is necessary to use the measurement method with the smallest error, but it is necessary to know the ammeter resistance value Ra and the voltmeter resistance RV in advance

The resistance range of the resistance to be measured is known.

Suppose that the resistance value of the resistor to be measured is about r, if r (ra*rv) then it is connected externally, and it is connected inversely (abbreviated as large inside and small outside).

The resistance range of the resistance to be measured is unknown.

If the value of r is not known, it is necessary to test the internal and external connection twice to obtain the internal voltage and current values u1 and i1; The external voltage and current values are u2 and i2. Compare the magnitude of the absolute value of the error if |u1-u2|/u1≤|i1-i2|i1 then it is connected externally, and it is connected internally (abbreviated as Anming outside, Fuming inside.) Clearly, the changes are obvious).

When the internal resistance of the ammeter is much smaller than the resistance to be measured, the internal connection method is adopted.

In this case, the measured data is on the large side.

Because the ammeter and the resistor under test are connected in series, the voltmeter measures the voltage of the voltmeter and the resistor under test, resulting in inaccurate voltage.

When the memory of the voltmeter is much larger than the resistance to be measured, the external connection method is used.

In this case, the measured data is on the small side.

Because the ammeter is connected in series with the voltmeter and the resistance to be measured, the ammeter measures the current of the voltmeter and the resistance to be measured, resulting in the inaccuracy of the measured current.

According to the specific accuracy requirements to decide, when the current accuracy is required, the external connection is selected.

External method: when the internal resistance of the voltmeter is much greater than the resistance value of the resistance to be measured, the external method is used, and the current flowing through the voltmeter is very small and negligible, so the current measured by the ammeter is basically the current flowing through the resistance to be measured.

Internal method: when the internal resistance of the ammeter is much less than the resistance value of the resistance to be measured, the internal method is used, and the voltage obtained by the ammeter is very small and negligible, so the voltage measured by the voltmeter is basically the voltage at both ends of the electrical resistor.

Both methods are designed to measure more accurately, and once you have mastered the principle, you will be able to use it easily.

There is a permanent magnet inside the ammeter, which generates a magnetic field between the poles, there is a coil in the magnetic field, there is a hairspring spring at each end of the coil, the spring is connected to a binding post of the ammeter, between the spring and the coil is connected by a rotating shaft, and there is a pointer at the front end of the rotating shaft relative to the ammeter.

When there is a current passing through, the current passes through the magnetic field along the spring and the rotating shaft, and the current cuts the magnetic inductance line, so it is affected by the magnetic field force, which makes the coil deflect, and drives the rotating shaft and pointer to deflect.

When r is a large resistance, the internal method is used, and when r is a small resistance, the internal method is used. The rv r to r ra ratio is used to determine which method to use.

When the size of the resistance is not known, the internal and external connection methods can be determined by the test contact method. In the circuit shown in the figure below, vacate a terminal of the voltmeter, and use the terminal to try to touch M and N respectively, and observe the change of the indication of the two meters, if it is <>

It shows that the influence of the internal resistance of the voltmeter is large, that is, the difference between the resistance and the resistance of the voltmeter is small, which belongs to the "large resistance", and the internal method should be used, if it is <>

It shows that the influence of the internal resistance of the ammeter is large, that is, the difference between the resistance and the resistance of the ammeter is small, and it belongs to the "small resistance" and should be connected externally. <>

1) When the ammeter is connected externally, due to the shunt effect of the voltmeter, the current value measured through the resistance is greater than the true value, and the resistance value measured according to the experiment is less than the real value.

When the ammeter is connected internally, due to the voltage division effect of the ammeter, the voltage value at both ends of the measured resistance is greater than the real value, and the resistance value measured according to the experiment is greater than the real value.

Why is this error occurring? In the final analysis, it all stems from the fact that voltmeters and ammeters are essentially resistors.

So, how do we choose the connection method of the ammeter in the experiment? It should be the one that has the smallest error. Which connection method has less error?

2) When externally connected, when the shunt effect of the voltmeter is very small, the error is small. At this time, rv (voltmeter resistance) >> rx (resistance to be measured) should be satisfiedIn this case, Rx can be considered as a small resistor.

When it is connected internally, when the voltage division effect of the ammeter is very small, the error is small. At this time, the resistance to be measured rx>>ra (ammeter resistance) should be satisfiedIn this case, Rx can be considered as a small resistor.

It can be summed up in one sentence: small resistance is external, and large resistance is internal. (Remember the big inner master, the big resistance internal connection).

It's OK to remember this sentence, it's not that difficult to ask for this aspect now.

Maybe you have this question, so what can be considered a large resistance and what can be considered a small resistance?

Theoretically deduced conclusions:

1) When rx> (rvra) (this is the meaning of the open root number), large resistance, internal connection;

2) When rx

Measure small resistance with an external connection.

For large resistances, internal connections are used.

Because the voltmeter is connected in parallel, it has a shunt effect.

And the ammeter series has the role of voltage division.

So. When it is a small resistor.

The external connection method can make the current obtained by the voltmeter very small.

The impact on the experiment is also very small.

Equally. When the resistance is large.

With an inline. The voltage distributed by the ammeter is very small.

The error is also very small.

As for what is a big resistance and what is a small resistance.

It should be determined by the ratio of resistance to the internal resistance of the ammeter.

One for ridicule, nine to distinguish by about 10 at the end of the day.

The difference between the internal method of ammeter and the external Fengying method: internal and external connection are relative to the ammeter, the ammeter is externally connected outside the voltmeter, the ammeter is internally connected in the voltmeter, the measurement of small resistance is external, the measured value is smaller than the actual value, and the internal connection is used to measure the large resistance, and the measured value is larger than the actual value.

Ammeter internal connection method: The voltmeter measures the sum of the voltage of the resistance and the electric start-up current meter, which is larger. The measured current is accurate to the voltage.

Since the ammeter is connected in series with the resistance, the measured current is the current through the resistor, and the measured current is accurate, but the measured voltage is the voltage at both ends of the resistance and the ammeter in series, and the measured voltage is relatively wrong for measuring the voltage at both ends of the resistor.

When the measured resistance value is very large or the internal resistance of the ammeter is very quiet relative to the resistance value, the voltage error measured by this connection method is relatively small. Otherwise, the error will be large.

External ammeter method: The ammeter measures the sum of the current of the voltmeter and the resistance, which is larger. The measured voltage is more accurate than the current.

Because the voltmeter is connected in parallel with the resistor, the measured voltage is the voltage at both ends of the resistor, and the measured voltage is accurate, but the measured current is the current that is connected in parallel through the resistance and the voltmeter.

When the resistance value to be measured is very small or the internal resistance of the voltmeter is large relative to the resistance value, the flow error measured by this connection method is relatively small. Otherwise, the error will be large.

When the resistance value of the resistance to be measured is comparable with the resistance value of the ammeter, the external connection method is used;

When the resistance value of the resistor to be measured is much greater than that of the ammeter, the internal connection method is used;

The measured resistance r is relatively large, so the ammeter internal connection method is used; r is relatively small, then the ammeter external method is used.

There are two basic circuits for measuring resistance by voltammetry: the internal side of the ammeter and the external connection of the ammeter.

Let the resistance to be measured be r, the voltage on it is ur, and the current passing through it is ir; The voltmeter resistor is RV, the voltage on it is UV, and the current passing through it is IV; The ammeter resistor is RA, the voltage on it is UA, and the current passing through it is IA

In the internal circuit of the ammeter, UA UR UA, IA IR, the first measurement of the matching error is that the voltmeter reading is greater than the voltage at both ends of R, and the resistance calculated by R uv ia is larger than the true value. When R>>Ra, U>>UA, i.e., U UV, does not cause much error in the measurement results.

In the galvanometer external circuit, UV UR, IA IR IVThe ** of the measurement error is that the ammeter reading IA is greater than the current IR passing through R, so the resistance value calculated by R UV IA is smaller than the true value. When Rv >> R, IA>>IV, i.e., IA IR, does not cause much error in the measurement results.

In the experiment, if you do not know the resistance range of the resistance to be measured, you can first connect the ammeter and the resistance to be measured in series to the circuit, turn on the power supply, and write down the ammeter reading. Then connect the voltmeter to both ends of the resistance to be measured, if the ammeter reading changes greatly at this time, the ammeter internal connection method is adopted; If the ammeter reading changes very little, an external method is used.

There are two basic circuits for other common voltammetry resistors: internal ammeter and external ammeter.

Let the resistance to be measured be r, the voltage on it is ur, and the current passing through it is ir; The voltmeter resistor is RV, the voltage on it is UV, and the current passing through it is IV; The ammeter resistor is RA, the voltage on it is UA, and the current passing through it is IA

In the internal circuit of the ammeter, UA UR UA, IA IR, the measurement error is that the voltmeter reading is greater than the voltage at both ends of R, and the resistance calculated by R uv ia is larger than the true value. When R>>Ra, U>>UA, i.e., U UV, does not cause much error in the measurement results.

In the galvanometer external circuit, UV UR, IA IR IVThe ** of the measurement error is that the ammeter reading IA is greater than the current IR passing through R, so the resistance value calculated by R UV IA is smaller than the true value. When Rv >> R, IA>>IV, i.e., IA IR, does not cause much error in the measurement results.

In the experiment, if you do not know the resistance range of the resistance to be measured, you can first connect the ammeter and the resistance to be measured in series to the circuit, turn on the power supply, and write down the ammeter reading. Then connect the voltmeter to both ends of the resistance to be measured, if the ammeter reading changes greatly at this time, the ammeter internal connection method is adopted; If the ammeter reading changes very little, an external method is used.