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# How the resistance of an ammeter changed when introduced into a circuit.

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Introduction

DC MEASUREMENTS

The experiment was to find out how the resistance of both an ammeter and voltmeter, using both an ammeter and voltmeter changed when introduced into a circuit. From doing this it was found that the resistance of the ammeter decreases as both the emf and current decrease, however the avometer suggested that internal resistance increases with the voltage.

## Introduction

Voltmeter’s and ammeter’s use a moving coil galvanometer to take the current and a mechanical pointer to display the current. When using analogue components an important factor to consider is full-scale deflection, which is when the maximum deflection of an analogue instrument such as a moving coil meter is reached. At present the full-scale deflection of most instruments is 10-50 μA, but to overcome this shunting resistors are placed in parallel to carry the excess current.

Middle

E was increased to 1v, which was measured by the digital voltmeter. The current I was recorded and compared to the expected value of 100μA. Then using the following formula, , Ra was calculated, and then the experiment was repeated with E=0.1v, range 50μ A and 10v, range 1mA.

## Results

These results were taken using the avometer, and the intention was to find Rm, from the results.

 Range (V) Max E (V) E/2 Rm (Ω) 3.0 3.0 1.5 60x103 10 10 5 20x104 30 30 15 60x104

Conclusion

When using the digital voltmeter it has an error of ±1 digit. Whereas the avometer is a moving needle, which means the error is much greater as the needle can seem in two different positions from different angles.

## Conclusion

In conclusion it was found that the resistances for both the ammeter and voltmeter behave differently, with the ammeter decreasing as the range is increased and the voltmeter increasing with range. This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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