Worksheet: Design of the Ammeter

In this worksheet, we will practice describing the combining of a galvanometer with a shunt resistor to design a DC ammeter.

Q1:

A galvanometer has a resistance of 12 mΩ. A current of 150 mA produces full-scale deflection of the galvanometer. A shunt is connected in parallel with the galvanometer to convert it into an ammeter. The resistance of the shunt is 70 µΩ. What is the greatest current that the ammeter can measure? Answer to one decimal place.

Q2:

Which of the following is the most correct description of how the range of values of currents that a galvanometer can produce is extended when it is converted into an ammeter using a shunt resistor?

  • AA shunt resistor with a resistance much greater than that of the galvanometer is connected in parallel with the galvanometer.
  • BA shunt resistor with a resistance much smaller than that of the galvanometer is connected in parallel with the galvanometer.
  • CA shunt resistor with a resistance equal to that of the galvanometer is connected in parallel with the galvanometer.
  • DA shunt resistor with a resistance much greater than that of the galvanometer is connected in series with the galvanometer.
  • EA shunt resistor with a resistance much smaller than that of the galvanometer is connected in series with the galvanometer.

Q3:

The current 𝐼 in the circuit shown is 2.5 mA, which is the greatest current that can be measured using the circuit as an ammeter. The resistance of the galvanometer is ten times the resistance of the shunt.

Find 𝐼, the current through the galvanometer. Answer to the nearest microampere.

Find 𝐼, the current through the shunt. Answer to two decimal places.

Q4:

Which of the following circuit diagrams most correctly represents a galvanometer combined with a shunt resistor being used as an ammeter to measure the current through a circuit that has a direct-current source?

  • A
  • B
  • C

Q5:

The diagram shows a galvanometer that has two scales. One of the scales is a galvanometer scale and the other scale is a direct-current ammeter scale. When a current is measured, the galvanometer arm deflects so that it points at the position indicating the maximum magnitude of current on both scales. On the galvanometer scale, this value is 𝑋 μA and on the ammeter scale, this value is 𝑌 μA. What is the ratio of 𝑋 to 𝑌?

Q6:

An ammeter is used to measure the current drawn from a direct-current source with an emf of several volts. The ammeter is connected in series with a resistor that has a resistance of a few ohms. The galvanometer in the ammeter has a resistance of several milliohms and the shunt resistor in the ammeter has a resistance of several microhms. Which of the following correctly explains why the shunt resistor in such an ammeter must have a resistance much smaller than the resistance of the galvanometer that the shunt is connected in parallel to?

  • AIf the shunt resistor has a resistance of a magnitude comparable with or greater than the resistance of the galvanometer, enough of the current through the ammeter will pass through the galvanometer to make the current through the galvanometer greater than the current that would produce full-scale deflection of the galvanometer arm.
  • BIf the shunt resistor has a resistance of a magnitude comparable with or greater than the resistance of the galvanometer, the direction of the deflection of the galvanometer arm will reverse and no reading will be displayed on the ammeter.
  • CIf the shunt resistor has a resistance of a magnitude comparable with the resistance of the galvanometer, the current drawn from the source will be significantly reduced.
  • DIf the shunt resistor has a resistance of a magnitude comparable with or greater than the resistance of the galvanometer, the resistor will generate a magnetic field that significantly changes the deflection of the galvanometer arm.

Q7:

A galvanometer has a resistance of 15 mΩ. A current of 125 mA produces a full-scale deflection of the galvanometer. Find the resistance of a shunt that when connected in parallel with the galvanometer, allows it to be used as an ammeter that can measure a maximum current of 12 A. Answer to the nearest microohm.

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