21.5 Null Measurements

21.5 Null Measurements

• The circuit being measured is altered by standard measurements.
• Ammeters reduce current flow.

• The use of a null measurement is generally more accurate but it is also more complex and has limits to their precision.
• In this module, we will look at a few types of null measurements that are common and interesting.

• Do you want to measure the emf of the battery?
• The terminal voltage is related to the emf of the battery and the current that flows and is the internal resistance of the battery.

• The emf is usually not calculated accurately.
• There is another technique that needs to be used since the standard voltmeters need a current to operate.

• A terminal voltage that is different from the emf of the battery is measured by an analog voltmeter attached to it.
• It is not possible to calculate the emf precisely because the internal resistance of the battery is not known.

• A long wire is connected to a voltage source that passes a constant current through it.
• A variable potential can be obtained by making contact with different locations along the wire.

• Figure 21.35(b) shows an unknown connected in series with a galvanometer.
• The other voltage source is opposed to that note.
• The location of the contact point is adjusted until the galvanometer reads zero.
• Where is the resistance of the wire up to the contact point when the galvanometer reads zero?
• Since no current flows through the galvanometer, no current flows through the unknown emf and so so on.

• The contact point is adjusted until the galvanometer reads zero again.
• The current through the long wire is the same as it is through the galvanometer.

• Since no current flows through the galvanometer, the segment of wire has a resistance and script.
• The wire segment's resistance is proportional to the unknown emf.

• The ratio of resistances is the same as the ratio of lengths of wire that do not have a galvanometer.
• The quantities on the right-hand side of the equation can now be calculated.
• The uncertainty in this calculation is not zero, but it can be considerably smaller.
• There is always uncertainty in the standard ratio of resistances.
• It is not possible to tell when the galvanometer reads zero, which introduces error into both and may affect the current.

• The most common way for an ohmmeter to do its job is to apply a voltage to a resistance, measure the current, and calculate the resistance using Ohm's law.
• This is the calculated resistance.
• The meters alter both the voltage applied to the resistor and the current that flows through it, so the configurations are limited in accuracy.

• There are two methods for measuring resistance.

• The galvanometer is called a bridge because it forms a bridge between two branches.

• It is possible to read the value precisely.
• The resistance is adjusted until the galvanometer reads zero.
• The potential difference between points b and d is zero.
• The galvanometer has no effect on the rest of the circuit.
• Each branch has the full power of the source, and they are in parallel.
• The drops along abc and adc are the same.
• Since b and d are at the same potential, the drop along ad must be equal.

• Since b and d are at the same potential, the drop along dc must match the drop along bc.

• The Wheatstone bridge is used to calculate resistances.
• The resistance is adjusted until the galvanometer reads zero.
• The circuit can be calculated based on the drops in the text.

• The unknown resistance is calculated using this equation.
• The method can be very accurate, but it is limited by two factors.
• It is not possible to get the current through the galvanometer to be zero.

• Other factors might affect the accuracy of null measurements.

• Resistance in the wires and connections is a factor.
• They can change over time and are impossible to make zero.
• The temperature variations in resistance can be reduced but not completely eliminated by choice of material.
• Digital devices sensitive to smaller currents are more accurate than analog devices because they allow you to get the current closer to zero.