TEST OF PRACTICAL KNOWLEDGE QUESTION You are provided with two resistance wires labeled and B, a1\(\Omega\) standard resistor Rx, and other necessary appara...

Answer Details

Principle of the metre bridge. The bridge wire NQ is a uniform wire of length 100 cm. When the balance (null) point P is found, no current flows through the galvanometer and the bridge is balanced. If \(l_x = NP\) and \(l_y = PQ\), then for the standard resistor \(R_x\) in the left gap and the unknown in the right gap,

Thus each computed value \(R_1 = \dfrac{l_y}{l_x}R_x\) is the resistance of the length of wire in the right gap for that trial.

Method summary. For wire A the balance point is taken for lengths L = 100, 95, 85, 75 and 65 cm and \(R_1\) evaluated each time; the procedure is repeated for wire B to obtain the corresponding values (here labelled \(R_2\)). A graph of \(R_2\) (vertical) against \(R_1\) (horizontal) is a straight line whose slope s compares the two wires, and

Two precautions.

• The jockey was tapped gently (not dragged) on the bridge wire to avoid scraping and to protect the wire.
• Tight, clean connections were made and the balance point was kept near the middle of the wire for greatest sensitivity; readings were taken with the eye vertically above the wire to avoid parallax.

(b)(i) Two advantages of a potentiometer over a voltmeter.

• At balance the potentiometer draws no current from the cell, so it measures the true e.m.f. (no loading/"volt-drop" error), whereas a voltmeter always draws some current and reads slightly low.
• It can be made very sensitive and gives readings of high accuracy, and it has effectively infinite input resistance at balance.

(b)(ii) Internal resistance of a cell. The internal resistance of a cell is the opposition offered by the materials (electrolyte and electrodes) inside the cell to the flow of current through the cell itself.