Shortcut Methods

Wheatstone’s bridge:

• Shortcut:

$$ R_x = \frac{P}{Q}\times (R_1 + R_2)or \frac{R_2}{R_1}×R_x=P/Q$$

Where P and Q are the lengths of the two arms of the bridge on either side of the galvanometer.

• Tricks:

To remember the formula, use the mnemonic: “Right over left equals Wheatstone’s cleft” Where “Right” refers to Rx, “left” refers to R1, and “cleft” refers to the fraction P/Q.

Meter bridge:

• Shortcut:

$$R="/L×R$$ Where R is the unknown resistance, R is the known resistance, L is the length of the wire on the side of a known resistor, and L’ is the length on the side of an unknown resistor.

• Tricks:

To remember the formula, use the mnemonic: “Resistance equals Resistor times Length”

Potentiometer:

• Shortcut: $$V_x=E(l_2/L)$$ Where Vx is the potential difference across the unknown resistance, E is the total potential difference or cell potential, l2 is the length of the potentiometer wire between jockey and zero, and L is the total length of the potentiometer wire.

• Tricks:

To remember the formula, use the mnemonic: “Voltage_x_equals_E_times(length_2_divided_by_L)”