METHODS FOR RESISTANCE MEASUREMENT

Measurement of resistance starts with most basic Ohm’s Law (V=IR), and various method are designed for refinements for accuracy. Low resistance values and also high values pose hurdles in accurate measurements. Accordingly, low resistance, medium resistance and high resistance values are measured different techniques.

A] Medium Resistance Measurement:

Medium resistances (1 Ω to 100 kΩ) values can be measured with following methods.

1) Ammeter-Voltmeter Method:

This uses basic Ohm’s Law to calculate resistance. It measures voltage across resistance terminals when a certain current is passes through it or measures current when certain voltage is applied across it. Precision and accuracy depend on voltmeter and ammeter.

2) Substitution Method

This method involves replacing the unknown resistor with a known standard resistor to get same voltage and current.

With switch in position 1, current is adjusted to a suitable value and noted. Switch is then changed to position 2, and standard calibrated variable resistance is adjusted to ger same current. Value of this resistance is same as unknown resistance. 

 3) Wheatstone Bridge

This is ratio arm bridge. Uses a balancing circuit with four legs, having two fixed resistors, one known or calibrated variable resistance, and the unknown resistance under measurement. A galvanometer carries unbalanced current, which becomes zero at equilibrium or balanced condition. Resistance value is calculated from known ratios of legs. Current levels are low, and the bridge offers high accuracy. Principle and calculation for unknown resistance are given in the figure below. R₁, R₂ are fixed, while R₃ is adjustable resistance.

Accuracy of Wheatstone bridge depends on components.   General purpose bridge gives an accuracy of ±0.05% to ±1%, laboratory grade are accurate to ±0.1%, and precision bridges with standard resistors are accurate to ±0.001%.

B] Low Resistance Measurement

Low resistances face challenges in measuring accurately due to contact and lead resistances. Normal Wheatstone bridge is not useful for low resistances because lead resistance becomes comparable to the unknown resistance, contact resistance introduces large percentage errors and further, temperature effects become significant.
Common methods for low resistance measurement include:

a. Kelvin’s Bridge

Kelvin’s Bridge is a modification of Wheatstone bridge designed specifically for low‑resistance measurement (typically 0.001 Ω to 1 Ω). The meter uses 4-terminal method by providing separate leads for current and potential leads- this eliminates error due to leads. Accuracy varies between 0,1% to 1%, depending on lead resistances, contact resistances etc.

Low contact resistance is critical for efficient current flow and to prevent overheating or energy loss. These meters are widely used in electrical testing, quality control, maintenance. The circuit is a four terminal arrangement to offset the effect of lead wires. 

b. Kelvin’s Double bridge

Kelvin’s Double Bridge adds a second set of ratio arms for much higher accuracy where contact and lead resistances are significant. It is suitable for precise measurements for very low resistances from 0.1μΩ to 1Ω with an accuracy ± 0.05% to ±0.2%. The double bridge compensates for imperfections in main ratio arms. This makes it a choice for standard laboratory method for extremely low resistances

C) High Resistance Measurement

High resistances (>100 kΩ) require methods that minimize leakage currents and electrostatic effects.

a) Megger (Insulation Tester)

Applies a high voltage (500 V / 1000 V) to measure insulation resistance accurately. The source could be a battery or DC voltage from a hand cranking handle arrangement.

b) Direct Deflection Method

 Uses sensitive galvanometers to detect small currents through high-resistance materials. An Aryton shunt works as safety device to eliminate any chance of having the galvanometer in circuit without a shunt. This advantage is obtained at the cost of slightly higher overall resistance.

c) Megohm Bridge

Megohm bridge is a circuit designed for precise measurement of very high resistances. This is a modified version of Wheatstone bridge, with guard terminals added to eliminate effect of stray resistances. The ratio arm P is often provided with steps to select standard resistance and give different ranges of measurement, e.g. from 0.1 MΩ to 1000,000 MΩ. Accuracy of bridge can be from 1% to 10% (above 10,000 MΩ).

Guard, or shield (sheath) is generally provided to eliminate the effect of stray resistance / impedance on measurement. Million megohm meters are available to measure high resistances of up to millions of Megohms. There is often a provision to charge a capacitor to preset voltage to get dielectric insulation resistance of capacitor.

d) Loss of Charge Method

This method measures the rate at which a charged capacitor discharges through the resistor and is suitable for extremely high resistances. Loss of charge method measures very high resistances (typically above 100 kΩ) by charging a known capacitor to voltage V, allowing it to discharge through the unknown resistance.   Voltage drop over time is to used compute R using standard discharge characteristics curve for capacitor. It is possible to note time to drop voltage across resistance by 63.2%, representing RC time constant, and calculate capacitance from known value of C.

D) Multimeter

i) Multimeters are handy tools for most common measurements. They measure resistance.based on Ohm’s Law. A small known current is passed through the resistance, and voltage across it gives an indication of its value, and is calibrated accordingly.

ii) Advanced multimeters use a known resistance in series with unknown resistance. When voltage is applied, ratio of voltage on known and unknown resistance can be used to get the vakue of unknown resistance. The ratio metric method is highly precise because it relies on stability of a physical internal reference resistor- so it is free from any error due to battery voltage.opr test current.