Q5Electronic Measurement & Instrumentation

Low resistances ($<1\Omega$) require specialized methods to avoid errors from contact and lead resistances. The various methods are: 1. Kelvin's Double Bridge Method (most precise). 2. Potentiometer Method. 3. Voltmeter-Ammeter Method (for rough estimation).

The Voltmeter-Ammeter method uses Ohm's Law ($R=V/I$). However, it suffers from two major connection flaws when measuring very low resistances:

1. Ammeter connected outside: If the voltmeter is connected directly across the unknown low resistor, the ammeter measures the current flowing through the resistor plus the current flowing through the voltmeter. Because the unknown resistance is very low, the voltage drop is tiny, requiring a very sensitive voltmeter, which might draw a non-negligible current compared to the main current, skewing the $I$ reading.

2. Voltmeter connected outside: If the ammeter is connected directly in series with the resistor, the voltmeter measures the voltage drop across the resistor plus the voltage drop across the ammeter itself. Since the unknown resistance is very small, the internal resistance of the ammeter (and the contact leads) is comparable in magnitude. The voltmeter reading will be artificially high.

Furthermore, basic contact resistance at the terminals (which can be around $0.01\Omega$) is directly added to the measurement, causing catastrophic percentage errors when trying to measure a component that is itself only $0.01\Omega$.