Measurement of medium resistance, often referring to resistances in the range of several ohms to several kilohms, involves various techniques and instruments to accurately determine the resistance value. This type of measurement is commonly encountered in fields such as electronics, electrical engineering, and material science. There are several methods and instruments used for measuring medium resistance:
Ohmmeter: An ohmmeter is a simple instrument used to measure resistance. It typically consists of a battery, a galvanometer (current meter), and a variable resistor (potentiometer). The unknown resistance is connected in series with the potentiometer and the galvanometer. By adjusting the potentiometer until no current flows through the galvanometer, the resistance can be directly read from the potentiometer's scale.
Wheatstone Bridge: The Wheatstone bridge is a widely used method for accurately measuring resistances. It consists of four resistors arranged in a diamond shape. A voltage source is connected across one diagonal, and a galvanometer is connected across the other diagonal. The resistance to be measured is connected to one of the arms of the bridge. By adjusting the known resistors until the galvanometer reads zero, the resistance of the unknown resistor can be calculated using the bridge's balance equation.
Kelvin Bridge: The Kelvin bridge is an improved version of the Wheatstone bridge that eliminates the effects of lead resistance, which can introduce errors in resistance measurements. It is commonly used for accurate low-resistance measurements in the range of milliohms.
Four-Wire (4-Wire) Measurement: Also known as the Kelvin or Kelvin-Varley method, this technique involves using four leads to connect the resistance under measurement. Two leads carry the current through the resistor, and the other two leads are used to measure the voltage across the resistor. By separating the current-carrying leads from the voltage-measuring leads, the effects of lead resistance are minimized, enabling more accurate measurements.
Digital Multimeters (DMMs): Digital multimeters are versatile instruments that can measure various electrical quantities, including resistance. They typically use a low-voltage current source and measure the voltage drop across the resistance to calculate its value. DMMs are available with different accuracy levels and are suitable for a wide range of resistance measurements.
Bridge Circuits with AC Excitation: In some cases, medium resistance measurements might involve alternating current (AC) excitation. AC bridges, such as the Maxwell-Wien bridge or Anderson bridge, are used to measure resistances and impedances accurately at different frequencies.
Specialized Instruments: For more precise measurements in specific applications, specialized instruments like LCR meters (which measure inductance, capacitance, and resistance) or micro-ohmmeters (for very low resistance measurements) can be used.
When performing medium resistance measurements, factors like temperature, lead resistance, and the accuracy of the measurement instrument need to be considered to ensure accurate results. It's important to select the appropriate method and instrument based on the required accuracy and the resistance range you are working with.