# The Story of 4-Wire Measurement

GENEVA, OH, February 25, 2021

It is common knowledge in industry that 4-wire measurement is the preferred method for resistances under 1 Ω but what makes it so advantageous over 2- and 3- wire measurement? Let’s look at how the technique was invented and why it works.

## Building Bridges

William Tomson, later Lord Kelvin of temperature scale fame, was studying Wheatstone bridge errors in the mid-1800s (See Figure 1). His concern was that the ratio arms (Ra, Rb) of the bridges contributed more to measurement error as the value of the measured resistance became lower (Rx). Figure 1: Wheatstone Bridge

His solution to this problem became known as the Kelvin Double Bridge (See Figure 2). By integrating a second pair of ratio arms (Ra’ and Rb’) he divided the voltage drop proportionately so that Rx/Rs = Ra/Rb = Ra’/Rb’. Figure 2: Kelvin Double Bridge

Now that the bridge is mathematically balanced, the resistance of all arms of the bridge have been canceled out. This is the basic principle behind 4-wire measurement, or Kelvin sensing, used in equipment from handheld devices to laboratory grade Ohmmeters.

## Ohm Sweet Ohm

In Kelvin sensing, a known current source is applied to the component being measured and the voltage drop is measured at the lead tips, negating any lead resistance present. All  four measurement nodes are brought to the front of the instrument by the way of Kelvin clips or probes. Figure 3: Schematic Representation of Kelvin Sensing

The Drive side of the probe is the current source and the Sense side of the probe is where the voltage drop is measured. The V+ (Sense) and I+ (Drive) are paired in one probe and the V- (Sense) and I- (Drive) are paired in the other. When the probess are placed on either side of a component, an electric circuit is completed, and the resistance can be calculated as follows: If this formula looks familiar, it is because it simply Ohm’s Law.

Thanks to the ingenious method Kelvin devised over a century and a half ago, the resistance of 4-wire instrument leads will not affect the reading. While components have become smaller and faster, the idea remains the same.