- Kelvin Bridge Definition: Kelvin’s double bridge is a refined version of the Wheatstone bridge designed specifically for measuring low values of electrical resistance with greater accuracy.
- Measurement Categories: Resistance is categorized into high, medium, and low classes, which dictates the type of measurement device needed for accurate assessments.
- Null Point Technique: The null point in bridge circuits is achieved when there’s no measurable current or voltage, indicating perfect balance and accurate resistance measurement.
- Error Reduction in Kelvin Double Bridge: The double bridge configuration uses additional ratio arms to correctly position the galvanometer, effectively eliminating errors from lead resistance.
- Industrial Relevance: The precision of the Kelvin Bridge makes it invaluable in industrial applications where even small measurement errors can have large implications.
A Kelvin Bridge (Kelvin double bridge) measures low resistance by adding extra ratio arms so lead and contact resistance do not swamp the unknown. A Wheatstone bridge is the usual tool for medium resistance; a well-built one can reach about 0.1% on those values, not on milliohm samples.
Textbooks group electrical resistance into three bands so the right method is chosen:
- High Resistance: Resistance that is greater than 0.1 Mega-ohm.
- Medium Resistance: Resistance that ranges from 1 ohm to 0.1 Mega-ohm.
- Low Resistance: Under this category resistance value is lower than 1 ohm.
A meter that is accurate on high resistance can be useless on a low value once lead and contact drop sit in series with the sample.
Choose the method to match the resistance band. Ammeter-voltmeter and substitution methods still work, but lead drop and meter error are usually larger than on a balanced bridge.
As the unknown falls toward 1 ohm and below, lead resistance becomes a larger share of the reading, so a four-terminal or double-bridge method is used.
A Wheatstone bridge is sound from a few ohms up to several megohms. On low resistance the same leads and contacts that were negligible now add a large fraction of the reading.
That is the job of the Kelvin bridge: a Wheatstone layout with extra arms so low resistance can be measured without putting the yoke leads in series with the unknown.
Two terms used below:
Bridge :
A bridge usually has four arms, a balance detector and a source, and it is set to a null. The detector does not need a linear calibrated scale. There is no need to measure voltage and current; the operator only needs to see whether current or voltage is present. The detector must still resolve a small current at balance. Two voltage dividers in parallel, with their difference taken as the output, form a bridge. That arrangement measures electrical resistance, capacitance, an inductor and other circuit parameters. Accuracy follows the accuracy of the arms.
Null point:
The null is the balance at which the ammeter or voltmeter reads zero.
Kelvin Bridge Circuit
The Kelvin Bridge is a Wheatstone layout with extra arms, used where low resistance must be measured without counting the leads as part of the unknown.
The change is in how the leads and contacts are connected, so they do not add to the unknown.
Take the modified Wheatstone layout, the Kelvin bridge circuit below:
Here, t is the resistance of the lead.
C is the unknown resistance.
D is the standard resistance (whose value is known).
Mark points j and k. If the galvanometer is connected at j, t is added to D and C reads too low. If it is connected at k, C reads too high.
Connect the galvanometer at d between j and k so that d divides t into t1 and t2. From the figure,
The presence of t1 still causes no error, and we can write,
So t (the lead resistance) does not appear in the result. That exact split is hard to build by a tap on the yoke, but it shows why the galvanometer can sit between j and k at null.
Kelvin Double Bridge
It is called a double bridge because it adds a second set of ratio arms, as shown below:
Ratio arms p and q place the galvanometer at the right point between j and k so the connecting lead of electrical resistance t drops out. At balance the voltage drop between a and b (i.e. E) equals F (voltage drop between a and c)
For zero galvanometer deflection, E = F
The same conclusion follows: t does not appear. Equation (2) is still useful because it shows the error when:
