- 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.
Before exploring the Kelvin Bridge, it’s crucial to understand why this bridge is necessary, despite the existence of the Wheatstone bridge which measures electrical resistance with an accuracy of about 0.1%.
To understand the need of Kelvin bridge we must first recognize 3 important ways to categorize electrical resistance:
- 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.
Now the logic of doing this classification is that if we want to measure electrical resistance, we have to use different devices for different categories. It means if the device is used in measuring the high resistance gives high accuracy, it may or may not give such high accuracy in measuring the low value of resistance.
We must carefully choose the appropriate device for measuring specific resistance values. Other methods like the ammeter-voltmeter and substitution methods also exist but are less preferred in industries due to their higher error rates compared to bridge methods.
Recall the resistance classification: as resistance values decrease from high to low, the need for more accurate and precise measurement devices increases.
A major drawback of the Wheatstone bridge is its significant errors in measuring low resistances, although it can accurately measure from a few ohms to several megaohms.
So, we need some modification in Wheatstone bridge itself, and the modified bridge so obtained is Kelvin bridge, which is not only suitable for measuring low value of resistance but has wide range of applications in the industrial world.
Let us discuss few terms that will be very helpful to us in studying the Kelvin Bridge.
Bridge :
Bridge’s usually consists of four arms, balance detector and source. They work on the concept of null point technique. They are very useful in practical applications because there is no need of making the meter precise linear with an accurate scale. There is no requirement of measuring the voltage and current, the only need is to check the presence or absence of current or voltage. However the main concern is that during the null point meter must be able to pick up fairly small current. A bridge can be defined as the voltage dividers in parallel and the difference between the two dividers is our output. It is highly useful in measuring components like electrical resistance, capacitance, inductor and other circuit parameters. Accuracy of any bridge is directly related to bridge components.
Null point:
It can be defined as the point at which the null measurement occurs when the reading of ammeter or voltmeter is zero.
Kelvin Bridge Circuit
As we have discussed that Kelvin Bridge is a modified Wheatstone bridge and provides high accuracy especially in the measurement of low resistance.
A question may arise: where is the modification needed? The answer is straightforward—the modification is needed in the leads and contacts to avoid an increase in net resistance.
Let us consider the modified Wheatstone bridge or Kelvin bridge circuit given below:
Here, t is the resistance of the lead.
C is the unknown resistance.
D is the standard resistance (whose value is known).
Let us mark the two points j and k. If the galvanometer is connected to j point the resistance t is added to D which results in too low a value of C. Now we connect galvanometer to k point it would result in a high value of unknown resistance C.
Let us connect the galvanometer to point d which is lying in between j and k such that d divides t into ratio t1 and t2, now from the above figure it can be seen that
Then also the presence of t1 causes no error, we can write,
Thus we can conclude that there is no effect of t (i.e. the resistance of leads). Practically it is impossible to have such situation however the above simple modification suggests that the galvanometer can be connected between these points j and k so as to obtain the null point.
Kelvin Double Bridge
Why it is called double bridge? It is because it incorporates the second set of ratio arms as shown below:
In this the ratio arms p and q are used to connect the galvanometer at the correct point between j and k to remove the effect of connecting lead of electrical resistance t. Under balance condition voltage drop between a and b (i.e. E) is equal to F (voltage drop between a and c)
For zero galvanometer deflection, E = F
Again we reach the same result – t has no effect. However equation (2) is useful as it gives error when:
