26-02-2013, 10:58 AM
Study of Kelvin Bridge.
Study of Kelvin.docx (Size: 13.43 KB / Downloads: 22)
APPARATUS:
•Ammeter 1
•Function Generator 1
•Inductor Coil 1
•CRO 1
•Connecting wires
THEORY:
If a standard Wheatstone bridge is used to measure low resistance, it would look
something like this:
When the null detector indicates zero voltage, the bridge is balanced and the ratios Ra/Rx
and RM/RN are mathematically equal to each other. Knowing the values of Ra, RM, and RN
therefore provides the necessary data to solve for Rx.
There is a problem, in that the connections and connecting wires between Ra and Rx
possess resistance as well, and this stray resistance may be substantial compared to the
low resistances of Ra and Rx. These stray resistances will drop substantial voltage, given
the high current through them, and thus will affect the null detector's indication and thus
the balance of the bridge
Since there is no need to measure these stray wire and connection resistances, but only
measure Rx, there must be some way to connect the null detector so that it won't be
influenced by voltage dropped across them.
A specialized version of the Wheatstone bridge network designed to eliminate, or greatly
reduce, the effect of lead and contact resistance, and thus permit accurate measurement of
low resistance is called Kelvin Bridge. Also known as double bridge, Kelvin network or
Thomson Bridge.
As with the Wheatstone bridge, the Kelvin bridge for routine engineering measurements
is constructed using both adjustable-ratio arms and adjustable standards. However, the
ratio is usually continuously adjustable, over a short span, and the standard is adjustable
in appropriate steps to cover the required range.
PROCEDURE:
1. Place the BA switch to OFF position, open the Rx terminals.
32
2. Set the GA sensitivity dial to CH position. By doing this check that the
galvanometer driving battery is good. When the pointer of the galvanometer
deflects to the blue zone on the scale, this shows that the battery is good.
3. Set the GA sensitivity dial to G2, and check that the galvanometer indication is in
the zero adjusting screw of the galvanometer to obtain a true zero point.
4. Connect an unknown resistance to the Rx terminal.
5. Select a multiplying factor by using the plug, to fit the approximating value of the
unknown resistance.
6. Turn the BA switch ON.
7. Set the measuring dial to a position near the center, push the GA button switch
momentarily, and observe the galvanometer. If the deflection is on the (+) side,
increase the dial value to obtain zero indication by repeating the above operation.
When the indication comes near the zero position, push and turn the GA button
switch either clockwise or counter clockwise, to lock the switch. Then, move the
measuring dial to obtain zero indication on the galvanometer. If the indication is
on the (-) side in the beginning of this adjustment, reduce the dial value and obtain
zero indication in the same manner as above. If more sensitivity is required, select
G1 position first, and then G0.
8. When the galvanometer indication comes to zero through adjustment of the
measuring dial, the resistance value of Rx is calculated from the following
equation.