3-6
Resistance values ranging from 1 ohm to 1 megohm can be measured with an accuracy of
approximately 0.1%. However, difficulties are encountered when very high and very low resistances are
measured. Resistances less than 1 ohm are difficult to measure accurately because of uncertainty arising
from the contact resistance present between the resistor to be measured and the binding posts of the
bridge. Measurement of resistances greater than 1 megohm becomes difficult because of two factors: (1)
The ratio of standard resistances RA and RB involve a ratio on the order of 1,000 to 1, and (2) the voltage
applied to the bridge must be substantially increased to obtain definite galvanometer action. The result is
that an increase in the supply voltage increases the power dissipation (heat) of the bridge resistors. The
change in resistance RB, because of the heat, is sufficient to produce an appreciable error. A Kelvin bridge
is recommended for measuring resistances lower than 1 ohm. An electronic multimeter is recommended
for the indicating device in bridges used for the measurement of very high resistances.
One of the most elementary precautions concerning the use of a bridge, when measuring low
resistance, is to tighten the binding posts securely so that the contact resistance between the binding posts
and the resistance to be measured is minimum. Leakage paths between the resistor leads along the outside
surface of the resistor body must be avoided when resistances greater than 0.1 megohm are measured.
Search for defective solder joints or broken strands in stranded wire leads; these defects can cause erratic
galvanometer indications. In those cases where wire leads must be used to reach from the resistance under
test to the bridge terminals, measure the ohmic value of those leads prior to further measurements.
Q-3.
How does the supply voltage affect the accuracy of Wheatstone bridge measurements?
Kelvin Bridge
It is often necessary to make rapid measurements of low resistances, such as samples of wire or low
values of meter shunt resistors. A frequently used instrument that is capable of good precision is the
Kelvin bridge, shown in figure 3-1. Note the similarity between this and the Wheatstone bridge. Two
additional resistances, R1 and R2, are connected in series and shunted across resistance R, which is the
circuit resistance existing between the standard and unknown resistances, RS and R
X, respectively. In
performing the adjustment for balance, you must make the ratio of R1 to R2 equal to the ratio of RA to R
B. When this is done, the unknown resistance can be computed in the same manner as that for the
Wheatstone bridge, because resistance R is effectively eliminated.
In using a Kelvin bridge, you must follow precautions similar to those given for the Wheatstone
bridge. A rheostat is usually placed in series with the battery so that bridge current can be conveniently
limited to the maximum current allowable. This value of current, which affects the sensitivity of the
bridge, is determined by the largest amount of heat that can be sustained by the bridge resistances without
causing a change in their values. All connections must be firm and electrically perfect so that contact
resistances are held to a minimum. The use of point and knife-edge clamps is recommended.
Commercially manufactured Kelvin bridges have accuracies of approximately 2% for resistance ranges
from 0.001 ohm to 25 ohms.
Q-4.
Kelvin bridges are well suited for what type of measurements?
Resistance-Ratio Bridge
The resistance-ratio bridge, shown in figure 3-1, may be used to measure capacitance, inductance, or
resistance so long as the electronic part to be measured is compared with a similar standard. The
measurement of the value of a capacitor must be made in terms of another capacitor of known
characteristics, termed the STANDARD CAPACITOR. The same requirement is necessary for an
inductance measurement. The standard of comparison is designated as XX, and the losses of the standard
are represented as R
X. If you experience difficulty in obtaining a balanced bridge condition, insert
