3B Scientific Resistance Bridge User Manual

Page 2

age U is applied to this circuit. The ammeter is
used to measure the current flowing between
the point D and the moveable tapping point C
located on the resistance wire.
The partial resistances of the wire R

and R

can

be varied using the slide contact on the resis-
tance wire.
Now it is important to calibrate the measurement
bridge, i.e. to adjust the slide contact so that
there is no voltage between points C and D and
thus a current no longer flows. The partial resis-
tances are:

⋅

and

⋅

whereby F is the cross-sectional area of the
wire.

For the resistance ratios the following then holds
true:

From this we can deductively compute the un-
known resistance:

⋅

The resistor R

should be selected so that upon

calibration of the bridge l

and l

are approxi-

mately equal, in order to keep the error to a
minimum.

Fig. 1

5. Sample experiments

5.1 Determining resistance in a Wheatstone

bridge circuit

Additionally required:
1 AC/DC Power Supply 12 V, 3 A (230 V, 50/60 Hz)

1002776

or
1 AC/DC Power Supply 12 V, 3 A (115 V, 50/60 Hz)

1002775

1 Zero Galvanometer CA 403

1002726

1 Resistance Decade 1 Ω 1002730
or
1 Resistance Decade 10 Ω 1002731
or
1 Resistance Decade 100 Ω 1002732
1 Incandescent lamp with socket
8 Experiment cables (500 mm)
1 Switch (optional)

• Connect up the experiment setup as illus-

trated (see Fig. 1).

• An incandescent lamp is used as the un-

known resistance.

• A voltage of 4 to 6 V is applied.
• Close switch K and slowly move the slide

contact from A to B to A again.

• At the same time observe the deflection of

the ammeter. When the pointer deflection in
the proximity of point A is zero, this means
that the value of R

is very high and that it

must be reduced. If the zero value is in the
proximity of point B, then the value of R

too low and must be increased.

• Select

the

value so that when the power

is switched on again the pointer of the am-
meter does not deflect when the slide con-
tact is in the middle of the wire, i.e. the
measurement bridge is calibrated.

• If there is no appropriate resistance avail-

able, use a resistor R

, for which the

pointer's deflection is smallest and then
carry out the calibration.

• Obtain readings of partial lengths of the

resistance wire.

• Repeat the experiment with varied voltage

levels, enter your findings in a table and
compute the resistance R

5.2 Determine the specific resistivity ρ of a

wire

• Experiment set-up according to Fig. 1, but

this time use a resistance wire with a length
from 1 to 3 m instead of the incandescent
lamp.