Again, since b and d are at the same potential, the
drop along dc must equal the
drop along bc. Thus,
Taking the ratio of these last two expressions gives
Canceling the currents and solving for R
x yields
The Wheatstone bridge is used to calculate unknown resistances. The variable resistance
is adjusted until the galvanometer reads zero with the switch closed. This simplifies the circuit, allowing
to be calculated based on the
drops as discussed in the text.
This equation is used to calculate the unknown resistance when current through the galvanometer is zero. This method can be very accurate (often to four significant digits), but it is limited by two factors. First, it is not possible to get the current through the galvanometer to be exactly zero. Second, there are always uncertainties in
,
, and
, which contribute to the uncertainty in
.
Identify other factors that might limit the accuracy of null measurements. Would the use of a digital device that is more sensitive than a galvanometer improve the accuracy of null measurements?
One factor would be resistance in the wires and connections in a null measurement. These are impossible to make zero, and they can change over time. Another factor would be temperature variations in resistance, which can be reduced but not completely eliminated by choice of material. Digital devices sensitive to smaller currents than analog devices do improve the accuracy of null measurements because they allow you to get the current closer to zero.
Section summary
Null measurement techniques achieve greater accuracy by balancing a circuit so that no current flows through the measuring device.
One such device, for determining voltage, is a potentiometer.
Another null measurement device, for determining resistance, is the Wheatstone bridge.
Other physical quantities can also be measured with null measurement techniques.
Conceptual questions
Why can a null measurement be more accurate than one using standard voltmeters and ammeters? What factors limit the accuracy of null measurements?
If a potentiometer is used to measure cell emfs on the order of a few volts, why is it most accurate for the standard
to be the same order of magnitude and the resistances to be in the range of a few ohms?
Problem exercises
What is the
of a cell being measured in a potentiometer, if the standard cell’s emf is 12.0 V and the potentiometer balances for
and
?
24.0 V
Calculate the
of a dry cell for which a potentiometer is balanced when
, while an alkaline standard cell with an emf of 1.600 V requires
to balance the potentiometer.
When an unknown resistance
is placed in a Wheatstone bridge, it is possible to balance the bridge by adjusting
to be
. What is
if
?
To what value must you adjust
to balance a Wheatstone bridge, if the unknown resistance
is
,
is
, and
is
?
(a) What is the unknown
in a potentiometer that balances when
is
, and balances when
is
for a standard 3.000-V emf? (b) The same
is placed in the same potentiometer, which now balances when
is
for a standard emf of 3.100 V. At what resistance
will the potentiometer balance?
(a) 2.00 V
(b)
Suppose you want to measure resistances in the range from
to
using a Wheatstone bridge that has
. Over what range should
be adjustable?
Questions & Answers
where we get a research paper on Nano chemistry....?
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest.
Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.?
How this robot is carried to required site of body cell.?
what will be the carrier material and how can be detected that correct delivery of drug is done
Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?