- voltmeters, ammeters and ohmmeters were made from
galvanometers connected in series or parallel to circuit branches. Galvanometers are simply
current measuring devices. They are composed of a metal core with wire wound around the core
to make an N turn circuit loop of some cross-sectional area A which when it carries a current I in
the presence of an external magnetic field (between the poles of a permanent magnet) a torque
given by:




= NIAB sin .

The needle connected to the metal core with the wound wire rotates to an angle which is
proportional to the current flowing through the galvanometer, the needle points in the direction of
the inward flowing current. A restoring set of small coil springs is connected to the rotation axis of
the metal core and needle to insure a smooth rotation of the needle. Due to the "wiring" in the
galvanometer, there is some internal resistance. The internal resistance depends on which
manufacturer made the galvanometer. The Pasco galvanometers for student use have internal
resistance of about 1.7 k and full scale deflection currents in the range of a fraction of a
milliampere and a potential drop of a few millivolts.

Experiment:

A. First determine the characteristics of the galvanometer. The galvanometers are marked with
the polarization of (+) and (-) poles.

You must hold the reading button down when operating these galvanometers.

1. Determine the resistance of the galvanometer coil by using the digital multimeter. Set the
Ohmmeter to measure in the 4 k range, place the probes and press the button on the
galvanometer. Measure the resistance and enter the value in the Data Table.

2. Connect the galvanometer to the Power Supply through a resistance substitution box, initially
set to zero resistance.

G
R
B
V
V = Vg
at full deflection
- +
g
Power Supply
R
R
G
R
B
Resistance
substitution box
set initially to "zero"



The current is too small to be read accurately on a milliammeter (it is something like 0.05 mA).
Slowly adjust the voltage from the power supply, while holding the button on the galvanometer
down, until the galvanometer needle is at full deflection. Measure the voltage drop across the
galvanometer (as shown in the right hand picture above) and calculate the current as:



Ig = Vg/Rg.

The value of the voltage drop across the galvanometer at full deflection should be on the order of
90 mV and the current for full deflection should be around 0.05 mA. If your values are much
different from these, something is wrong.

3. You have now measured the galvanometer's particular characteristics.

B. Construct a 1.0 Volt voltmeter:

1. To construct a 1.0 volt voltmeter means that when there is a 1.0 volt potential across the
galvanometer, the needle will indicate a full deflection. Since Vg is much less than 1.0 volt, a
larger series resistor needs to be inserted with the galvanometer, so at Ig current,




Ig (Rg + R) = 1.0 V.

Solve for R, enter the value in the Data Table. R should be a fairly large resistance, usually on the
order of 16 k , use a precision on the order of what you can actually measure.

2. Using the resistance substitution box, set the needed resistance and check it with the digital
multimeter, as a double check, enter the DMM value - ***** Measure the resistance BEFORE you
connect the box to the circuit.

3. Connected the galvanometer and R together in series. Make sure the Power Supply rheostat is
adjusted to zero and is switched off. Connect the circuit to the Power Supply and connect the
Digital Multimeter as a voltmeter across the Power Supply. Turn on the Power Supply and
gradually increase the rheostat setting, always monitoring the galvanometer needle. Hopefully,
the galvanometer needle will indicate full deflection at 1.0 volt.



- +
V
= 1 Volt
G
Rg
R


4. Verify you get a full deflection for an applied potential of 1.0 volt, enter in the table and
calculate a percent difference.

C. 20.0 milliamp Ammeter:

1. Since the galvanometers have a full deflection at a fraction of a milliamp, not all the 20 mA can
be permitted to flow through the galvanometer. To remedy this, a small resistance is used to
shunt the bulk of the current around the galvanometer, while only allowing the small Ig to pass
through. The shunt resistance Rs is placed in parallel with the galvanometer, so the potential drop
across the galvanometer is the same as that across the shunt resistor.





( )
V
I I R
g
g
s
=




G
Rg
Rs
20 mA
(20mA-Ig)
Ig


2. Calculate the required shunt resistor . Enter the value in the Data Table.


(
)
(
)
I
R
V
20mA I
R
R
V
20mA I
g
g
g
g
S
S
g
g
×
=
= ×
=

3. There is a selection of shunt resistors varying from 0.10 to 3 .
4. Connect the following circuit with the 100 mA ammeter.






Power Supply
- +
A
s
R
G Rg
set to "zero"
resistance
Load


5. With the Power Supply turned off, zero the rheostat. Connect to the Power Supply and slowly
increase the voltage to the circuit. Monitor both the milliammeter and the galvanometer. Make
sure the galvanometer indicates a full deflection for about 20 mA total current.

6. Verify the 20 mA ammeter you have constructed. Slowly increase the current by slowly
adjusting the power supply voltage until you have achieved 20.0 mA.

7. Compute the % error in your predicted maximum deflection at 20 mA current and the actual
maximum deflection.
DATA TABLE

A. Galvanometer:

Internal Resistance = _______ k


Current at Full Deflection = _____________ milliamps


Potential Difference at Full Deflection ____________ millivolts


B. 1.0 Volt Voltmeter

R (by calculation) = _____________

R (substitution box by multimeter) = _____________







C. 20 mA Ammeter:

Rs (calculated) = ___________


Rs (actually used) = ___________






100
50
50
-

deflection

actual


volt

1

at

diffrence

Percent
x
=
100
50
50
-

deflection

actual

mA

20

at

diffrence

Percent
x
=