Pasco Scientific SE-9641 User manual

InstructionManual
and Experiment Guide
for the
PASCO scientific
Model
SE-9640 Franck - Hertz Tube
Model
SE - 9641 Franck - Hertz Oven
Model
SE - 9642 Franck - HertzControl Unit
10101 FoothillsBlvd. P. 0. B OX 619011 Roseville, CA 95661 USA
012-03498C
2/90
Franck - Hertz Experiment
S E - 9641
Franck
- HertzOven
(with tube)
0 C
5
1
SE - 9640
Franck
- HertzTube
SE
- 9642
Franck
- HertzControl
Unit
Copyright 0 December 1989 $5.00
I PASCO scientific

012-03498C
Table of Contents
Page
..
Equipment Return ........................................................................................ .ii
Introduction ................................................................................................... 1
The Franck - HertzApparatus ......................................................................... 2
Important Cautions and Tips ........................................................................ .3
The Experiment: Setup, Operation, and Analysis ........................................ 4
1
Please - Feelfree to duplicate this manual subject to
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The PASCO scientificModel SE - 9640, SE - 9641, and SE-
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i

012-03498C
Equipmen t Re turn
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instrumentcome in contactwith the edge of the
box.
ii

0 12-03498C
-
'
In troduction
Ma
- (Accelerating
Voltage:
0 - 30 VDC)
In 1914, J. Franck and G. Hertz performed a landmark
experiment,providing important empirical support for
Max
Planck's quantum theory and for the model of the atom
suggestedby NielsBohr. Through the study of collisions
between electronsand gas molecules,Franck and Hertz
demonstratedthat energy
is indeed quantized in atomic
interactions.For this work, they shared the1925Nobel Prize
in physics.
Because of the importance of their experiment,
and because of itsconceptualsimplicity, it has become an
important experiment in the undergraduatelaboratory.
A simplified diagram of the Franck - Hertz experiment is
shown in Figure 1. In an oven - heated vacuum tube contain -
ing mercury gas, electrons are emittedby a heated cathode,
and are then accelerated toward agrid that isat apotential
Va relative to the cathode. Just beyond the grid isan anode,
which isat a slightly lower potential than thatof the grid (V
P
= Va - AV; AV = 1.5 V; where V is the anodepotential,and
Va isthe grid potential, ).
If the acceleratedelectronshave sufficientenergy when they
reach the grid,some of them will pass through and will reach
the anode. They will be measured
as current Icby the
ammeter.
If the electronsdon't have sufficientenergy when
they reach the grid,they will be slowed by
AV, and will fall
back onto
the grid.
Whether the electronshave sufficient energy
to reach the
anodedepends on three factors: the acceleratingpotential
(VJ, the opposingpotential (AV), and the nature of the
collisionsbetween the electrons and thegas molecules in the
tube. As long as the electron/moleculecollisionsare elastic,
the collector currentdepends only
on Va and AV, because
the electrons
lose no energy to the gas. However, Franck and
Hertz discoveredthat Icwent through a seriesof maxima and
minima
as theacceleratingpotentialwas varied. This implies
that the gasmolecules absorbenergy from
the electronsonly
at specificelectronenergies(resonantenergies).
For example, the first excited state of mercury is4.9 eV
above the ground state.This is the minimum amount of
energy that the mercury atoms
can absorbin collisionswith
the acceleratedelectrons. When
Va is less than 4.9 V, the
electron/moleculecollisions
are therefore elastic, so the
electrons loose no energy to the gas and arriveat the grid
with kineticenergy equal
to eVa. If Va > AV, the electrons
Rave sufficientenergy
to overcome the opposing voltage,
and many of them will pass through the grid andreach the
anode, tobe measured
as current.However, if Va equals4.9
V, the electrons gain enoughkinetic energy to collide inelas-
tically with the mercury atomsjust as they reach the grid. In
these collisions, the mercury atoms absorb the
full 4.9 eV
P
-.
Anode
Grid
Cathode
ct 7""'
T
AV (~1.5 V)
Ic
(AnodeCurrent:
0-5 PA)
Figure 1 SimplifiedDiagram of Franck - Hertz
Experiment
carriedby the electrons.The electrons no longer have suffi -
cient energy to overcome AV, and they fallback onto the
grid. Ic
isthen a minimum.
As Va is raised beyond 4.9 V, I, increasesagain. However,
when
Va reaches 9.8 V, the electrons can lose all their
energy
to the gas molecules in two collisions.One collision
is likely
to occur midway between the cathodeand the grid,
the other will occurjustas the electrons reach the grid.
Again, the electronsloose
all their kinetic energy in the
inelastic collisions,
so they fall back onto the grid, and Ic
again falls to a minimum. Because of multiple inelasticcolli -
sionsbetween the acceleratedelectrons and the mercury
atoms,current minima are found whenever
Va isa multiple
of 4.9
V.
Note: The abovedescription is somewhat simpli-
fied. Due to contactpotentials,the total energy
gain of the electronsisnot quite equal
to eVa.
Therefore, Va will be somewhat higher than 4.9 V
when the firstcurrent minimum occurs. However,
the contactpotentials are a constantin the experi-
ment,
so at successivecurrentminima Va will
alwaysbe a multiple of 4.9
V higher than it was
when the firstcurrent minimum occured.

012-03498C
regionsin the tube wherethe electronsare collidinginelasti-
cally with the gas, raising the mercury atoms to
their first
levelof excitation. Theblue light is causedby the emission
If theacceleratingvoltageistoo high, the electrons can still
gain excessiveenergy before strikingMercury atoms,even if
the temperature is correct,and the sameproblem
can occur.
How the Tube Should Look Duringthe
Experiment
When the tube is properly heated and the accelerating
voltage is
within theproperrange, there will be a deep blue
cloud between the cathodeand the grid (asopposed
to the
blue
- green glow causedby ionization). You should see dark
bands forming
near the grid and moving towards the cathode
as the accelerating voltage is increased. Thesebands are the
-
of a photon as the atoms return to the ground state (Ephom =
4.9 eV; XPhom = 254 nm).
'
The Experiment: Setup, Operation,andAnalysis
TheFranck - Hertzexperiment can be performed using three
different setups:
A. Using the ControlUnit and an Oscilloscope
B. Using the ControlUnit and Voltmeters
C. Without the ControlUnit (usingseparatepower supplies
and an ammeter or voltmeter)
Theprocedure for each setup is describedbelow. Data
tables and information on how
to analyze the experimentare
provided at the endof the manual.
A
1.
2.
3.
Using the Control Unitand an
Oscilloscope
Remove the six thumbscrewsthat securethe frontpanel
of theFranck
- Hertz Oven. Replace the front panel with
the Franck
- Hertz Tube as shown in Figure 4.
Insert a centigradethermometer (0 - 200 degrees)intothe
hole in the top
of theoven. Insert it so the tipof the
thermometer
is near the centerof the tube.
Check the label and plug the oven intoan appropriate
AC power outlet, then turn the thermostat dial to 180
"C. Let the oven heat up for 10 - 15 minutes. Keep an eye
on the thermometer. Do not let the oven temperature
exceed
205 OC.
4. Connect the tube, controlunit, and oscilloscope as
shown in Figure 4. (The voltmeters that areshown in
the figureare not needed when an oscilloscopeis used.)
5. Switchon the controlunit, but only AFTER the tube has
warmed up for
10 - 15 minutes. Set the controlsas
follows:
Heater: midrange (about 5.5 V)
Amplifier: Gain
- midrange
Zero
- midrange
Va: Adjust - not applicable
Switch
- ramp (4 )
6. Wait about 90 secondsfor the cathode to warm up, then
adjust
the gain controlson the oscilloscope to get a
trace asshownin Figure 4. (The x - channelgain should
be approximately 0.5 volts/cm; the y - channel gain
should
be approximately 0.5 volts/cm.)
7. From the oscilloscopetrace, measure the valuesof Va
forwhich the collectorcurrent (I2 is a minimum.
Note:
The input to the y - channelof the oscilloscope is
proportionalto Ic. Theinput to the x-channel of the
oscilloscope is equal to Va/10.
8. See Analyzing the Experiment atthe end of the manual
fordetailsaboutmanipulating and analyzing your
data.

012-03498C
Shieldedcablewith
~ a I
A, ,BNCconnectors
Voltmeter: 0 - 1 VDC
I,, the anodecurrent)
(Voltage proportional
t
Important:
If an oscilloscope is
used in this setup, the
Voltmeters are not
needed.
If the voltme -
ters are used, the
oscilloscope is not
needed.
I I'
oscilloscope:
y-input
u3
Voltmeter: 0-30 VDC
(V,-Acce leratingVohag e)
Sample of oscilloscopedisplay I
Figure 4 Franck - Hertz Experiment Using the ControlUnit
B. Usingthe Control Unit and Voltmeters
1. Remove the six thumbscrews that secure the frontpanel
of the Franck
- Hertz Oven. Replace the frontpanel with
the Franck
- Hertz Tube as shown in Figure 4.
2. Insert a centigradethermometer (0 - 200 degrees)into the
hole in the top
of the oven. Insert it so the tip of the
thermometer
is near the center of the tube.
3. Check the label and plug the oven into an appropriate
AC power outlet, then
turn the thermostat dial to 180
"C. Let the oven heat up for 10 - 15minutes. Keep an eye
on the thermometer.
Do not let the oven temperature
exceed
205 OC.
4. Connect the tube, control unit, and voltmetersas shown
in Figure 4 (theoscilloscope connections that are shown
in the figure are not necessary when using voltmeters).
5. Switchon the controlunit, but only AFTER the tube has
been allowed to warmp up for 10 - 15minutes. Initially,
set the controls
as follows:
6.
7.
8.
9.
Heater: midrange (about 5.5 V)
Amplifier: Gain - maximum gain (fully clockwise)
Va: Switch - Man. ( - )
Zero - midrange
Adjust
- zero (fully counterclockwise)
Allow at least
90 secondsfor the cathode to heat up.
Adjust the Amplifier
Zero so that the amplifier output is
zero volts (if it won't go to zero,justadjustit as close to
zero as possible).
Increasethe accelerating voltage in increments
of about
0.5 volts,by turning the
Va Adjust knob clockwise. At
each value of Va, record Va and the correspondingvalue
forthe amplifieroutput.
Note: The amplifier output is not equal to Ic. It is a
voltage that isproportional to Ic.
See Analyzing the Experiment at the end of the manual
fordetailsabout manipulating and analyzing your
data.

012-03498C
C. Without the Control Unit
I 17. Increasetheacceleratingvoltage, V-, in increments of
1.
2.
3.
4.
6.
Remove the six thumbscrews that securethe frontpanel
of theFranck - Hertz Oven. Replace the front panel with
theFranck
- HertzTube as shown in Figure 4.
Insert a centigradethermometer (0 - 200 degrees) into the
hole
in the top of the oven. Insert it so the tip of the
thermometer
is near the center of the tube.
Check the label andplug the oven into an appropriate
AC power outlet,
then turn the thermostat dial to
180 "C. Let the oven heat up for 10 - 15minutes. Keep an
eye on thethermometer. Do not let the oven tempera -
tureexceed 205 "C.
Connect the tube, power supplies,and ammeter* as
shown
in Figure 5.
Important: All power suppliesmust be floating
with respect
to ground.
Set
all the power supplies to zero volts. Then turn them
on and set the heater voltage to 5.5 volts, theaccelerat -
ing voltage to zero volts, and theopposingvoltage to 1.5
volts.
DO NOT LET THE HEATER VOLTAGE
EXCEED 7 VOLTS, or the tube may be damaged.
about0.5 volts. At each valueof
Va: record Vaand the
correspondingvalue for the anode current, I,. Repeat
your measurements,but this time
try to locate the exact
values of Va for which currentminima occur. Record
these values of Va and the correspondingvalues of I,.
18. See Analyzing the Experiment at the end of themanual
fordetailsaboutmanipulating and analyzingyour
data.
I
*Note: If you don'thave a sufficiently sensitive
ammeterto accurately measure
the anodecurrent,
you
can use a voltmeterinstead. Thevoltmeter
must have
0.1 mV resolution and a 10 Msz input
impedance. Connect itjust
as the ammeteris
connectedin Figure 5. Thevoltagereading
of the
voltmeterwill then
be equal to the anodecurrent,in
units of
lo-" amperes.
Ammeter:
*If you don't have a
sufficiently sensitive
ammeter,see the
above note.
Power
Supply:
5.5 VDC or VAC
(Heatervoltage)
supplies must be floating
with respect to ground.
Flgure 5 Franck-HertzExperimentWithout the Control Unit
6 RAscoscientmc

012-03498C
Analyzingthe Experiment
In performingthe experiment (described in the previous
section forthe various setups),you will have collected
values of
Va and correspondingvalues of Ic. To analyze this
data:
1. Subtract 2.0 voltsfrom each of your recorded values for
V andrecord these value as Va(adjusted) in your data
table. Va(adjusted) is proportional to the energy supplied
to the electronsby the field
between the cathode and the
grid.
Explanation: The electrons emitted by the cathode lose
a
certain amount of energy in escapingand gain some
energy when they are absorbedintothe plate. The net
energylost or gained in this transition
is just the
differencebetween the work functionsof the
two
metals-in this case the electronsloseapproximately
2.0 eV. This energy can be accounted for by subtracting
2.0 volts from the measured values of Va.
at which I, is a minimum
for "a(adjusted)
va
Voltage differencebetweenadjacent minima
'a(adjusted)
2.
3.
4.
If you did not use the oscilloscopeto make your
measurements,plot a graph
of plate current versus
plate currentminima and record the corresponding
You should find that the current minima are spaced at
intervals of 4.9 volts,showing that the excitation energy
of the mercury atom is 4.9 eV. The spectral frequency
corresponding
to this energy is :
on the x - axis. Locate the
'a(adjusted) ' with 'a(adjusted)
Of 'a(adjusted) in Table 2.
= 1.18 x 1015 Hz.
4.9 eV
4.133 x 1015eV
v=E/h=
The correspondingwavelength is:
h = c/v = 253.7 nm.
In their original experiments,Franck and Hertz verified
the presence of this ultraviolet radiation with the aid of a
quartz spectrometer.
Table 1 AccelerationVoltage and Anode Current
IC IC
Table 2 Resonance Data
v.
'a(adjusted)
IC

8

.
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