PASCO SE-9639 Product Manual

Franck-Hertz Experiment
Introduction
In 1914, in the course of their investigations, James Franck and
Gustav Hertz discovered an "energy loss in distinct steps for
electrons passing through mercury vapor", and a corresponding
emission at the ultraviolet line ( = 254 nm) of mercury. As it is
not possible to observe the light emission directly, demonstrating
this phenomenon requires an extensive and cumbersome
experiment apparatus. Performance of this experiment has
become one of the classic demonstrations of the quantization of
atomic energy levels. Franck and Hertz were awarded the Nobel
Prize for this work in 1925.
In this experiment, we will repeat Franck and Hertz's energy-loss
observations, using argon, and interpret the data in the context
of modern atomic physics. We will not attempt the spectroscopic
measurements, as the emissions are weak and in the extreme
ultraviolet portion of the spectrum.
Equipment list
DC Current Amplifier (SE-6621)
Tunable DC (Constant Voltage) Power Supply I
(SE-6615)
Tunable DC (Constant Voltage) Power Supply II
(SE-9644)
Franck-Hertz Enclosure (SE-9660)
5× Connecting cable, 850 mm, red (EM-9740)
5× Connecting cable, 850 mm, black (EM-9745)
Not pictured:
Franck-Hertz Argon Tube (SE-9645A)
3× Power cord (not pictured)
BNC cable (not pictured)
2× 8-pin DIN Extension Cable (UI-5218; not pictured)
(SE-9639)
Recommended Items:
• 850 Universal Interface (UI-5000) or 550 Universal
Interface (UI-5001)
• PASCO Capstone data collection software
Principle of the experiment
Figure 1. Layout of a Franck-Hertz tube
the cathode (K), with a voltage of about 1.2 V between them. A
variable potential difference is applied between the cathode and
Grid 2 (G ) so that electrons emitted from the cathode can be
2
accelerated to a range of electron energies. The distance
between the cathode and the anode is large compared to the
mean free path length in the argon, in order to ensure a high
collision probability. On the other hand, the separation between
G and the collector anode (A) is small.
2
A small constant negative potential U
potential") is applied between G
words, A is less positive than G
between G and anode A opposes the motion of electrons to the
2
collector electrode, preventing electrons with kinetic energy less
than e•U at Grid 2 from reaching anode A. As will be shown
G2A
later, this retarding voltage helps to differentiate the electrons
that experience inelastic collisions from electrons that do not.
A sensitive current amplifier is connected to the collector
electrode so that the current due to electrons reaching the
collector plate may be measured. As the accelerating voltage is
increased, the following is expected to happen: Up to a certain
voltage (which we will call V
as more electrons reach the plate. When the voltage V
reached, it is noted that the plate current I
is due to the fact that, at this voltage, the electrons have gained
just enough energy to collide inelastically with the argon atoms
before reaching the grid G
atoms, these electrons do not have sufficient energy to
overcome the retarding voltage between G
decrease in the plate current I
However, as the voltage is further increased, the electrons
continue to gain energy until they have enough to reach the
anode even after an inelastic collision with an argon atom. As
such, I will increase again as more and more electrons arrive at
A
the plate. This continues until another specific voltage V
reached, at which point I sharply decreases again. This second
A
drop indicates that the electrons have now obtained enough
Product Guide | 012-14264B
The Franck-Hertz tube
is an evacuated glass
cylinder containing
argon, with four
electrodes (collectively
called a "tetrode"). The
four electrodes consist
of: an indirectly heated
oxide-coated cathode
as an electron source
(cathode K); two grids,
G and G ; and a plate,
1 2
A, which serves as an
electron collector
(anode A). Grid 1 (G
positive with respect to
(the "retarding
G2A
and collector plate A (in other
2
). The resulting electric field
2
), the plate current I will increase
1 A
suddenly drops. This
A
. Having lost energy to the argon
2
and A, causing the
2
.
A
) is
1
is
1
is
2
1