Celebrating 100 years of Frank

James Frank and Gustav Hertz in 1914 demonstrated experimentally the existence of discrete

or quantized energy levels in atoms, as proposed by Niels Bohr’s atomic model. This is the

100th

year of their experimental demonstration.

The experiment demonstrates that electron can occupy only discrete or quantized energy

states as per quantum theory. The experimental

pressure mercury gas filled in i

electron emitter), anode (collector) and grid (for acceleration and reducing space charge

effect). The electrons are thermionically emitted by a heated cathode, and then accelerated

toward the grid which is at a positive potential, relative to the cathode. The collecting plate is at

a lower potential and is negative with respect to mesh grid. A few electrons have sufficient

energy on reaching the grid, will pass through the grid, and reach colle

measured as current. Electrons which do not have sufficient energy will be reflected backward

at the grid.

As long as the electron collision is elastic, the electrons will not lose energy on colliding with

gas molecules in tube. As the accelerating potential increases, the current also increases.

A plot derived from measurements of accelerating voltage versus anode current shows a

sharp drop of current at regular intervals of applied accelerating voltage

19V for neon). Typically, the

mercury atom is 4.9 eV.

At accelerating voltage of 4.9 V, free electron possesses 4.9 eV of kinetic energy. As a result

the inelastic collision between free electron and mercury atom, free electron’s kinetic energy

converted into potential energy by raising bound electron of mercury atom to higher energy

state. Due to the loss of kinetic energy of free electron, it can

required to reach at the ground electrode, which in turn leads to sharp drop in the current. If

voltage is increased above 4.9 V, electron will participate in one inelastic collision, lose their 4.9

eV, but then continue to be ac

Theme: RMC 2014

Celebrating 100 years of Frank – Hertz Experiment.

James Frank and Gustav Hertz in 1914 demonstrated experimentally the existence of discrete

or quantized energy levels in atoms, as proposed by Niels Bohr’s atomic model. This is the

year of their experimental demonstration.

The experiment demonstrates that electron can occupy only discrete or quantized energy

states as per quantum theory. The experimental setup comprises an evacuated

in it. The tube has a triode arrangement with

electron emitter), anode (collector) and grid (for acceleration and reducing space charge

The electrons are thermionically emitted by a heated cathode, and then accelerated

rid which is at a positive potential, relative to the cathode. The collecting plate is at

a lower potential and is negative with respect to mesh grid. A few electrons have sufficient

energy on reaching the grid, will pass through the grid, and reach collecting plate, and will be

measured as current. Electrons which do not have sufficient energy will be reflected backward

As long as the electron collision is elastic, the electrons will not lose energy on colliding with

As the accelerating potential increases, the current also increases.

A plot derived from measurements of accelerating voltage versus anode current shows a

sharp drop of current at regular intervals of applied accelerating voltage

the threshold potential required for the electronic excitation of

accelerating voltage of 4.9 V, free electron possesses 4.9 eV of kinetic energy. As a result

the inelastic collision between free electron and mercury atom, free electron’s kinetic energy

into potential energy by raising bound electron of mercury atom to higher energy

state. Due to the loss of kinetic energy of free electron, it can not overcome the potential

required to reach at the ground electrode, which in turn leads to sharp drop in the current. If

voltage is increased above 4.9 V, electron will participate in one inelastic collision, lose their 4.9

eV, but then continue to be accelerated with the remaining kinetic energy

James Frank and Gustav Hertz in 1914 demonstrated experimentally the existence of discrete

or quantized energy levels in atoms, as proposed by Niels Bohr’s atomic model. This is the

The experiment demonstrates that electron can occupy only discrete or quantized energy

n evacuated tube with low-

with cathode (thermionic

electron emitter), anode (collector) and grid (for acceleration and reducing space charge

The electrons are thermionically emitted by a heated cathode, and then accelerated

rid which is at a positive potential, relative to the cathode. The collecting plate is at

a lower potential and is negative with respect to mesh grid. A few electrons have sufficient

cting plate, and will be

measured as current. Electrons which do not have sufficient energy will be reflected backward

As long as the electron collision is elastic, the electrons will not lose energy on colliding with

As the accelerating potential increases, the current also increases.

A plot derived from measurements of accelerating voltage versus anode current shows a

sharp drop of current at regular intervals of applied accelerating voltage (4.9V for mercury,

required for the electronic excitation of

accelerating voltage of 4.9 V, free electron possesses 4.9 eV of kinetic energy. As a result of

the inelastic collision between free electron and mercury atom, free electron’s kinetic energy is

into potential energy by raising bound electron of mercury atom to higher energy

not overcome the potential

required to reach at the ground electrode, which in turn leads to sharp drop in the current. If

voltage is increased above 4.9 V, electron will participate in one inelastic collision, lose their 4.9

with the remaining kinetic energy. In this manner, the

current rises again after the accelerating potential exceeds 4.9 V. At 9.8 V, the situation changes

again. There, each electron now has just enough energy to participate in two inelastic coll

excite two mercury atoms, and then be left with no kinetic energy. Once again,

is observed at this accelerating voltage

process repeats, each time the electrons will undergo

This experimental observations are in well agreement with quantum mechanical predication

that atom can absorb energy if and only if it exceeds than required to lift an electron from

lower into higher energy state. Thus,

James Frank and Gustav Hertz were awarded Nobel Prize in Physics, in 1925 for their work.

current rises again after the accelerating potential exceeds 4.9 V. At 9.8 V, the situation changes

again. There, each electron now has just enough energy to participate in two inelastic coll

excite two mercury atoms, and then be left with no kinetic energy. Once again,

at this accelerating voltage. Thus it is observed that, at intervals of 4.9 volts the

each time the electrons will undergo one additional inelastic collision.

This experimental observations are in well agreement with quantum mechanical predication

that atom can absorb energy if and only if it exceeds than required to lift an electron from

higher energy state. Thus, energy levels of electron in atom are discrete.

James Frank and Gustav Hertz were awarded Nobel Prize in Physics, in 1925 for their work.

current rises again after the accelerating potential exceeds 4.9 V. At 9.8 V, the situation changes

again. There, each electron now has just enough energy to participate in two inelastic collisions,

excite two mercury atoms, and then be left with no kinetic energy. Once again, the current drop

Thus it is observed that, at intervals of 4.9 volts the

one additional inelastic collision.

This experimental observations are in well agreement with quantum mechanical predication

that atom can absorb energy if and only if it exceeds than required to lift an electron from

energy levels of electron in atom are discrete.

James Frank and Gustav Hertz were awarded Nobel Prize in Physics, in 1925 for their work.