The distance between the filament F and the grid G was considerably greater than
the mean free path of the electrons in the gas at this pressure, so that many collisions were
made in this region. The distance from the grid to the anode was made relatively small,
however. The anode A was slightly negative compared with the grid G, so that electrons
were retarded between G and A. The accelerating potential between the filament and the
grid was slowly increased from zero and the current in the electrometer E rose. When a
potential Vc was reached, however, the current fell a little before rising again, and this also
happened for other values of the potential (2Vc, 3Vc and so on).
This can be
explained as follows. As the voltage across the tube is increased the current increases, as
the electrons collide elastically with the atoms of the gas,
but when it reaches Vc inelastic collisions occur, and the electrons are brought practically to
rest. The electric field in most of the tube is small since most of the potential drop occurs
near the very thin filament. Therefore when the electrons reach the grid they have insufficient
kinetic energy to overcome the retarding field between the gird and anode, and cannot reach
the anode, so the anode current falls.
This shows that no increase in the energy of
the atom can occur if the energy of the electrons is less than eVc. The existence of a definite
series of energies with no intermediate values suggests that the atom must have a set of
well-defined energy levels. A further drop at 2eVc shows electrons losing energy to two
atoms in successive collisions.
The transition of electrons from one energy
level to another gives the characteristic spectrum of the material. No two elements have
identical energy level structures and therefore the spectrum of an element is unique. Notice
that energy must be put in to raise the electrons within
the atom to higher energy states.