Experiment / equipment suggestion:

The gas discharge tube



This a sealed glass tube filled with low-pressure air. At both ends there are electrodes so one can apply a high DC-voltage over the length of the tube.

One will observe a series of glowing rings successively further apart from one another as one looks towards the anode.

The glowing appears because of collisions between the accelerating electrons and the atoms in the air. These collisions excite the atoms, which in turn de-excite by emitting photons. The reason we see this in the series of rings, is twofold:

1)     Firstly the electrons need a certain amount of energy to ionize the atoms. This means they will have to travel a certain distance along the tube to gather enough energy.

2)     When the electrons collide with the atoms however, they will lose energy and thus be returned to an energy-state to low to excite the atoms. They will then have to accelerate some distance before further ionization takes place.



This experiment can be used both as a demonstration of accelerating electrons in an electric field as well as a good demonstration of Bohrís model of the atom. Especially the last point is an important one as there are so few good demonstrations of this idea.

Using physical tables one can perform simple calculations of both energy and kinematics.


Depending on the national curricula one would use this in either in a first or a second level physics course