Teltron tube

A fine beam is a physical test structure in which accelerated electrons move in a ball due to the Lorentz force, which is directed into the ball inside, on a circular path. When colliding with gas molecules generated the light, which makes the circular path of the electrons visible.

Experimental setup

In an evacuated glass bulb some hydrogen gas is filled (H2 ), so that in the piston a hydrogen atmosphere at low pressure ( about 1 Pa) is produced. The pressure is such that the electrons are slowed down by collisions as little as possible ( change of the kinetic energy ), the number of collisions, but is sufficient to form a visible glow. In the test tube is an electron gun. This consists of a heating coil, a cathode and an anode hole. From the cathode emerge from electrons that are accelerated toward the positively charged anode. Most of the electrons are absorbed on the anode again, but through a hole in the anode, leaving a small part of the electron beam generation system.

Result

When you turn on the heater voltage to the heating coil and heated electrons emerge from her. In the electric field between anode and cathode, the electric field on the electrons which these are accelerated to a high velocity, so that leave a part of the electrons pass through a small opening in the anode such as an electron beam acts. Since the coil current is not on, no force acts on the beam, and it maintains its direction at. But the coil current will now be additionally turned on, the Lorentz force deflects the electrons to a circular path.

Determination of the specific charge of the electron

The higher is the coil current, the smaller the radius of the orbit, since the magnetic field is stronger. The strength of the magnetic field and the Lorentz force are proportional to each other, so that the Lorentz force increases. A greater Lorentz force deflects the electrons strongly, and therefore the orbit is smaller.

The Lorentz force is always instantaneous movement direction perpendicular and allows as a centripetal force circular motion. The magnitude of the velocity and hence the kinetic energy can not change:

This yields for the amount of the specific electron charge

The determination of the speed is effected by means of the energy conservation law

So finally follows

The specific electron charge has the value

Since the charge of an electron from the Millikan experiment is available, the study of electrons in the magnetic field of the determination of their mass is in accordance with:

Similar concepts for the weighing of charged particles can be found in the mass spectrometer.