plasmaspherebeam

Plasma sphere

In the Exploratorium there is an exhibit named Quiet Lightning.
Examine the glowing beams created by this exhibit.

Material

The Exploratorium Exhibit Quiet Lightning.

A Plasma Sphere from Radio Shack with a current adjustment.

To Do and Notice

At low current levels a uniform glow surrounds the central ball.

Turn the beam intensity on the plasma sphere down to its lowest level then increase the intensity until a diffuse glow of light comes from the center of the ball.

Increase the current in the sphere.

Notice that at some current level the diffuse glow gathers into beams.

Continue to increase the current, watch how the beams get narrower.

What's Going On?

The glow is caused by electrons accelerating from the center of the ball toward the outside.

At low currents the electrons, which are all negative, repel each other electrically and spread out in their race toward the outside of the ball. (When they collide with the gas atoms in the ball those atoms emit the light we see.)

However, an electric current also makes a magnetic field.
Two wires side by side carrying electric current in the same direction attract each other.
Two electrons moving in the same direction also attract each other magnetically.

When the magnetic attraction of a moving stream of electrons exceeds the electric repulsion, the stream pulls together into a narrow, self-focussed beam.