Lab Snapshots

by Marek Ples

Historical experiments and apparatus

A few examples

Table of contents

Lebiez machine
Glow discharge
Expansion cloud chamber
Diffusion cloud chamber
Miniature Solid State Tesla Coil
Further readings

Lebiez machine

This electrostatic influence machine is a variation of the Holtz machine and a simplified version of the Voss machine. It consists of a rotating disk with metallic tabs and a fixed disk with two inductor plates that make contact with the tabs through brushes. The output is taken directly from the inductor plates. The machine is made of copper clad laminate (CCL), aluminum foil, copper wire, two plastic jars and wood. The main problem with this device is periodic polarity reversals, but self-starts more easily than Wimshurst machine.

On Vid.1 you can see the operation of the Lebiez machine powering Volta's Hailstorm.


Here (Vid.2) I present a slightly "heavier" version of this demonstration.


Glow discharge

A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.

In Vid.3, you can see what the glow discharge looks like in a glass tube included electrodes with high voltage as air is evacuated with a vacuum pump. As the pressure of the gas decreases, the character of the discharge changes and takes on a characteristic stratified appearance.


Gases under low pressure - in this case air in a ampoule made from a regular test tube - also glow in a strong electromagnetic field (Vid.4), such as that generated by a Tesla coil (also built by author).


There is something truly fascinating about the appearance of glow discharges Vid.5).


Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry.

Expansion cloud chamber

Charles Thomson Rees Wilson, a Scottish physicist, is credited with inventing the cloud chamber. Looking into a cloud chamber you can see the tracks of electrically charged particles as they pass through device. The space inside the chamber is filled with oversaturated vapour and, as a particle passes through, tiny droplets of alcohol form, showing up its track. It’s a bit like the vapour trails you see when a jet aircraft flies through the atmosphere.

In Wilson's original chamber the air inside the sealed device was saturated with water vapor, then a diaphragm was used to expand the air inside the chamber (adiabatic expansion), cooling the air and starting to condense water vapor (expansion cloud chamber).

I present below (Vid.6) the results obtained in a simple Wilson expansion chamber built in my workshop. The source of alpha radiation is americium-241. Alpha particle tracks are visible as thick, distinct trails.


The Wilson chamber allows the demonstration of various properties of nuclear radiation, for example by placing a barier made of paper inside (Fig.4).


You can see that alpha radiation is blocked even by a thin layer of paper (Fig.5).


Diffusion cloud chamber

The diffusion cloud chamber was developed in 1936 by Alexander Langsdorf. This chamber differs from the expansion cloud chamber in that it is continuously sensitized to radiation, and in that the bottom must be cooled to a rather low temperature, generally colder than −26°C (−15°F). Instead of water vapor, alcohol is used because of its lower freezing point.

A simple cloud chamber of this type consists of the sealed environment, a warm top plate and a cold bottom plate. It requires a source of liquid alcohol at the warm side of the chamber where the liquid evaporates, forming a vapor that cools as it falls through the gas and condenses on the cold bottom plate. Isopropanol, methanol, or other alcohol vapor saturates the chamber. The alcohol falls as it cools down and the cold condenser provides a temperature gradient. The result is a supersaturated environment. As energetic charged particles pass through the gas they leave ionization trails. The alcohol vapor condenses around gaseous ion trails left behind by the ionizing particles. This occurs because alcohol and water molecules are polar, resulting in a net attractive force toward a nearby free charge. The result is a misty cloud-like formation, seen by the presence of droplets falling down to the condenser. When the tracks are emitted from a source, their point of origin can easily be determined (Fig.6, natural uraninite as source).


The effect of injecting radioactive gas (Ra-220) into my diffusion cloud chamber is quite interresting and somehow beautiful (Vid.7).


From time to time, we can observe the effects of the interaction of nuclear radiation and matter. One example of this is the sudden bending of the alpha particle track, most likely caused by a collision with other atom nucleus (Fig.7, red circle).


Miniature Solid State Tesla Coil

A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891. It is used to produce high-voltage, low-current, high-frequency alternating-current electricity. Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits. Originally, Tesla coils used fixed spark gaps or rotary spark gaps to provide intermittent excitation of the resonant circuit but more recently, electronic devices are used to provide the switching action required.

Solid State Tesla Coil (SSTC) use power semiconductor devices, usually thyristors or transistors such as MOSFETs or IGBTs, triggered by a solid state oscillator circuit to switch pulses of voltage from a DC power supply through the primary winding.

I have built many different constructions of this device, but one of my favorite is a miniature (fitting in the palm of the hand) Tesla coil powered by a 9V battery (Fig.8).


The electromagnetic field around the secondary winding is so strong that it easily induces glow discharges in a neon lamp (Fig. 9) and a small fluorescent tube (Fig. 10). See also Vid.8.



This simple and safe device has a great didactic value and I often use it during my pop-sci activities.

That's not all

For more information about these and other experiments or constructions, please visit You can also contact me by email (

Further readings: