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Cathode ray tubes


For non-physics techs these are funny oversized lightbulbs that lurk with, what is normally referred to as, the atomic physics apparatus. These are used at “A” level to illustrate many properties of electrons. They are very delicate and either contain gas at low pressure or a vacuum. The best known are the “TELTRON” range but other manufacturers also exist. These tend to have dedicated stands that allow orientation to be changed and special “Helmholtz coils” used to provide a magnetic field. If your equipment is from one range/supplier, the colour of the end of the tube will indicate if it is gas or vacuum filled.


All of the tubes are based on a principle of “thermionic emission” and so at least one part of their function can justify spending some time getting to grips with.
Electrons are (by convention) emitted from the end called the cathode. The process is fairly simple to understand. Heat is applied to the cathode and electrons in the metal of the cathode are given energy. (If you want think of the electrons as being like a gas in the metal). As they are more energetic they are more likely to be located on near the surface. All that is then required is to efficiently pull these energetic electrons off of the cathode. A voltage is required to do this. As an electron is negatively charged a positive voltage, with respect to the cathode, is required. This is applied to the terminal referred to as the anode. That is it.
Okay, so how is the heat supplied to the cathode? Well if anyone has used an electric kettle you are one step ahead. The process is called ohmic heating. An alternating current is passed through the cathode. After a short time the resistance of the cathode produces heat. Convention has that the EHT and HT units have a 6.3V supply specifically for this.
Things to consider:

  • As there is a time delay the power should be switched on to heat the cathode before the anode voltage is raised.
  • Occasionally the current may need to be monitored (on an a.c. Miliammeter) to ensure that it isn't too large.

Knowing if your tube is filled with a low pressure gas or vacuum will give you an idea of the voltages required to operate these devices successfully. The tubes that are gas filled require the lower HT supplies. An inert gas is used where electrons need to interact with the gas. The two tubes where this is common is the “fine beam” tube and the “Frank-Hertz” tube. The fine beam tube has the electrons ionising the gas so that a visible beam can be seen. Far less common, the Frank-Hertz tube has the gas undergoing various ionisations.

Types of tube and function
(links to a more detailed description to be added)

Maltese Cross electrons influenced by magnetic fields:
Perrin shows charge on an electron
Planar diode shows conduction in one direction
fine beam or double fine beam shows e/m motion in a circle
deflection e/m e/m ratio using magnetic electric fields
Electron diffraction wave nature of electrons
Frank_Hertz/Critical potentials Ionization in gases

The term “optical activity” is used to describe how a (usually) liquid alters the orientation of polarised light.


It is of importance in physics where the interaction between light and the solution is of consequence. In chemistry its use is where isomerism and concentration are investigated. Although not necessarily optical, it is in biology especially pharmacology where the greatest impact may be felt. Synthetic chemicals tend to differ to their “natural” counterparts, and engineering may allow these differences to be applied to treat medical problems.


When polarised light enters an optically active medium the plane of polarisation is changed. Why this happens is complex, but can be attributed to the nature of the molecules in the solution through which the light passes. These molecules are “chiral”, that is they have no mirror symmetry. Mixing a “left handed” molecular solution with its “right handed” counterpart will decrease the effect. The standard piece of apparatus used to measure optical activity is called a polarimeter. Many schools may have a commercially produced demonstration model or one that has been made “in-house”.
A technician may be asked to prepare this equipment to illustrate the principles described above.


THIS PART REQUIRES CONFIRMATION OF METHOD,-as a physicist null detection is a standard method.
The polarimeter consists of a tube in which the solution under test is placed. If air or water were used in the tube a “null” or minima of intensity occurs when the analysing filter is perpendicular to the plane of polarisation of the incident light. The presence of the active solution causes the null to move as perceived by the observer to the Right (dextrorotatory or +) or Left (levorotatory or - ).
Common chemicals showing optical activity: Sugar/sucrose/glucose/Dextrose (+), Fructose(-) check your bottles to see if they are natural or this may not be the case!
A solution with equal quantities of + and – is entomers called a “racemix”



Barber's Pole

The above may be summarised in an elegant demonstration called the “barber's pole”. There are various configurations possible, however the following is reasonably simple and less likely for errors (such as over heating the polarising filter) to occur. A slide projector is used with a blank slide with a small hole (possibly use a drill in thin aluminium the size of a standard slide if one isn't available). The light from this is directed onto a rotating mirror (the motor should turn reasonably slowly at, say, 10 r.p.m). The rotating mirror is held above a measuring cylinder containing a sugar strong sugar solution (250g in 250ml) with a few drops of milk added. The milk is added to allow scattering and only a few drops are necessary as too much will not allow light to penetrate into the column of syrup to any significant depth. The observer will see colours apparently moving up the column of liquid as the plane of polarisation rotates around the cylinder. This is partially due to the dispersive nature of the polarisation in the column.
Barberspole.jpg and photo required

Construction notes:

  • raise the projector on a suitable stand.
  • If making a temporary device hot glue can be used to fix a glass mirror to the shaft. Care miust be taken to have the mirror centralised and perpendicular to the shaft.
  • A flat mirror or slightly convex is best as the polarising filter can be held in place (using tape over the back) without introducing adhesives between the mirror and the lens.
  • The mirror should be angled at 45 degrees to the parallel light beam for best results,
  • A dark room is essential.

Further reading view/bp/1organic/chirality.html [1] [2]

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