Folder 7


This Folder was created long before we were swamped with the corona pandemic. However, the same name happens to be tagged to the virulant and benign "coronas" and so you may be interested in reading about a benign corona, namely, corona discharge. There is a chance that the ozone generated by the discharge may be able to kill the COVID 19 virus and thus act as a non-chemical disinfectant. (details given at the end of the folder)

Disclaimer: Do not attempt to experiment with high voltages unless suitably trained. Caution advised whenever using high voltage or when in the presence of ozone-rich environments. Such experiments are undertaken at the viewer's own risk.

Corona discharge

The discharge of electricity through air at normal temperature and pressure is familiar to most people during a thunder storm; lightening flashes to earth in a spectacular way. But much closer to home are the common gas lighters which produce a spark from a piezo-electric material .

 NOW,.....  ANOTHER DISCHARGE is possible and this is called CORONA DISCHARGE. It is not as spectacular as a spark but it has many uses in industry - removing particles out of a chimney stack, purifying air or killing mites not to mention the whole area of XEROX copying and printing equipment.

A YouTube presentation will describe what happens - 

                                                                                                     click here  .

So we see that a corona discharge produces a faint blueish glow and charged particles. Several consumer items have started to employ the technique for improvements to their products, eg hair dryers and air fresheners.

A useful publication was given several decades ago in SCIENTIFIC AMERICAN. This explains the corona form of discharge very well   Sci_Am_Adam1.pdf

The following small apparatus gives a demonstration of the corona effect. It is a simple set up that can be constructed by anyone with modest workshop skills

A wire brush (with wooden handle) for cleaning spark-plugs was purchased from a local DIY shop. This had brass wire bristles which were 0.15 mm in diameter and so were VERY pointed. A wire was soldered to each of the bristle clumps and the 25 kV was connected to this wire.  If clamped by the wooden handle there was "LEAKAGE" to earth when the 25 kV was applied so it was necessary to add a PTFE spacer onto the brush and clamp the assembly via the PTFE spacer.

If the Aluminium sheet was place less then 3 cm away there were occasional sparks so the separation was 3.2 cm and, at this separation, only corotron discharge was possible.

Thus we have a wire brush electode held at a HIGH VOLTAGE of 25 kV and this is separated by MORE THAN 3 cm from a plane electrode at earth potential, the latter is simply an aluminium sheet.  The illuminated electrode is shown as the view from a webcam attached to a telescope. (Accurate focussing on the tips of the bristles was enabled in good illumination).

( Note some time ago an investigation was carried out with Multiple-Needle Electrodes by Dan Rafirolu and fellow workers. The reference is available free  "on-line". The journal reference is as follows:- IEEE Transactions on Industry Applications (1999), "Multiple-needle corona electrodes for electrostatic process applications"  Vol 35 (issue3) p 543 - 548   by   Dan Rafirolu, Alexandru Iuga,  Lucian Dascalescu, Adrian Samuila and Roman Morar)

The discharge is shown in the following photograph

The photograph does not give a TRUE RENDERING OF the colour  or extent of the discharge. With the naked eye there was much more Purple (or crimson) in the glow which extended throughout the whole region.

A video is included   coron . CLICK on "coron" and then one can see a camera shot of a discharge from the brush held at a voltage of 25 kV. This is only visible in a darkened room ---- note, there is quite a loud HISS. With an ammeter in series with the discharge path (on the earth side so as not to endanger the meter) it is possible to record the current. With an electrode separation close to 3 cm the current is approximately 120 microA and the hiss is quite loud. When the separation is increased to 3.5 cm the current falls to 20 microA and the hiss is just audible.

Countless experiments  involving Corona Discharge have been carried out. The following is a novel investigation into photoconductivity in polymer films where the need to made contacts to the film is not needed since CHARGE is deposited on the upper surface.  Scott_Adam2.pdf

High Voltage engineering is a very specialized subject and is not usually well covered in the curriculum of schools and colleges. However, the subject is going to get more important in the future as the National Grid will have to become more flexible to accomodate green energy inputs. A good text is given below:


A discussion on corona discharge is given at page 58 in the above text and several other sections are devoted to electrical beakdown in gases, liquids and solids. NOTE on page 59 the equation given is limited as it just specifies the radius of the inner cylinder as "a" and the outer cylinder as "b" and with b = 12 and a = 2 the field is 41.8 kV/cm . With a general radius in the equation the whole curve can be plotted -- as descdribed in the following .pdf file


As so rightly pointed out in the text on page 59 the average field would be about 15 kV/cm and such a low field would not cause a discharge. BUT, with the smaller electrode in a cylindrical configuration the field rises to 41.8 kV/cm at the electrode and, for all the blue region, the field is above 30 kV/cm. Thus a sheath corona discharge occurs round the central electrode. In the article in the Physics Education journal : F.Thompson "Getting charged up - the corona way" , Physics Education 54(6) 065005 (2019) the size  (diameter) of the wire is shown to be important and further experimental data is given below

The LHS figure is for a single corotron wire whereas the RHS figure is for a dual wire arrangement.

Commercial air cleaner have been available for some time and these use corona discharge

Part of a video clip is shown to explain the mechanism   click . Please be patient as the up-load may take 5 mins or so depending on your computer. (Streaming is not used here)

The year 2020 will be remembered as the Coronavirus year- so much saddness and turbulance has been caused as the COVID-19  virus roams around the world unchecked. We can turn to another form of CORONA to assist us in destroying the virus. The ozone generated by corona discharge will kill the virus and therefore may prove to be one of many tools which we can use to stamp out this evil beast  ---- watch the following Youtube . 

Details of respiratory function are given in the following video :

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