Investigation into the luminescent effects of high voltage on citrus fruits.
DA PIPET, JS PIPET, CE HOWMAN, JD HALL, C REED, AJ PENTON, et Al. University of Cambridge. May 1999.



In an extension to the well-known phenomenon of electro-luminescence in pickled foodstuffs, an attempt was made to show that other foodstuffs containing a large quantity of acid also glow when mains current is passed through them. A variety of citrus fruits were tested for electro-luminescence in a similar way to that used to test pickles. The results were remarkably positive, not only do citrus fruits glow in a more impressive way, but they also smell much nicer while they are glowing.


Following the success of the group in establishing the luminescent efficiency of picked gherkins and pickled onions, several proposals were made about the luminescence mechanism. Most of these related to the forced movement of ions through the vegetable, and are dependent on the large amount of ion-generating acid (acetic acid from the vinegar) present.

It was proposed that maybe citrus fruit, which also contain large amount of acid (citric acid) might display similar properties. This provided an excellent excuse to repeat the experiment (fun!) and take it to a wider audience, as well as gaining more insight into the mechanism of operation.

As an extension to the experiment, some quantitative measurements were also made, including the current through the device.

Apparatus Used


The apparatus and method were in general the same as for the gherkin experiment. For parts of the experiment, and ammeter was included in series with the electrodes, connected with small jump leads. Otherwise the method of inserting straightened paper clips, connected to a mains lead, was the same.

The equipment set up

Since the venue was different on this occasion (the Lillypad garden), there was no balcony to dangle the wire from. Instead the tree was used, and the branches of this were found convenient for routing cables over the heads of the experimenters.

The first experiment was to connect a pickled gherkin and a pickled onion, to check that the previously observed results still held. Subsequently slices of citrus fruits were taken (so as to be able to see inside), and connected to the circuit. A lemon, grapefruit and a lime were all tested.

Connecting the subjects


As previously, an RCD was connected into the circuit and a fire extinguisher (powder) was to hand. It was noted that no-one knew how to operate the fire extinguisher prior to the experiment. This was deemed to be the common operating practise for most fire extinguishers. The large and sharp knife was handled with great care (well, John Hall didn't throw it about).


Both the gherkin and the onion glowed in a similar manner to before. The current through the onion was found to be a maximum of 2A rms. It was noted that once again, they smelt bad, especially since they were from the same jars as the original experiment and were decidedly off.

The slice of lemon used was about 1.5 cm thick, upon initial power-up it showed no change. It was only after about a minute that the juice around one electrode began to fizz. This was shortly followed by the production of smoke from the same area and illumination. Individual (and changing) segments of the lemon were illuminated for several minutes. After the illumination ceased, it did not occur again. The lemon was then rather warm to the touch.

The grapefruit slice (about 4 cm thick) behaved in a similar manner. It took much longer to turn on, somewhere between five and ten minutes (we got rather bored). However, once on, the light produced was rather spectacular compared to any previous experiments. Like the lemon, the area illuminated varied over time, and the grapefruit eventually turned off. Unlike the lemon, the grapefruit came back on again after a period of time, glowing at the opposite electrode. It turned on and off several times during the course of the experiment.

The grapefruit in an illuminated phase

While it was on, the grapefruit consumed a maximum of 70mA rms current. The off current ("dark" current?) was less than 0.5mA rms, the minimum measurable current. After the experiment the grapefruit was hot to the touch.

It was decided to make some time measurements while testing the lime. A slice about 2 cm thick was connected to the supply. The times of the various reactions are noted:

Bubbling at electrode.
Fizzing at the electrode and associated noise.
Sparks being produced at the electrode.
Illumination of the lime.

Click here to see more pictures of the results

As previously, the electrodes in the gherkin and the onion showed a significant tendency to move towards each other, a process known as Electrode Drift (E.D.), which eventually curtailed the experiments after about a few minutes. There was very little E.D. in all of the fruits, the only noticeable movement being downwards due to gravity. This allowed the fruit to be kept illuminated for much longer, typically an order of magnitude.

Lime after test, showing lack of Electrode Drift


In almost all respects the light-producing properties of citrus fruit were superior to pickled vegetables. They produced more light, consumed less current and lasted longer because of the far smaller amount of Electrode Drift. Electrode Drift was reduced by the strength and thickness of the peel that the electrodes were wedged in. Most significantly, the lack of bad odour in most cases made them much more pleasant to operate.

In contrast, citrus fruits showed the disadvantage of taking some time to "warm up" before emitting light. The analogy might be drawn between citrus fruits and fluorescent light bulbs. They are superior in almost every way, except they take much longer to light up. Clearly both would be totally unsuited to high-speed applications such as disco lighting or fibre-optic communication.

The hypothesis in this case was most definitely correct. Theories about the light-producing effect based on ion-transport mechanisms using acids could be well-founded. Larger research budgets for further work would be easily justified.

Further Research

Electro-luminescence in fruit and vegetables is now well-established. The experiments have been performed many times the world round and there is no doubt that it occurs. The physicists among us would now like to go and produce detailed theories of how it works, but can't be bothered. The engineers, on the other-hand are going to start designing garden lights based on grapefruits (the best so far) right away, without wondering why. The rest appear to be happy to sit around watching the experimental versions glow.

On a more serious note, further experiments will continue the search for more brightly glowing vegetables, in the same way the Edison first searched for the appropriate material for the filament in light bulbs.

Further Reading

The original gherkins experiment is essential reading. Further experiments into diverse and neglected phenomena have been performed in Cambridge and around the world.

The People


The team consisted of John Penton, Clare Howman, John Hall, Dot Pipet, Chris Reed, and John Pipet (left to for web pages).


The spectators consisted of Anna Ruff, Joanna Swanson, Judith Kyte, Craig Watson and the Lillypad goldfish (again, left to right...when I get all the images).

Page written by John Penton and John Hall
Last updated 15/9/99.

With thanks to Richard Fisher for the loan of a really cool digital camera

Any comments or suggestions then please e-mail TonyJohn.