THE WEIRD BUBBLE

CURRENT GENERATION BY FRICTION

Fig. 1 shows the basic implementation of a friction generator. After a few preliminary tests it was found that nickel-copper coins were the ideal choice and henceforth adopted as reference discs. They are readily available and of the right size, and what is left to do is just some polishing of the rim with a fine sandpaper. The metal to be tested was placed at the outer edge of the coin and the voltage measured while the disc was running. Several types of metals were tested with the aim to find the one giving the highest output. The result is reported in the table together with the triboelectric series and the thermoelectric series. The tested metals do not seem to follow neither of these series although a modest contribution from a thermoelectric effect is to be expected as the metal under test is bound to become slightly warmer than the rotating disc. The output terminals were loaded with a 2200μF capacitor in parallel with a 4.7KΩ resistor and the current measurement was taken by connecting directly a digital meter to the output terminals; the reading is thus comprehensive of the contacts voltage drop and internal resistance of the instrument.

The voltage is positive and proportional to the relative speed of the disc, no matter which direction is rotated, but also to the pressure applied to the contact being tested. Actually the higher the polishing of the sliding surfaces the higher the pressure required to get the same voltage. A rough surface needs a lower pressure but the internal resistance of the generator is increased.

Eventually it was found that bismuth gave the highest reading. Yet, only a small number of metals and alloys were tested and probably there is a combination of materials that would give a much better performance. For example, it was rather by accident that a chunk of conductive ferrite was available for testing giving the highest negative reading; however the sample had a resistance of a few KΩ, not quite a good conductor and definitely not good for a current generator.

The electrofriction series

Voltage output was measured for many metals (table 1). The output is purely indicative, as consistency could not be kept across all measurements especially concerning pressure and finish of the surface. They seem to follow their own rule although more accurate measurements might give results closer to the triboelectric series (table 2).

In order to see how local heating was influencing the output voltage, a test (fig. 2) was carried out. It was expected that a small positive voltage would develop due to heating generated by the friction between the metal under test and the Ni-Cu disc. The experiment partly confirmed this because a similar positive voltage was indeed produced but only when the sample was heated at a temperature of about 80-100°C above ambient temperature. The normal heating due to friction in our generator was measured at only a few degrees above ambient temperature. The conclusion is that most of the voltage comes from friction and not from the heating effect, besides, it was also found that the conductive ferrite gave a positive result when running the experiment of fig. 2 while in the friction generator it gives a negative voltage. Exactly the opposite happens with bismuth: in the experiment of fig. 2 it gives a negative voltage but it is positive in the friction generator. This is rather puzzling and it has not been investigated further. It must be pointed out that the experiment of fig. 2 does not follow the thermoelectric series (table 3): here we have that the highest voltage is obtained by different metals heated at the same temperature, for example a bismuth-iron junction is most effective while a zinc-aluminium junction is not. In fig. 2 we have different metals heated at different temperature, actually, in this experiment, even the same metal but at a different temperature gives a voltage output.

Other configurations

The voltage of these devices is usually fairly low and it is not feasible to wire more units in series, as they will add extra pairs of sliding contacts.

One solution to the above problem is to have two discs instead of one: with suitable choice of metals you can double the output. Fig. 3 show how a set of “complementary” metals, in this case bismuth and nickel-copper alloy, will give twice the voltage using one single pair of sliding contacts. The problem of these configurations, and all the ones using bismuth, is that this metal is rather brittle and it is a pain to cut it as a round disc, it will just brake apart. The best solution is to cast it using a suitable form but this is not always feasible and the only other alternative is to melt it on a flat metal surface and then cut it out using the hot tip of a soldering iron or chipping it off a bit at the time. Bismuth has a low melting point, similar to lead, and you can do the casting operation in your kitchen, using all the proper precautions.

Another way to raise the low voltage is using a transformer but in order to do this you need an AC source and fig. 4 shows a solution: the disc is split in two halves while both contacts, of the same material, are placed at opposite places on the disc. The frequency of operation was 25Hz, and with a 1:100 transformer you get a more useful voltage. A soldering gun transformer was used in this test.

Type of material on the homopolar Faraday generator

Having experimented with Faraday homopolar generators in the past, I was interested to see the influence of the contact material on the performance of these generators. In fig. 5 there is a classic configuration with a strong gold plated neodymium magnet. No appreciable difference could be detected when contact A was copper, aluminium or gold but the difference was clearly detected when contact A was bismuth. The voltage created by friction by the metal pair gold-bismuth adds algebraically to the voltage generated by the homopolar effect. In the example of fig. 5 the voltage generated by friction is +1.5mV and will add in one case and subtract if you reverse the magnet rotation or polarity. Not every mechanical configuration will show this effect: when the contacts are symmetric there is no influence of the material used but it will be evident in case of asymmetric contacts as in fig. 5.

In all experiments we have seen that the material used is what makes the difference and there are metals or combination thereof that will work better than others; constantan, for example, seemed a better alternative to nickel based coins, but only wires were available for experiments and could not be used as a reference disc. Highly conductive ceramic materials are also interesting and some surprise might come from mercury, easy to find but problematic to use in this application because it is a liquid. It is felt that the best material is going to be some sort of hitherto unknown alloy or superconductive compound with the possibility to have friction without any physical contact, as investigated by some researchers. In the meantime, if you like to experiment with bismuth you may find it in some ammunition shops as it is used occasionally as an alternative to lead shots or order it online by some suitable suppliers such as www.scitoys.com. 

Related documents

1)  Thermoelectric series - http://www.xyroth-enterprises.co.uk/thermser.htm

2)  Triboelectric series - http://www.ece.rochester.edu/~jones/demos/triboseries.html

4)  Di Mario, D. 2001, Faraday's Homopolar Generator, Electronics World, (vol. 107-1786), Highbury Business Communications, Swanley, UK

what else?   I rather go back to the surface

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