The generator was invented by Robert Van De Graaff in 1930. As can be seen it is made from a 568ml lager can and some plastic pipe fittings. The motor is from an old tape recorder and runs with an 8 volt PSU.
The top electrode is made from a 4mm brass rod and a glass tube from an old fuse. The lower electrode (right) is made from a 4mm brass rod which fits on to the motor and slides into a short length of PVC rod, the static charge is picked up using a short length of flex. The charge is transferred between the 2 electrodes using a 6″ half inch rubber band. It is a bit fiddly to get the band to run straight, so a few turns of PVC tape are added to the centre of the PVC rod.
The capacitor (*condenser) is made from 4 cans, 2 inside and 2 outside with a PVC tube 360mm long and 65mm diameter which gives a capacitance of 425pF. The spark gap is about 19mm so the voltage is around 20kV and takes about 25s to charge depending on the humidity.
- * I can see why they called it a condenser, the spark is a lovely thick blue colour but without it the spark is very thin! They were orginally called Leyden jars, and I think Marconi used the word ‘jar’ as a unit of capacitace.
For such a simple device the spark is quite impressive!
More about the electrodes: Although the generator looks simple the theory is quite complicated. It works by making use of the triboelectric effect. Materials that gain -ve charges are Teflon, Silicon and PVC, materials that gain a +ve charge (ie tendency to give up electrons) are Air, Rabbit fur and Glass. Because the lower roller is removing electrons to ground the top roller becomes +ve and so does the “dome”.
In the future I would like to increase the capacitace and make some more ‘condensers’ but of course that would entail drinking more Lager!
Nothing like a flashing flourescent tube…