About six years ago I purchased two 30cm stainless steel spheres from a
gift shop that was clearing out old stock very cheaply. As soon as I saw
them, I knew I would build a Van de Graaff generator using them.

It has taken me a long time to realise the machines (I have built two
identical machines, both negative output)

The generators are self excited with a lower nylon roller mounted
directly on the shaft of a 35W 230V shaded pole motor (~2000 RPM). A
Teflon insert insulates the roller from the motor shaft. The belt is
thin flexible PVC film sold here for protecting dining tables. The
column is 90mm PVC stormwater pipe and is 1 m tall, it has been sanded
and polished to a gloss finish.

The upper roller is Teflon, and rotates on ball bearing races (recovered
from hard disk drives). The terminal sits by its own weight on the top
of the column.

The charging current is 3 - 4 uA and the terminal attains the maximum
charge it can contain- at this point small purple discharges appear at
random points around the terminal with a popping sound. I believe this
to be 400 - 450kV.

Sparks in excess of 40cm are possible depending on the curvature of the
grounded electrode, although these can be faint. Brighter sparks between
20 and 30 cm are obtained with a spherical grounded electrode. The one
in the pictures is 20cm diameter.

Very long and bright sparks down the length of the column can be also be
obtained.

Most of the spark pictures have been obtained with 30 second exposure
times on the camera, showing multiple discharges.

Due to the small diameter of the rollers (20mm) idler pulleys were
required to maintain separation of the two sides of the belt- as charge
built up, the belt would stick together and stall the motor. The white
protrusions halfway up the column are the supports for the ends of the
pulley shaft.

Julian Phillips
New Zealand
February 2011