CONSTRUCTION HINTS                         (c)1996 William J. Beaty
amasci.com/emotor/vdg.html


USE A METER

When building or debugging a VDG machine, it helps to know that VDGs are
actually constant current sources.  Their fundamental goal is to always
produce the same current regardless of whatever electrical resistance is
placed between their terminals.  This is the opposite of a battery, which
is a constant voltage source.  A battery tries to keep the voltage across
its terminals always constant, and the SMALLER the resistance placed
across its terminals, the higher the current and the higher the wattage
produced. Shorting out a battery creates heat and "draws" a high current. 
On the other hand, a VDG tries to keep the current flowing across its
terminals always constant, and the LARGER the resistance placed across its
terminals, the higher the voltage and the higher the wattage produced. 
NOT shorting out a VDG creates heat and "draws" a high voltage.  To turn
off a battery, remove any load from it so that the battery sees infinite
resistance.  To turn off a VandeGraaff, connect a short from sphere to
base, so the VDG sees zero resistance. 

And so to work on optimizing your VDG, you should short out your VDG and
measure the current in the short.  To do this, simply connect a sensitive
current meter between the sphere and the base, or between the upper and
the lower combs.  A current meter IS a short circuit, so no other shorting
wire is needed.  Select a meter which can indicate around 100uA (100
microamperes).  When the VDG is operating, the meter will show you the
maximum current produced by the device.  A 5 microamp flow is OK for a
low-speed VandeGraaff.  A bigger one with a high speed belt might put out
several hundred microamps.  More is better.

So, when designing a new VandeGraaff, how can you pick the best belt
material, best comb spacing, etc?  Let your VDG tell you itself.  Simply
short out your generator with a microamp meter and measure the current.
Adjust things to get the highest value on your meter.  Will a plastic belt
work better?  Should you use many needles on the comb or few?  Just run
your VandeGraaff and measure its current, and you'll know.