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.