Circuit for measuring static charge
The MPF-102 used in the linked circuit is an N-channel FET see

http://en.wikipedia.org/wiki/Field_effect_transistor

Here is the MPF-102 data sheet where the manufacturer takes no responsibility for it surviving more than +/- 25V on the gate. With a 1 megohm gate resistor it will tolerate +10000V (forward gate current 10 mA) but we cannot predict what negative voltage it might tolerate. 

https://www.fairchildsemi.com/datasheets/MP/MPF102.pdf

For investigating static charges, consider making a small adjustable spark gap as an alternative to a transistor cuircuit that cannot be calibrated. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. Intensity of the spark is a guide to its shocking effect.

http://en.wikipedia.org/wiki/Spark_gap#Sphere_gap_for_voltage_measurement