This very high input impedance JFET circuit was found in the 1995 Encyclopedia of Electronic Circuits Volume 5. The JFET is a general-purpose transistor as is the output transistor.
This General Electric circuit is from the 80's. The PUTs used in this meter are not common...
This circuit provides a power of the order of 2 W, characterized by high performance. The output...
The sensor can be made using two bare wire or two screen with a piece of tisse between them. One...