We found this circuit in a 1967 RCA SCR manual. The circuit uses components from the era. The bridge diodes must withstand the current of the controlled load.
This circuit comes from an English publication of 1976, consisting of an experimental touch key....
This touch switch appears elsewhere on this site from various documentation. This is from an 80's...
We found this measurement circuit with a logarithmic scale in an Electronics Now from November 1995....