I accept what John G. writes about CBL because I had no prior knowledge
here. But what I read about MBL (ULI) conflict with two things:
1) About 3 years ago when I started using ULI I was told that the imput R
is at least 5 giga-ohms ("because it is the Field effect transistor").
2) I know that discharging a low leakage C through ULI, while measuring the
d.o.p., is much much slower that through most voltmeters. My observations
were that R is always of the orders of giga-ohms. Too bad that CBL do
work as very high input impedance voltmeters.