1. For this purpose an a.c. bridge (impedance comparator) and a measuring cell of glass were used. At the site of radiation entrance and exit the solution was bounded by 150μm polyethylene foils.

2. The main developments are the use of four electrodes, to reduce the effect of variations in electrode impedance; a method of recording output which does not require the balancing of an a.c. bridge; and a method of restoring base line with minimal change in sensitivity.