An improved method of testing paper insulation for moisture by the measurement of dielectric dispersion over the frequency range 0.5–50 c/s or over the time range 3–300 millisec has been investigated. A direct-reading portable instrument giving immediately-significant results has been developed for dispersion testing, and its suitability for the testing of paper-insulated bushings, busbars, transformers and cables is now under investigation. An alternative method of detecting moisture in paper insulation which has been examined is the determination of non-linearity in the relationship between voltage and current. Although comparable with the dispersion method in sensitivity the non-linearity method has not been developed because of the greater difficulty in adapting it to field use.The testing of installed insulation for discharges in voids involves problems of interpretation and of portability. The nature of these problems is examined particularly in relation to a portable a.c. discharge detector developed for this kind of testing.