It is frequently necessary for economic reasons to carry out tests on short cable samples. Measurements of a.c. leakance and power factor on short lengths of power or submarine cables may be subject to considerable error unless due allowance is made for end effects, which are of minor importance on long lengths. The application of the simple guard wire, which is adequate for d.c. testing, is not sufficient to secure accurate results. The nature of the errors which arise is discussed, a theory of the cause of the errors is developed, and recommendations are made as to the form of guard which will enable accurate results to be obtained.