Power transformers are likely to break down at service voltage due to excessive water in the solid insulation. It is shown that insulation measurements between the transformer terminals and tank do not give a reliable indication of the condition of the solid insulation of a transformer in service. It is suggested that a paper probe or image would give the required information. A design of probe is described and some test results are given.A simple method of measuring the degree of water saturation of oil is described in an Appendix.