The relationship of EerNisse’s technique for measuring small dilatations caused by irradiation with short‐range particles, which utilizes a metallized thin plate mounted as a cantilever below whose free end an electrode is placed (forming a capacitor), to a photoelastic technique and to an interferometric technique are derived. The effects of stray capacitance, the fringing field of the capacitor, the clamping stress on the cantilever plate, the electrical resistance of the metallic coating, the charging of the tank circuit of which the capacitor is an element, the flange bolting stress, and the beam heating are assessed, and examples of the manner in which they contaminate the data are given.