A description is given of a three‐terminal cell for dielectric measurements on polymeric materials over a wide temperature range. Use is made of a lightly spring‐loaded, floating electrode to maintain good electrical contact with the specimen when it expands or contracts thermally. A differential‐capacitor, built into the movable electrode, is used with a simple detector circuit to indicate the position of the electrode with respect to its micrometer‐driven carriage. Simultaneous measurements can be made of cell capacitance and electrode separation. The latter can be determined with a precision of better than 10−4inch. Provision is made to confine the specimen when it softens at temperatures above its glass‐transition temperature. The cell has been operated successfully at frequencies between 20 cps and 1 Mc and at temperatures between 20 and 200°C.