Abstract: A test vessel assembly comprises a central test vessel defining a chamber in which a sample to be tested may be stored. A pair of side adjustable electrodes is received in the chamber and immersed in the sample under test to determine the breakdown voltage of the sample. A gap between the electrodes can be adjusted by respective electrode adjusting moved in and out of the test vessel by rotation of an associated adjusting wheel. To prevent the breakdown in air rather than in the sample, care is taken to ensure sufficiently large creepage and clearance distances between the connections to the electrodes and from the connections to the electrodes to a wall of the test chamber. To this end, the adjusting wheels include convolutions that mesh with corresponding convolutions in the test vessel.