In diagnostic radiology the practical peak voltage was initially derived by postulating that, for a given combination of X-ray tube and contrast geometry, a constant X-ray tube voltage should produce the same low level contrast as an arbitrarily pulsating X-ray tube voltage. It has been shown previously that the practical peak voltage can be properly defined as a weighted average of the X-ray tube voltage. Up to now the concept of practical peak voltage was based entirely on the results of calculations. The present paper describes the experimental investigations for measuring and comparing the contrast-equivalent X-ray tube voltage and practical peak voltage derived from an invasive measurement of the time-dependent X-ray tube voltage. Within the experimental uncertainties, the results demonstrate the mutual equivalence of the practical peak voltage and the contrast-equivalent X-ray tube voltage.