Abstract: An apparatus and method for checking and measuring the momentary value of a variable physical magnitude, such as a volume of liquid in a container or tank, or a temperature, viscosity, or velocity, for example. The invention comprises a transducer free to follow without friction the variations of the physical magnitude, such as for example the level of a liquid fluid in a reservoir, the transducer having a portion of progressively varying cross-section displaceable as a function of the changes in the physical magnitude disposed in alignment with a sensor normally positioned in a reference position out of contact from the transducer.