There is provided a reference voltage generating apparatus including: a reference voltage source, a voltage retaining circuit, a switch and a controller. Also described are devices comprising calibration phantoms; methods of using these devices; methods of formulating databases containing information regarding x-ray images; the databases themselves; and methods of manipulating the information and databases. The method can be used to create tangible items like a book or game that are written according to the method of the invention. There is a radial gap between the thermal mass and the support structure. The secondary circuit comprises a compensation component for producing a phase shift between the power-supply voltage and the power-supply current, so as to reduce the phase difference that the inductance of the induction coil produces. Efficient operation is attained automatically in that the frictional resistance applied and also the pulling profile exerted on the material during unwinding of the roll are changed as a function of weight andor other characteristics of the roll of material to be converted into cushioning product.