a layer arranged between the layers 27 28 this of course assumes that at least one layer is also transparent to beams it may also be the case that the electrode 29 is one of the layers necessary for forming the beams and hence performs two functions this structure is equally suitable for beam sources and for beam sinks detectors fig4 shows another part of a device wherein several elements are arranged behind or above one another here it is possible to discern for example two radiators 30 and 31 each of which emits beams having definite properties such properties are for example different wavelengths of the radiation different frequencies or modulations etc fig5 shows a device for a test material 32 with a beam source 33 and a beam sink 34 which are positioned in relation to one another in such a way that a measuring gap 35 is formed for the test material 32 which is preferably moved in its longitudinal direction so that a relative movement corresponding to an arrow 36 arises between the device or the measuring gap 35 and the test material 32 the mode of operation of the device according to the invention is as follows with a view to registering parameters in respect of an elongated test material 1 the latter is moved in a manner known as such and therefore not described here in any detail past a sensor as represented by elements having reference symbols 2 5 8 17 etc with this device it is intended to register the bulk capacitively for example and optically the diameter or cross section of the test material or alternatively the irregularities thereof over the length however instead of or in addition to the irregularities it is also.