slug 105 is exposed to the magnetic field of proper intensity and rf tone bursts to accomplish the requisite polarization the slug 105 is relatively narrow and is more sharply defined while there may be other intensities such as field strength h 1 and h 2 the interaction between the rf frequency and field intensity is not accomplished except at the slug 105 within the gradient magnetic field defining the slug 105 the narrow width obtained by field gradient is true of the tag coil 106 this narrow width can also be obtained in the detector coil 107 while the coil 107 may be quite long there is a region within the coil 107 where the magnetic field strength again is the requisite value h o this defines a flowing slug 108 which is exposed to the interacting field strength and rf frequency so to speak the use of magnetic field gradients narrows or shortens the effective length of both coils the incorporation of additional coil turns and the extension of the coils lengthwise along the pipe does not increase their size with the requisite decrease in accuracy or sensitivity accordingly fig3 shows a system whereby magnetization across the pipe 103 is accomplished in the region of the tag coil 106 there is a gradient in the magnetic field which provides the requisite field strength h o to interact with the coil frequency f o to shorten or narrow the flowing slug of material excited by the transmitted rf pulse this shortened flowing slug of material.