nuclear magnetic resonance apparatus for measuring properties of a fluid stream flowing within a pipeline has one or more magnet systems for applying magnetic field to the fluid stream and also has means for inducing and observing magnetic resonance within the fluid stream as it passes through a said magnetic field . the apparatus may also include a polarizing magnetic field upstream of the magnetic field in which resonance is observed . the fluid stream may be hydrocarbon from an underground reservoir . in order to guard against accumulation of magnetisable iron debris particles entrained in the fluid flow , the apparatus comprises one or more upstream traps having a magnetic field to attract and hold solid magnetizable material and an exit path for the removal of the solid magnetizable material so that it does not continue towards any polarizing field and the field where resonance is observed .