embodiments of the apparatus and method disclosed herein and further features which may be used will now be described with reference to the accompanying drawings . this description is exemplary in nature and is not intended to limit the scope of the subject matter disclosed and claimed . fig1 shows a general arrangement of nmr apparatus for examining properties of fluid flowing along a pipeline 10 in the direction indicated by arrows . in the portion a , the pipeline 10 is made of non - magnetic electrically insulating material such as fibre reinforced polymer . the pipeline 10 extends through a uniform magnetic field between a pair of permanent magnets 11 . within this field there is at least one radio - frequency coil 12 encircling the pipeline and used to emit radiofrequency pulses to induce magnetic resonance and also to receive signals from nuclei undergoing resonance . the magnets 11 , the coil 12 and associated electronics for creating radiofrequency pulses , observing radiofrequency emissions and storing data may all be conventional in construction and operation . upstream of this portion a of the apparatus there is a polarizing portion b . here too the pipeline 10 is made of non - magnetic material although electrically conducting material such as aluminium or stainless steel may be used . magnets 13 provide a magnetic field to polarize resonant nuclei in the liquid flowing in the pipeline 10 , before the liquid reaches the magnetic resonance portion a . these magnets may be made of material with so - called high - temperature superconducting properties ( superconductivity above 77 k ) such as yttrium barium copper oxide ( ybco ), bismuth strontium calcium copper oxide ( bscco ) and the materials mentioned in w02007 / 045929 and in coombs et al , superconductor sci . & amp ; tech . volume 21 , article 034001 ( 2008 ). the portion c is a trapping section for magnetizable material in the flowing fluid . possible embodiments of the trapping section c will next be described . fig2 shows a simple possibility for trapping magnetizable solids . a branch pipe 14 is connected to the pipeline 10 through a gate valve 16 and is closed at its other end by gate valve 17 . the branch pipe 14 is surrounded by a solenoid coil 18 . in operation with valve 16 open , valve 17 closed and the solenoid coil 18 energised , the magnetic field within the solenoid coil attracts magnetizable solids out of the fluid stream in the pipeline 10 into the branch pipe 24 . periodically the valve 16 is closed , the coil 18 is turned off and magnetizable solids which have collected in the branch pipe 14 are discharged through valve 17 . during this removal of accumulated magnetizable solids , flow along the pipeline 10 may be temporarily halted by closing valves 19 . fig3 shows a possible arrangement of the trapping section c with traps in parallel . the incoming pipeline splits into two parts 22 which lead to valves 24 , 25 . further on the two flow paths pass through valves 26 , 27 and then rejoin . flow in the rejoined path 28 continues into the polarizing section b and the magnetic resonance section a . between the valves 24 and 26 and between the valves 25 and 27 there are lengths of pipe 30 , 31 made of non - magnetic material such as aluminium or stainless steel and attached at flanges 32 , 33 . these lengths of pipe 30 , 31 pass through magnetic fields transverse to the pipe created by pairs of magnets , one above and one below the pipe . the magnets above the pipes are indicated 34 in fig2 . while flow is passing through a pipe 30 or 31 , the magnetic field draws any magnetizable contaminants to the pipe wall and holds them there . thus , each length of pipe 30 , 31 functions as a trap for magnetizable contaminants . the valves 24 - 27 are used to direct flow alternately through the lengths of pipe 30 and 31 . so , when valves 24 and 26 are open and flow passes through pipe 30 , the valves 25 and 27 may be closed . the length of pipe 31 can then be temporarily detached by unbolting at the flanges 33 , removed from the magnetic field , cleaned out and replaced . subsequently , when valves 25 and 27 are open to direct flow through pipe 31 , the valves 24 and 26 can be closed , allowing pipe 30 to be unbolted at flanges 32 , removed , cleaned out and replaced . fig4 shows another possible arrangement for the trapping section c . the pipeline 40 follows a serpentine path and has two traps 41 , 42 in sequence at low points of the pipeline . each of these traps has a gate valve 44 opening and closing connection between the pipeline 40 and a length of pipe 46 connected at flanges 48 . to provide a magnetic field for trapping magnetizable contaminants , a probe consisting of rod shaped steel pole piece 50 attached to a permanent magnet 52 is mounted so that the pole piece 50 extends through a seal 54 into the pipe 46 . each probe is movable between two positions . as shown in the trap 41 at the left of fig3 , the probe can be pushed inwardly towards the pipeline 40 so that the distal part 56 of the pole piece 50 projects through the gate valve 44 into the pipeline 40 . in this position the magnetic field from the magnet 52 attracts any magnetizable material in the flowing stream onto the pole piece 50 and holds them on it . when it is desired to remove accumulated contaminants from one of the traps , the probe is withdrawn longitudinally to a position as shown in the trap 42 at the right of fig3 . the distal part 56 of the pole piece 50 has moved down into the pipe 46 , taking accumulated magnetizable material with it . the valve 44 can then be closed and the pipe 46 can be detached at the flanges 48 allowing the pipe 46 and the probe 50 , 52 to be removed , cleaned of accumulated magnetizable material and then replaced . the two traps 41 , 42 are operated alternately . at all times one or other of the traps has its magnetic pole piece 50 pushed in , as shown in trap 41 at the left of fig3 , so that it can attract and retain magnetizable material . periodically , the probe 52 , 50 of one or other of the two traps is moved to the position as shown by the trap 42 , for the accumulated magnetizable material to be removed from it . fig5 and 6 show a further possibility for a trapping section c . a pipeline 60 carrying flow in the direction shown by arrows has two traps at position 62 and two more at position 64 . fig6 is a cross section at position 62 showing that the two traps at this position extend downwardly at an angle at each side of the pipeline 60 . the traps are somewhat similar to the traps in fig4 . each trap communicates with the pipeline 60 through a gate valve 44 and has a pipe 46 attached at flanges 48 . the traps have probes comprising electromagnets 69 and pole pieces 66 , 67 which extend through seals 54 . the probes are movable between two positions . a pushed - in position is shown by pole piece 67 and also at the right of fig5 . in this position the distal part 68 of the pole piece extends across the pipeline 60 and its magnetic field attracts and holds any magnetizable solid material . the pole piece can also be drawn back into the pipe 46 , through gate valve 44 , as illustrated by pole piece 66 . when the pole piece is drawn back , as in the case of pole piece 66 , it carries accumulated magnetizable material with it into the pipe 46 . after a pole piece has been withdrawn as shown by pole piece 66 , the gate valve 44 is closed and the electromagnet 69 for that pole piece is switched off . accumulated magnetizable material on the pole piece 66 then falls off into the pipe 46 , and can be discharged through a valve 71 . the four traps are operated in a sequence such that at any time at least one trap has its pole piece pushed in and extending across the pipeline 60 . while the pole pieces of the two traps at position 62 are moved to allow one of the traps to be cleaned out , one trap at position 64 always has its pole piece pushed in and functioning to collect magnetizable material . correspondingly one trap at position 62 has its pole piece in the pushed in position throughout the time that pole pieces of the traps at position 64 are moved between their two positions . fig7 shows a single trap with some resemblance to the traps in fig4 . as in fig3 , a low point of pipeline 40 is connected to a branch pipe 46 through a gate valve 44 . a probe consisting of a pole piece 73 and an electromagnet 69 is located above the pipeline and extends into the pipeline through a seal 75 , so that the distal part 76 of the pole piece 73 , magnetised by the electromagnet 69 arrests and holds magnetizable solids entrained in the flow stream along the pipeline 40 . periodically , flow is stopped by closing valves 78 . the pole piece 73 and the electromagnet 69 are lowered , so that the distal part 76 of the pole piece carrying magnetizable solids accumulated on it passes through the open valve 44 into the branch pipe 46 . the electromagnet 69 is then switched off , so that the magnetizable solids fall from the pole piece 73 into the branch pipe . the pole piece and electromagnet are then raised back to the position shown , with the electromagnet still switched off . the valve 44 is closed and the magnetizable solids in the branch pipe 46 are discharged by opening the valve 79 . fig8 shows a further possibility for a single trap . the pipeline 80 has a downwardly inclined tubular section 82 at the lower end of which a branch pipe 46 is connected to the pipeline 80 through a gate valve 44 . a magnetic field to arrest magnetizable solids is provided by a pair of permanent magnets 84 located at either side of the inclined section 82 . these magnets 84 are supported on pivoted arms 86 . periodically , to remove accumulated magnetizable solids from the inclined section 82 , flow is stopped with by closing valves 88 and the arms 86 and magnets 84 are swung to put the magnets 84 at the position shown by a dotted circle 87 , with the magnetic field now extending through the branch pipe 46 . this movement of the magnetic field allows and assists the accumulated magnetizable solids to slide down the inclined section 82 into the branch pipe 46 . the gate valve 44 is then closed , the magnets 84 are then swung back to their original position and the magnetizable solids are discharged from the branch pipe 46 by opening valve 89 . it will be appreciated that the diagrams shown here are schematic and do not show the equipment used to move the magnets 52 , 69 and pole pieces 50 , 66 , 67 73 between positions , nor the mechanical handling equipment used to move detached pipes 30 , 31 or 46 . many modifications are possible and features used in one embodiment illustrated here may be utilised in another embodiment . specifically , the single traps shown in fig2 , 7 and 8 could be used in an arrangement with two traps in parallel or in sequence . any of the trapping sections shown in fig2 to 8 could be used with the resonance section a but without a polarising section b if so desired . magnetic fields of any of the traps could be provided by electromagnets instead of permanent magnets , or could be provided by movably mounted permanent magnets in place of electromagnets . all such modifications are intended to be included within the scope of this disclosure .