magnetic assay signal and then display and store the assay results in the appropriate context for example an assay to measure the presence of e coli would likely have results displayed in a different form than an assay testing for the presence of binding of oligonucleotides since the substrate may be prepared specifically for each kind of assay this information can be encoded on the substrate as a bar code or using one of the techniques described above in this particular exemplary embodiment motor 16 rotates wheel 14 which is connected to disc 12 by shaft 13 through a 120 tooth worm gear reduction of course rotational drives having different particulars could also be employed a magnetic field source 21 may be moved linearly with respect to disc 12 by a rotational device such as a stepper motor 22 having a 40 turn per circle lead screw 23 on a motor shaft 24 a boss 25 is configured with a hole having internal threads to which the spiral lead screw threads are coupled the control signals are applied from microcomputer 65 to motor 22 through a number of wires 26 again the specifics of the rotational drive are set out here as an example only other appropriate elements having different characteristics could also be used for example while the above system describes a situation where the magnetic field source is moved linearly with respect to the sample holder another embodiment may be used in which the sample holder is moved relative to the magnetic field source in this latter embodiment the sample holder may be mounted to a shaft and mechanical drive system similar to the drive system shown in fig3 the drive.