An instrument (100) monitors the coagulation of coagulable fluid blood, blood fraction or a control (514) by combining (101) the coagulable fluid (514) with particles which are affected by a magnetic field so that the particles become suspended relatively freely in the fluid. A time-varying magnetic field (2 Hz) causes the particles to be reoriented as the magnetic field varies. The reorientation changes as the fluid (514) coagulates owing to the fluid's (514's) changing viscosity. An electromagnet (140) creates the time-varying magnetic field. The electromagnet (140) includes a core (150) comprising a first leg (148), an electrically conductive coil (144) for creating a time-varying magnetic flux in the core (150), and a return (152) for the time-varying magnetic flux. The first leg (148) and the flux return (152) together comprise a generally E-shaped core (150) of ferromagnetic material. A bobbin (146) is provided on the first leg (148) and the electrically conductive coil (144) is wound on the bobbin (146).
