An instrument monitors the coangulation of coagulable fluid blood, blood fraction or a control by combining the coagulable fluid 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 causes the particles to be reoriented as the magnetic field varies. The reorientation changes as the fluid coagulates owing to the fluid's changing viscosity. An electromagnet creates the time-varying magnetic field. The electromagnet includes a core comprising a first leg an electrically conductive coil for creating a time-varying magnetic flux in the core, and a return for the time-varying magnetic flux. The first leg and the flux return together comprise a generally H-shaped core of ferromagnetic material. A bobbin is provided on the first leg and the electrically conductive coil is wound on the bobbin.