The Toroid Illustration (Fig. 8.26)

In Chapter 8 (Biomagnetism) of Intermediate Physics for Medicine and Biology, Russ Hobbie and I show an illustration of a nerve axon threaded through a magnetic toroid to measure its magnetic field (Fig. 8.26).

Fig. 8.26. A nerve cell preparation is threaded through the magnetic
toroid to measure the magnetic field. The changing magnetic flux in
the toroid induces an electromotive force in the winding. Any external
current that flows through the hole in the toroid diminishes the magnetic field.

While this figure is clear and correct, I wonder if we could do better? I started with a figure of a toroidal coil from a paper I published with my PhD advisor John Wikswo and his postdoc Frans Gielen.

Gielen FLH, Roth BJ, Wikswo JP Jr (1986) Capabilities of a Toroid-Amplifier System for Magnetic Measurements of Current in Biological Tissue. IEEE Trans. Biomed. Eng. 33:910-921.

Starting with Figure 1 from that paper (you can find a copy of that figure in a previous post), I modified it to resemble Fig. 8.26, but with a three-dimensional appearance. I also added color. The result is shown below.

An axon (purple) is threaded through a toroid to measure the magnetic field.
The toroid has a ferrite core (green) that is wound with insulated copper wire
(blue). It is then sealed in a coating of epoxy (pink). The entire preparation
is submerged in a saline bath. The changing magnetic flux in the ferrite induces
an electromotive force in the winding. Any current in the bath that flows
through the hole in the toroid diminishes the magnetic field.

Do you like it?

To learn more about how I wound the wire and applied the epoxy coating, see my earlier post about The Magnetic Field of a Single Axon. The part about "any current in the bath that flows through the hole in the toroid diminishes the magnetic field" is described in more detail in my post about the Bubble Experiment.