As a follow up to this question of primary and secondary currents and their ratios, you could do the following exercise, using the spherical head model in Brainstorm, to demonstrate the role of primary and secondary currents, without the need to program the primary model. You will need to adjust the dipole orientation or the sensor orientations.
Let's put a dipole on the z-axis ("upward" axis), in a hemispherical array of MEG sensors, spherical head centered on 0,0,0. (See our conference paper https://neuroimage.usc.edu/paperspdf/error_92.pdf or the full journal paper https://www.sciencedirect.com/science/article/abs/pii/001346949390043U where we discuss such theoretical arrays.)
P = scalar magnetic field at the MEG sensor due to the contribution of the primary current.
V = scalar magnetic field at the MEG sensor due to the contribution of the volume current.
T = scalar magnetic field at the MEG sensor due to the contribution of the total current.
From your reference:
Eq(12) is the ratio T/P
Eq(13) is the ratio P/V
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Regardless of the sensor orientations, orient the dipole upwards (z orientation), i.e. a radial dipole in a perfect sphere. Then P = V, but T = 0, since V exactly cancels P. The total field is exactly zero everywhere, regardless of sensor location or orientation, because volume current contributions exactly cancel primary current contributions. So your Eq(12) is identically 0, and Eq(13) is identically 1.
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Now orient the dipole in the x-direction (i.e. a tangential dipole). Now orient all of the MEG sensors in the radial direction. In this special well-known configuration (radial sensors outside a perfect sphere), the volume currents contribute absolutely nothing to the radial field, so T = P, and V = 0. So (12) = 1, and (13) is undefined.
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For the same dipole and the exact same MEG sensor locations as in 2), simply orient all MEG sensors in the x-direction. Now the primary current contributes exactly nothing (because the dipole is also oriented in the x-direction), and the total field is just due to the volume currents. So (12) is undefined, and (13) = 0.
Any other orientation of the MEG sensors is a variation of these two extremes. The Sarvas model used in Brainstorm (eq (10) of the above reference paper https://neuroimage.usc.edu/paperspdf/IEEEBME99.pdf) calculates the magnetic field for any arbitrarily oriented MEG sensor outside of a sphere, which is always due to the total current field.
If you want to study the vector magnetic field (not just the scalar magnetic field in the direction of the MEG sensor), duplicate each sensor location to be actually three collocated sensors, oriented in the three x, y, and z directions. The head model calculation now gives you the vector magnetic field at each point outside the head, due to the total current flowing in the sphere.
But I'm uncertain as to what additional insights you hope to gain by attempting to parse out volume from primary in this external vector field.