In other words the injured area must fall within the (accurate) maximum penetration range of the product. If the magnetic field doesn't reach the area it won't be effective (see diagram below).

Magnetic field strengths can accurately be determined using standard mathematical formulas based on the laws of physics. Please see the Article "Comparing Magnetic Therapy Products" for a chart that provides accurate maximum penetration depths for different types and sizes of magnets.

Therion defines maximum penetration depth as the distance at which the magnetic field strength drops to a level of 1 gauss, which is twice the strength of the earth's magnetic field.

We use 1 gauss as a basis for a very good reason. Since a stronger magnetic field will always overpower a weaker magnetic field, any magnetic field stronger than the earth's magnetic field (approx. 0.5 gauss) will have some therapeutic effect.

Unfortunately many companies state penetration depths that sound impressive, but are either inflated or completely inaccurate for health purposes.

They use a much lower basis (0 or 0.5 gauss), but don't tell you. While this method increases the stated maximum penetration depth by up to 6 times, it is meaningless because the magnetic field drops below therapeutic levels long before that distance is reached.

For example a 1/2" diameter x 1/16" thick neodymium magnet has a maximum penetration depth of 2.8" using Therion's method. If zero gauss is used as the basis, the penetration depth becomes 16"! But in reality, the product is not effective for therapeutic purposes beyond 2.8".