Click here to purchase the entire book in PDF format. Magnetic flux density If you have a permanent magnet, you can measure not only its strength, but how strong it is for a given surface area. For example, you can have two magnets one big and one small, both able to lift a block of iron that weighs exactly 1 kg. Both magnets have the same strength, but the small one has a higher density of magnetic flux, because it has the same total flux distributed over a smaller area. This is also true of the magnetism stored on a piece of analog tape (which can also be considered to be a permanent magnet). Since there is a magnetic signal stored on the coating of the tape, we can think of it as a permanent magnet that has a different flux in different locations (if it didn't, the signal wouldn't change in time and we would just have a DC component). So, the magnetic flux density on the tape is a measure of the magnetic flux (in Wb) per area of tape (in m$^{2}$). Therefore magnetic flux density is measured in Wb/m$^{2}$