The Science Behind It
The Tone Magnet was invented through a long process of experimentation, followed by a short process of lots of things coming together at once!
The original idea came simply from the popular conviction that, aside from pickup output, the main variations in tone from different guitars had something to do with the mass of the body. The debate about 'tonewoods' and aftermarket parts could be boiled down to the question: does my guitar sound different with extra weight on it?
Christopher Ferguson saw an opportunity to make a temporary adjustment to a guitar by applying a magnet to the bridge of his stratocaster. In the summer of 2015, he began with a rectangular ferrite magnet, weighing about 80 grammes. While he thought he could discern a difference, he was frustrated that the pull of the ferrite magnet was not enough to keep it on the bridge.
Seeking confirmation, Ferguson brought his idea along to the DMEM department at the University of Strathclyde. There, under the direction of the inventor of the Gripster, Jose Hernandez, John Shiell began experimentation to determine the effectiveness and parameters of a magnetic attachment to the bridge of the guitar.
Late on in the experimentation, Shiell decided to test with a lighter and stronger neodymium magnet and was surprised to find that the effect was similar to having a more massive, weaker magnet. He presented his findings to Ferguson, hypothesising that the magnetic flux was having an effect on the bridge and strings of the guitar. The exact nature of this effect became the basis for the patent application now pending on the Tone Magnet.
Working together, Shiell and Ferguson came up with a design that allowed the use of rare-earth neodymium magnets in a non-reactive shell. Using aluminium and ABS plastic, the Tone Magnet's shape is intended to recall the space-age sleekness of the Roger Meyer Octavia.
How does it produce the effect that has so interested players? Well, the most likely explanation comes from the effect of the vibration of strings affected by the magnetic field of the Tone Magnet. Consider the Tone Magnet as an open surface, and imagine that the vibration of the guitar's strings are moving the surface boundary. We can therefore intuit that the resulting change in magnetic flux will cause an electromotive force. This is expressed in Faraday's Law of Induction, which is often expressed as:
The induced electromotive force in any closed circuit is equal to the negative of the time rate of change of the magnetic flux enclosed by the circuit.
Jordan, Edward; Balmain, Keith G. (1968). Electromagnetic Waves and Radiating Systems (2nd ed.). Prentice-Hall. p. 100.
This is a simplification of the law, which is shown in notation as:
Essentially this means that the strings moving within the surface of a magnetic field would generate an electrical charge. This charge, for our purposes, would be very small, but offers an explanation for the change in tone heard when the Tone Magnet is applied to a guitar.
The oddest coincidence in the design of the Tone Magnet is, while Ferguson did not even consider it as a factor in the original conception, Faraday's law did actually feature in his 2012 thesis as part of a discussion of the science around the hydro-electrics of the early 20th century. (http://hdl.handle.net/1842/7545 pp. 188-197)
ADDENDUM: Chris has blogged about the initial findings from DMEM at Strathclyde here.