The Facts About NMR
A common claim with MFD proponents is that "all elements have a natural frequency" and will either resonate with other like elements, or can be made to resonate with a properly tuned signal generator. MFD proponents often point to the fact that all elements have a property called "nuclear magnetic resonance" and, therefore, the concept of resonance is entirely scientific.
Yes, it is true that all elements have an NMR frequency. You can go to WebElements and click on an element, then click the NMR link to the left side. For gold, you will find that the NMR frequency is 1.754000MHz, and that this entry includes the statement "relative to 1H = 100 (MHz)". What does this mean?
It turns out the NMR frequency for any given element is dependent on the static magnetic field the element is exposed to. For gold, the magnetic field that produces an NMR frequency of 1.754MHz is the same field that produces an NMR frequency of 100MHz for hydrogen.
What is this field? With a little effort, you will find the field strength to be roughly 2.35 Teslas. So for hydrogen, 2.35T yields an NMR frequency of 100MHz. 4.7T will result in an NMR frequency of 200MHz. In other words, the NMR frequency is proportional to the magnetic field.
What about the Earth's natural magnetic field? This varies from place-to-place, but 50 microTeslas (uT) is a fair average strength. So the magnetic field strength of 2.35T is a whopping 47,000 times stronger than the Earth's field. Working the other way, we can find that the NMR frequency of hydrogen exposed to the Earth's field is a mere 2.13kHz. And guess what? That's exactly the frequency we get from a proton precession magnetometer! Most PPM's use hydrogen-rich water as the precession medium, and it is the hydrogen that is doing the precessing. Variations in the Earth's field due to iron targets change the precession frequency, exactly because the NMR frequency varies with field strength.
So gold has an NMR frequency of 1.754MHz at 2.35T, which means at 50uT it will have an NMR frequency of only 37 Hz or so. So if any MFD were being true to the NMR property, it would use 37Hz for gold. Not 5kHz. Now, some MFD manufacturers talk about resonating elements at a harmonic (or, sometimes, a "sub-harmonic") frequency. Although 5kHz is roughly the 135th harmonic of 37Hz, it is far, far less efficient to try to resonate something at a harmonic rather than the fundamental. Anyone who has used 3rd overtone crystals is aware of this, and claiming resonance at the 135th harmonic is just plain absurd.
So now that we know what the real frequency of resonance should be, we can proceed with a gold detector, right? Not so fast. Let's go back to the proton mag. How does a PPM detect the precession frequency of water? Well, typically a small bottle of water is placed INSIDE a fairly hefty coil. The coil serves two purposes. First, water just sitting around has its molecules all randomly oriented, so that even if they were "resonating", there would be no net signal due to an overall cancelation of all the little signals. So the coil is hit with a large transient current, which generates a large magnetic field, which serves to align at least some of the water molecules. Then, with the transient field removed, the coil becomes a receiver to detect the very, very weak precession signal from the water.
So in order to utilize NMR, we need to "ping" the target to get it to precess, much like hitting a bell with a clapper. Then, we need a way to detect the precession signal, which is incredibly weak. With PPM, both of these are only accomplished when the water is INSIDE the coil. The same is true with hospital MRI machines... the patient is slid INSIDE a humongous coil.
In the end, the concept of trying to remotely resonate buried targets is just bogus. It is like trying to boil water with a microwave oven, by placing the water 100 meters from the oven, and then running the oven on a 9-volt battery. Ain't gonna happen.
- Carl
But for 0,5T = 100Amper current or For 1,5T Need (=) 300 Amper electrical current.
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