A
Anonymous
Guest
In that long exchange over the past week about how targets respond to the field, all the talk was about conductive metal objects.
Then, there's dirt. Specifically, the stuff with magnetically lossy minerals in it, which for convenience I often lump together into the category "maghemite" or "the red stuff" although technically this is an oversimplification.
It isn't conductive, and it doesn't care about e.m.f. fields. It includes single-domain particles of varying sizes and multidomain particles with a variety of crystal lattice defects, and a variety of anisotropies. The microscopic magnetic domains in these minerals can exist in two or more states, which have different energy levels. The difference between some of these energy levels is small enough that they can be tripped over the edge by thermal agitation at ordinary temperatures.
When the dirt gets hit by a change in the magnetic field, a whole lot of realignment happens almost instantaneously, as all those little energy differences that are smaller than the change in the external magnetic get kicked past their limits and fall into new states which, on the average, will be more in alignment with the (new) external magnetic field. All this happens in synchronization with the field, and does not normally produce a signal in the receiver of a PI machine. (VLF/MF machines, whole different story.)
Due to thermal agitation, something else also happens. Suppose (for ease of explanation) that the transmit current is held constant. Some microscopic magnetic domains will have been pushed nearly to the trip point of their next energy state. The ones which are within the proverbial gnat's tail of the next energy state will quickly be kicked by thermal agitation into the next state. The ones which require more energy to be kicked into that next state will, on the average, have to wait longer until a thermal fluctuation with enough energy comes along and provides the kick. So, as the current is held steady, over time the response in the soil minerals continues to occur at a decreasing exponential rate, as the easier state transitions have already been kicked and all that remain are a few obstinate holdouts.
In most PI machines, we don't look at what happens during the transmit period. But, the same thing happens after flyback, and it's affected by what happened during transmit.
When the transmit field is removed by flyback, the external field (primarily earth field) which the soil magnetic particles are immersed in changes strength and orientation, and again, most of the energy state differences will be small enough to respond instantly. And, those energy states which have almost but not quite enough energy to come into realignment, will be kicked into alignment by thermal agitation, the easy ones first and then the tougher ones.
What may not be obvious at first, is that the longer the transmit current was held on, the more magnetic particles capable of being kicked by the combination of field difference and thermal agitation came into alignment with that condition. Therefore, when the transmit current is turned off (by flyback), a greater number of particles capable of being kicked is available. If the transmitter current was on for a long time, quite a few "toughies" managed to get kicked into new energy states by thermal agitation, and after the current is turned off, these same "toughies" are susceptible to getting kicked back to where they came from. This stretches out the tail seen after flyback.
All the above is a gross oversimplification, but it explains why transmit on-time is very important to ground balancing the red stuff in PI metal detectors, even though it isn't all that important to metal objects. The physics is entirely different.
--Dave J.
Then, there's dirt. Specifically, the stuff with magnetically lossy minerals in it, which for convenience I often lump together into the category "maghemite" or "the red stuff" although technically this is an oversimplification.
It isn't conductive, and it doesn't care about e.m.f. fields. It includes single-domain particles of varying sizes and multidomain particles with a variety of crystal lattice defects, and a variety of anisotropies. The microscopic magnetic domains in these minerals can exist in two or more states, which have different energy levels. The difference between some of these energy levels is small enough that they can be tripped over the edge by thermal agitation at ordinary temperatures.
When the dirt gets hit by a change in the magnetic field, a whole lot of realignment happens almost instantaneously, as all those little energy differences that are smaller than the change in the external magnetic get kicked past their limits and fall into new states which, on the average, will be more in alignment with the (new) external magnetic field. All this happens in synchronization with the field, and does not normally produce a signal in the receiver of a PI machine. (VLF/MF machines, whole different story.)
Due to thermal agitation, something else also happens. Suppose (for ease of explanation) that the transmit current is held constant. Some microscopic magnetic domains will have been pushed nearly to the trip point of their next energy state. The ones which are within the proverbial gnat's tail of the next energy state will quickly be kicked by thermal agitation into the next state. The ones which require more energy to be kicked into that next state will, on the average, have to wait longer until a thermal fluctuation with enough energy comes along and provides the kick. So, as the current is held steady, over time the response in the soil minerals continues to occur at a decreasing exponential rate, as the easier state transitions have already been kicked and all that remain are a few obstinate holdouts.
In most PI machines, we don't look at what happens during the transmit period. But, the same thing happens after flyback, and it's affected by what happened during transmit.
When the transmit field is removed by flyback, the external field (primarily earth field) which the soil magnetic particles are immersed in changes strength and orientation, and again, most of the energy state differences will be small enough to respond instantly. And, those energy states which have almost but not quite enough energy to come into realignment, will be kicked into alignment by thermal agitation, the easy ones first and then the tougher ones.
What may not be obvious at first, is that the longer the transmit current was held on, the more magnetic particles capable of being kicked by the combination of field difference and thermal agitation came into alignment with that condition. Therefore, when the transmit current is turned off (by flyback), a greater number of particles capable of being kicked is available. If the transmitter current was on for a long time, quite a few "toughies" managed to get kicked into new energy states by thermal agitation, and after the current is turned off, these same "toughies" are susceptible to getting kicked back to where they came from. This stretches out the tail seen after flyback.
All the above is a gross oversimplification, but it explains why transmit on-time is very important to ground balancing the red stuff in PI metal detectors, even though it isn't all that important to metal objects. The physics is entirely different.
--Dave J.