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Hi Chris and All,
A website worth looking at is:- www.bartington.co.uk
They make susceptibility meters and have lots of info and application notes
available. From what I remember about magnetic impurities in copper wire used
in proton mag sensors there are two separate problems. A relatively large
ferrous inclusion can cause a faster than expected decay due to the magnetic
gradient it generates in the proton fluid. i.e. different parts of the fluid
try to precess at different rates thereby quickly killing the signal. The
other, more usual, problem is much finer impurities disseminated throughout
the copper. This builds up what is called a heading error in the sensor. The
effect of this is that if you rotate the sensor through 180 degrees you get a
different field reading. Over a repeated number of polarisation cycles the Fe
impurities take on a weak remanent field in the direction of the polarising
field and this adds or subtracts from the Earth's field. This effect should
be much less with a toroid than with a solenoid.
A wire carrying a current is surrounded by circular flux lines but if you
place a compass over the wire, you will find that the needle lines up along
the flux line in a certain direction. The flux line is like an infinite
number of minute SN poles in series. Same as you can put a number of bar
magnets in a ring and have no exterior poles. In a solenoid (or toroid) you
have many conductors side by side and above and below. The fields around each
wire add or subtract to give the net field through the centre of the
solenoid. It is this net field and its direction which can magnetise any Fe
impurities.
Apologies if these topics have already been covered at some time in this
forum as I only visit now and again.
Eric.
A website worth looking at is:- www.bartington.co.uk
They make susceptibility meters and have lots of info and application notes
available. From what I remember about magnetic impurities in copper wire used
in proton mag sensors there are two separate problems. A relatively large
ferrous inclusion can cause a faster than expected decay due to the magnetic
gradient it generates in the proton fluid. i.e. different parts of the fluid
try to precess at different rates thereby quickly killing the signal. The
other, more usual, problem is much finer impurities disseminated throughout
the copper. This builds up what is called a heading error in the sensor. The
effect of this is that if you rotate the sensor through 180 degrees you get a
different field reading. Over a repeated number of polarisation cycles the Fe
impurities take on a weak remanent field in the direction of the polarising
field and this adds or subtracts from the Earth's field. This effect should
be much less with a toroid than with a solenoid.
A wire carrying a current is surrounded by circular flux lines but if you
place a compass over the wire, you will find that the needle lines up along
the flux line in a certain direction. The flux line is like an infinite
number of minute SN poles in series. Same as you can put a number of bar
magnets in a ring and have no exterior poles. In a solenoid (or toroid) you
have many conductors side by side and above and below. The fields around each
wire add or subtract to give the net field through the centre of the
solenoid. It is this net field and its direction which can magnetise any Fe
impurities.
Apologies if these topics have already been covered at some time in this
forum as I only visit now and again.
Eric.