Effect of parmanent and induced magnetism with change of latitude

Effect of parmanent and induced magnetism with change of latitude

Effect of parmanent magnetism:
PQR force – Constant deviating force.
PQR – Will not change with change of latitude.
For a constant PQR deviating force,
If magnetic latitude increase – DF(H) get smaller (deviating force get bigger)
If magnetic latitude decrease – DF(H) get bigger (deviating force get smaller)
So, Dev varies as 1/H.
At equator, with strong DF(H), it is harder to deviate compass needle.

Effect of induced magnetism:
Vertical soft iron (VSI):
Ships VSI induced by only Z.
We know, If Magnetic latitude change – both Z change and DF(H) change.
If magnetic latitude increase – DF(H) get smaller (deviating force get bigger)
Also, if magnetic latitude increase – Z get bigger (deviating force get bigger)
So, dev varies as tan dip (Z/H)
If magnetic latitude decrease – opposite.

Horizontal soft iron (HSI):
Ships HSI (F&A, athwartship) induced by only H.
We know, If magnetic latitude change – DF(H) change.
But, deviating force due to DF(H) also change.
Therefore, we can tell, if DF(H) get bigger deviating force get bigger.
If DF(H) get smaller deviating force get smaller.
Again, we also know, If magnetic latitude increase – DF(H) get smaller (deviating force get bigger)
So, resulting deviation from HSI remain same in all latitude.