coil welded type
These terms apply to the coils inside a wattmeter. 'Pressure coil' is an archaic term for 'voltage coil', which is connected in parallel with the supply, while the 'current coil' is connected in series with the load.
in many ways..by adding coil,increasing frequency ,made coil smaller,inserting rods inside the coil,physical position adjustings..and so on.the fact is many factors are involving in inductions.
The strength of the magnetic field inside the coil increases.
Geophone Are Moving Coil Electromagnetic type principle is contained