Some interesting transformer experiments ...

[exnihiloest]

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But in transformers 95% of the field is contained in the core all the time and still the secondary 'generates' an output just fine.
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It doesn't work this way.
The field is not permanently contained in the core. It enters and escapes, crossing the core:
A field is generated around a conductor carrying current. The field lines appear as circles around a conductor when current is flowing. These circles grow and expand when the current is increasing and they reduce and collapse when the current is decreasing.
When the wire is wound around a core, and the current is switched on or increasing, the field lines circling the conductor expand. When they (rapidly) attain the core, they concentrate in the core due to its much higher "penetrability" (higher permeability). At this moment, the field lines still encircle the conductor wire, but their path is through the core, not just around the wire.
As the expanding circles start outside the core just around the conductor, we see that they have to cross the core walls to enter the core.
A varying magnetic field in a core should not be thought as something permanently contained in the core and flowing along the core with an increasing and decreasing strength. It must be thought as an external field crossing the core walls each time its strength changes, depending on the varying current. This is the reason why we can apply to the transformer, the rule of flux crossing through the core section (near 100%) in order to calculate all what we need.
The ignorance of this phenomenon has led to many reasoning errors ending with the conception of fabulous FE devices but... not working.

[SchubertReijiMaigo]

So you are saying this experiment is not valid ? Why when he put his magnet bar on the trafo the current droop and the motor accelerate !? Measurement errors with an analogue Ammeter ? I don't undertsand here...