3v OU Flashlight

[kolbacict]

At least such a core can actually be induced from under the table
powerful transmitter.

[rakarskiy]

At least such a core can actually be induced from under the table
powerful transmitter.

Of course, you can even insert a micro battery inside the winding, it will be easier.     
But this will not solve the problem of creating a real device.

[Sergh]

I think a regular bolt with nut (annealed in fire) would be more effective than ferrite.

It will not be better. Even an annealed iron wire with a diameter of 0.8 millimeters for welding is bad as the magnetic core of a transformer.
Ordinary electrical steel for transformer cores is a complex product.

https://en.wikipedia.org/wiki/Electrical_steel
Special ferrites for transformer cores are high-tech products with minimal losses. Just do not use ferrites from the trash.

Of course, you can even insert a micro battery inside the winding, it will be easier.     

- people think that small watch batteries are hidden in transistors KT603A 

[rakarskiy]

Who disputes, but there are such interesting data: ССЫЛКА
It all depends on the task, in our case, the material should rest on demagnetization. 

[Sergh]

High permeability - and what's the point?
Metglas alloy has even higher permeability than ultrapure iron, and low losses. Transformers on such cores don't produce free energy.
https://web.archive.org/web/20120206100947/http://www.metglas.com/products/page5_1_2_6.htm