A common sentiment from atheists is that they won’t believe in things that can’t be shown.
We cannot see it, we can’t touch it. In the case of electromagnetic devices it is not always there. Yet one can observe its effects being inline with a given theory.
So is it believed in?
First off, atheism is disbelief on gods, not magentism. And your premise is wrong, it is not disbelief in things that cannot be shown but things that can't be evidenced.
There is falsifiable evidence that magnetism exists,it can be accurately measured.and used for all sorts of practical applications, after all, you used yoir computing device to post this thread.
Is there falsifiable evidence for gods existence?
Of course if such evidence should turn up then it will destroy atheism at a stroke. But so far, despite thousands of years and literally billions of people making the claim of gods existence no falsifiable evidence has neen forthcoming.