It makes a lot of sense when we look at it energetically, because if we think about a 1 s core electron, that's going to be held really, really tightly to the nucleus.

The new type DC solid state relay is a kind of new contactless switching device, with the new type IGBT and power field effect tube as the core and composed completely by solid state electron devices.

These tubules were surrounded with an electron dense membranous sheath leaving the core semi translucent.