Instead , the air gap is provided between the top of the teeth and one of the PPWG plates .
In this design it is not convenient to provide a gap through the centre of the teeth as is done in the microwave version .
The air gap is provided between the top of the teeth and one of the PPWG plates .
In this design it is not convenient to provide a gap through the centre of the teeth as is done in the microwave version .