What condition must be true about the opening between base metals for effective capillary attraction?

For effective capillary attraction in brazing, the opening between the base metals must be quite small. Capillary action relies on the ability of the liquid filler metal to be drawn into narrow spaces through surface tension. When the gap between the base metals is small, the liquid can be effectively pulled into the joint by this capillary action, allowing for a strong bond to form as the filler metal solidifies.

A larger or wider opening would not create the same effect; the forces at play with capillary action diminish as the space increases, making it difficult for the filler metal to flow and fill the joint adequately. If the gap were to be filled, it would negate the effectiveness of the capillary action, which relies on the filler metal being drawn in rather than already occupying the space. Therefore, a small opening is critical for the successful application of capillary attraction in the brazing process.