A 115 lb rail, properly crowned, will have a gap under each end of the magnetic straight edge of ___________.

In the context of thermite welding, the term "properly crowned" refers to the slight arch shape that is intentionally formed at the top of the rail. This design is essential for ensuring proper drainage and reducing wear over time. When a magnetic straight edge is placed across the ends of the rail, it is expected to rest on the crowned surface.

The gap that is observed under each end of the magnetic straight edge is critical for confirming that the rail is indeed properly crowned. With a 115 lb rail, a gap in the range of .065 to .070 inches indicates that the crowning is adequate. This range reflects the need for a precise curvature; if the gap is too small, the rail may be too flat, and if it's too large, it could indicate excessive crowning, both of which could lead to operational issues.

Understanding these tolerances helps ensure that the rail meets both safety and performance standards, thus highlighting the significance of the correct gap measurement.