The tape measure and its loose hook

Look closely at a tape measure and you will see that the metal hook at the end is not firmly fixed. It slides a little. Many people assume it is loose by mistake. It is not.

The reason#

The hook is designed to move by exactly its own thickness.

• Measuring something from the outside. You hook the end over an edge and pull. The hook moves outward, away from the case, and the measurement starts at the inside of the hook.
• Measuring something on the inside. You push the end against a surface. The hook moves inward, and the measurement starts at the outside of the hook.

In both cases the hook slides so the zero mark lines up with the point you are measuring from. Without the movement, every measurement would be off by the thickness of the metal.

What the slots do#

The hook is attached with rivets that sit in slots. The slots allow the hook to slide. Over time they can wear or get bent, and then the tape starts to give wrong readings.

How to check yours#

1. Hook the tape on a firm edge and note a reading.
2. Measure the same thing from the one-inch or ten-centimetre mark instead of the end.
3. Subtract the starting mark. The two numbers should agree.

If they do not, the hook may be damaged, and an inexpensive replacement is the sensible fix.

A small habit#

When you need to be exact, measure from a mark in the tape rather than the end. And, as the old advice says, measure twice and cut once.

A tool that looks sloppy often hides a careful idea. This is a good example.