Grades K–2 · Everyday Life
Why does a safety pin have a curl and a little cap?
A safety pin has two long wire arms joined by a little curl. One arm ends in a point. The other has a small clasp that can cover the point when the pin is closed.
In the diagramThe curl is a spring; the clasp is a catchOne wire fastener · enlarged to reveal its partsSpring curlClaspPoint coveredThe clasp holds the pointed arm against the spring’s push.
The curl is part of a spring. Bending the arms changes the shape of the springy wire. It pushes back toward its resting shape. The clasp catches the pointed arm and keeps the spring from spreading the pin open.
A closed pin can join pieces of cloth because its wire passes through them and returns to the clasp. The spring helps hold the arm against the clasp, while the clasp hides the sharp tip. The curl and cap have different jobs, even though both are part of one small fastener.
About this explanation
Starting knowledge: Recognize cloth, a metal arm and a springy bend.
Sources
- Lemelson-MIT Program — Walter Hunt: Safety Pin
- Walter Hunt · US patent disclosure — Dress-pin · US6281A