Because after a period of contact, here forms a pit, which leads to a poor contact of the switch.自己翻译戚棚的。隐仔芹。灶毕。
A pit was formed here after long-time touch, this casued the loose contact of switch.
Because after a period of contact, it forms a concave point. So the switch contact
Because after a period of contact, there is a concave point formed and that results in poor contact switch