Three Dots

Metal contacts on Alps Alpine's TACT Switch™ and Contactsheet™ products have three small protuberances, or dots, on them.
They are really quite small, but sometimes these dots are what keep the electrical contact closed.

Behavior of Foreign Matter

Foreign matter that gets inside a contact tends to migrate towards the center during use.
The photo below is from an experiment we performed. You can see foreign matter in the middle of the contact.

We decided to add three protuberances to the metal contacts, positioning them a little off center.

Advantages of a Metal Contact with Three Dots

Needless to say, first you should be taking steps to prevent contamination by foreign matter.
Alps Alpine has a variety of Dustproof and Waterproof TACT Switch™ products on offer.

Dustproof and Waterproof TACT Switch™ Lineup (IEC IP67 equivalent)

Dustproof and Waterproof TACT Switch™ Lineup (IEC IP68 equivalent)