Dallas i-Button

  
The iButton® is a computer chip enclosed in a 16mm stainless steel can. Because of this unique and durable stainless steel can, up-to-date information can travel with a person or object anywhere they go. The steel button can be mounted virtually anywhere because it is rugged enough to withstand harsh environments, indoors or outdoors. It is durable enough to attach to a key fob, ring, watch, or other personal items and used daily for applications such as access control to buildings and computers.
Physical
The unit is 50mm long by 5mm deep

Information and photos from www.ibutton.com

Specifications are subject to change without notice.