I'm looking at this circuit for a 555 trigger plate based on human body inductance. First off, touching a piece of metal isn't going to do anything unless a circuit is completed right? So how is this circuit completed? It looks like the plate is hanging out there in space. Does it assume the human is grounded? Or is that not necessary?

Also, would say a mouse have enough inductance to trigger this plate you think?

 

The effect that turns on the 2N2222 is capacitance - http://pcbheaven.com/wikipages/How_a_Touch_Button_works/

 

I'm looking at this circuit for a 555 trigger plate based on human body inductance.

The link that beenthere posted is a very good student teaching aid. To answer your question: The circuit is completed to GND capacitively. There is no DC path. It's AC hum that forward biases the 2N2222 on the positive going waves. The Human body acts like an antenna. Fact is, a long wire connected to the base would also forward bias the transistor. FYI, FETS are far better suited for this circuit than BJTs.