http://th.wikipedia.org/wiki/%E0%B9%84%E0%B8%9F%E0%B8%9F%E0%B9%89%E0%B8%B2%E0%B8%AA%E0%B8%96%E0%B8%B4%E0%B8%95

http://pifloors.com/Default.aspx?pageid=15

http://www.rmutphysics.com/physics/oldfront/51/index51%20static%20electric.htm