“In later years, Franklin devised an amusing experiment to signal the passing of charged clouds by ringing a pair of bells located in his study.  The first bell was connected with a rod ending in a sharp point on the roof, and the second bell was grounded by a rod going into the earth.  In between the two bells, he hung a little metal ball on an insulated thread.”

Padamsee, H. (1993).  Humanistic Perspectives for Introductory Physics, 334.  Ithaca NY: PI Productions. 

Sketch a diagram of Franklin’s charged cloud detector, and explain how it works.  Sketch charge distributions on the parts of the device as it starts, and for both directions of motion of the small ball.  (Hint: recall the ping pong ball / charge ferry class demo.)