RAM A 9/16" Ball with Camera Thread
The RAM-A-237U, universal camera and camcorder base, consists of a 9/16" diameter rubber ball, connected to a 1/4"-20 camera thread. The male thread matches most standard camera and camcorder female threads found on the bottom of the camera.

Warning!
Most people find that the "A" sized components work fine for holding a small phone in an aircraft or automobile.  However, I have received a significant number of complaints about the rubber balls detaching from the bases.  I therefore recommend the larger "B" sized arms for most soaring applications.  In fact, one customer temporarily lost control of his sailplane when his PDA cradle fell off the RAM A arm and jammed the control stick.  I have never heard of a RAM "B" size arm component failing.