Especially good for arthritis, these accessories are textured for increased sensation/grip... length is 3 1/4 inch, diameter at widest point is 1.5 inches. These are unipolar, plug a standard 2mm pin into either end. 

The safest way to use is to ground each with a large pad somewhere on the forearm rather than using just the massagers themselves (because the electrical circuit has to go up the arms and back to complete).