It was the predecessor to the tattoo machine.. So Yes Machine Is The Correct Answer... http://en.wikipedia.org/wiki/Tattoo_machine Just Helpin A Debate... I Was Wrong Also. Lol
The electric pen invented by Thomas Alva Edison and patented under the title Stencil-Pens in Newark, New Jersey, United States in 1876.[1] It was originally intended to be used as a duplicating device, but in 1891, Samuel O'Reilly discovered that Edison's machine could be modified and used to introduce ink into the skin, and later patented a tube and needle system to provide an ink reservoir.... So.... Now The Plot Thickens... :)
They say 7.5 volts is good for shading.. :) And dont let your gun run like a racer or you will shred the skin.. Slow and steady and low speed will win the race on shading...