Rotary tattoo machines and coil tattoo machines operate two different ways to achieve the same outcome of moving tattoo needles in and out of clients' skin to deposit ink into the dermis (secondary layer of tissue beneath the epidermis) with each insertion. It's what drives the needles that sets apart rotary and coil tattoo machines. The two images below illustrate the motions of rotary and coil tattoo machines.* The image on the left is of a rotary tattoo machine. The cylinder with a red arrow around it is the tattoo machine's rotary motor; it rotates clockwise, moving the needle bar attached to it in a linear fashion, driving it up and down so that the attached tattoo needles will move in and out of the skin smoothly. The image on the right is of a coil tattoo machine, which uses a more complicated setup to drive tattoo needles in and out of the skin. When direct current from the tattoo machine's power supply reaches the two coils, they're charged and an electromagnetic field is created. That electromagnetic field pulls the tattoo machine's armature bar down (i.e. the part the blue arrow points to in the photo), which breaks the circuit created between the contact screw on the top-right and the metal front spring that was touching it, causing the electromagnetic field to collapse. A nanosecond later, the spring draws the armature bar back up again, placing the front spring back against the contact screw and re-establishing the electromagnetic field, which starts the cycle all over again. The breaking and re-establishing of the electromagnetic field is what makes a coil machine choppier than a rotary tattoo machine, which is powered consistently and therefore results in a smoother needle motion.