A tattoo machine will run either way you connect your clip cord to it, but you should take care to connect it the right way, because of the capacitor that’s installed in the machine electrical circuit. Axial capacitors are the most used in tattoo machines, and they are polarized. They have positive and negative lead ends, and if you invert voltage, it will cause a disfunction that may destroy the center layer of dielectric material and cause a short circuit, even heating up and bursting.
To avoid this kind of problem, learn how to attach your clip cord correctly: it’s common between most tattoo machine builders to place the capacitor with the positive end pointing downwards, connected to the rear binding post, and the negative end pointing upwards, and connected to the front binding post. This means that you have to connect the positive hook of the clipcord (usually marked in red) to the rear binding post of the machine (bottom), and the negative (marked in black) to the hole underside of the spring deck part (top), as the following picture shows:
ALL MY MACHINES are set this way, so remember, black on top, red on bottom, always. In some cases, when I place the binding posts horizontally, the positive one is the left post, and the right post is the negative. Always connect your clipcord correctly, and your machine will perform better, dramatically extending the capacitor life.
Now, some of you will say… “but my clip cord hasn’t got any red / black distinctions”. Yes, some manufacturers don’t put any mark on them. To find which are the positive and negative ones, you can unscrew the cover on the jack connector (the cord part that you plug into the power supply), and check the two pins to which the two cables that compose the clip cord are soldered. The one in the center is the positive, so follow that cable and you’ll find the positive end on the other side of the clipcord (I’d suggest to mark it so you don’t have to check again). The negative pin is the longer one, or the “sleeve”; check this diagram for better understanding:
To end this tutorial, if you own other machines (not built by me) and you want to check if the capacitor is connected to the machine circuit in “the correct way”, take a look at it: capacitors usually show a groove on the positive end, and it should point downwards. Also, an arrow is usually printed on the capacitor, pointing the negative side, upwards. If you find that the capacitor is inverted (groove, positive on top, negative on the bottom), you just have to connect the clip cord inversely, too (red on top, black on bottom).