Think of voltage as "electromotive force." It describes the potential for current, not the actual current itself. If we use the formula E= IR, then we know we need to replace the E with electromotive force, and the R is obviously resistance. I, then, must be the current. Generally, you have the voltage as determined by your battery. And you have the resistance, as determined by your atomizer. How do you figure out how much current is flowing through your atomizer? Think of an Eagle, a Rabbit, and an Indian. When the indian looks out over the mid-western plains, he sees an eagle soaring high above a rabbit. So, I equals E over R, or I = E/R. This works no matter which property you are trying to solve for. Anyway, I hope this little picture sticks in your head and makes you life a little easier one day