Most important to remember is that it is not the fast moving potentials, but the depolarization of myocardium that shows up on the EKG. So, the first deflection visible corresponds to the depolarization of atrial myocardium. Because the source of the depolarization wave is the SA node which is the extreme rightmost part of the heart, we see the depolarization proceeds from right to left. Therefore the vector of the depolarization points to the right, and lead I senses this as a positive deflection. The deflection is small because a small amount of myocardium is involved. I am avoiding talking about the depolarization in the third dimension for two reasons. One, lead I only "sees" the "x" component of the vector. Two, it gets really confusing...