A graph. The horizontal scale is labeled Frequency in c p s and covers the range from 950 at the left to 1050 at the right. The vertical scale is labeled Impedance in ohms and ranges from 0 at the bottom to 10 at the top. The curve on the graph starts at the upper right at just above 10 ohms and 950 c p s. It slopes downward to the right and is almost straight. It begins to level off and is horizontal at the point 1000 c p s and 1 ohm. It curves upward again making a mirror image of the part on the left. The low point of the curve is labeled with a capitol F. Two points are marked on either side of F. One to the left at the point 995 c p s and 1.4 ohms, and the other at 1005 c p s and 1.4 ohms. The horizontal difference between these two points is labeled delta f.