College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 65

The student engineer of a campus radio station wishes to verify the effectiveness of the lightning rod on the antenna mast (Fig. P18.55). The unknown resistance R x is between points C and E. Point E is a "true ground," but is inaccessible for direct measurement because the stratum in which it is located is several meters below Earth's surface. Two identical rods are driven into the ground at A and B , introducing an unknown resistance R y. The procedure for finding the unknown resistance R x is as follows. Measure resistance R 1 between points A and B. Then connect A and B with a heavy conducting wire and measure resistance R 2 between points A and C. (a) Derive a formula for R x in terms of the observable resistances R 1 and R 2. (b) A satisfactory ground resistance would be R x 2.0 . Is the grounding of the station adequate if measurements give R 1 = 13 and R 2 = 6.0