RACTICE EXERCISE 5.4 If the hole in the lead bar of Example 5.4 is completely filled with copper (= 5.8 X 10'S/m), determine the resistance of the composite bar. Answer: 461.7 p. EXAMPLE 5.4 A lead ( 5 x 10" S/m) bar of square cross section has a hole bored along its length of 4 m so that its cross section becomes that of Figure 5.5. Find the resistance between the square ends. FIGURE 5.5 Cross section of the lead bar of Example 5.A 1 cm 3 cm Solution: Since the cross section of the bar is uniform, we may apply eq. (5.16); that is, R= where S---3- **-*-**- (9-) cm². Hence, 4 R= 5 × 1019 - 2/4) > 10 974 μ PRACTICE EXERCISE 5.4 If the hole in the lead bar of Example 5.4 is completely filled with copper (5.8 x 10 S/m), determine the resistance of the composite bar. Answer: 461.7 f