Exercise $2\mathrm{.}3$: An interesting practical problem is when the outside radius $($b$)$ is fixed, b $=10$ cm$,$ say, and the inner radius $($a$)$ is variable. What value of a gives the lowestmaximum field strength? Consider two concentric cylinders such as a cable and twoconcentric spheres. What is the optimum b$/$a ratio for each case? Take V $=100$ kV DC$.$Use calculus or trial and error. $($Answer: Cylinders: b$/$a $=$ e$=2\mathrm{.}718,$ Spheres: b$/$a $=2)$

Exercise $2\mathrm{.}3$: An interesting practical problem is when the outside radius $(b)$ is fixed, $b=10$ cm $,$ say, and the inner radius $(a)$ is variable. What value of a gives the lowest maximum field strength? Consider two concentric cylinders such as a cable and two concentric spheres. What is the optimum b$/$a ratio for each case? Take V $=100$ kV DC$.$ Use calculus or trial and error. $($Answer: Cylinders: $\frac{b}{a}=e=2\mathrm{.}718,$ Spheres: $\frac{b}{a}=2)$