( $a$ ) Compute the energy of an electron in the $n=1$ ( $K$ shell) of tungsten, using $Z-1$ for the effective nuclear charge. ( $b$ ) The experimental result for this energy is $69.5 \mathrm{keV}$. Assume that the effective nuclear charge is $Z-\sigma,$ where $\sigma$ is called the screening constant, and calculate $\sigma$ from the experimental result.