For the function of a complex variable $\text{W}=\ln \text{Z}\; ($where, $\mathrm{W}=u+j \mathrm{v}$ and $\mathrm{Z}=x+j y),$ the $u=$ constant lines get mapped in $\text{Z}$-plane as
- set of radial straight lines
- set of concentric circles
- set of confocal hyperbolas
- set of confocal ellipses