Field Lines lets you put charges on the screen to visualize an electrical or magnetic field. You can watch the field lines or arrows that are showing the direction of the field.
The simulation is using point charges which you can place by drag & drop. The "snap to grid" function helps to make it well-arranged but it can also be turned off.
This app is great if you want (or have) to deal with field lines. You can try out nearly endless constellations of charges.
If you are missing a feature feel free to use the support link below.