Why do lights get duller in a series circuit when more globes are added?

The lights get duller as the voltage (electrons-they hold the electricity) have to be shared evenly among all of the globes. Therefore, when more light bulbs are added the voltage becomes less as it has to be shared among more light bulbs.

The current does not change. You can think of electricity as a river. If there is a main river and the it splits in two, the amount of water in each split of the stream is half the amount of water that was originally in the main stream. (this can be thought of as the voltage) In Contrast, the speed that the water is flowing does not change (this can be thought of as the current)

I hope this helps:)