Deluminator
A silver device the size of a cigarette lighter that takes the light out of nearby lamps at the press of a button, and gives it back again.
About the size and shape of a silver cigarette lighter, the Deluminator takes light away and gives it back. Press the button and the nearest light source goes out, its light flying to the device as a small ball; press it again and the light is let out. If the original source is close by, the light returns to it; if not, it simply stays where it has been released. The device belongs to Albus Dumbledore, who has used it to put out a whole street of lamps at once and hold a conversation without being seen.
How far into Harry Potter have you read?
Set your place and this page rewrites itself to exactly that point. Nothing ahead of you will show.