Undetectable Extension Charm

Undetectable Extension Charm
Type	Charm
Effect	Simultaneously makes things bigger on the inside whilst magically disguising this effect.

Harry: "When you say you've got the Cloak, and clothes... "

Hermione: "Yes, they're here."

Ron: "How the ruddy hell – ?"

Hermione: "Undetectable Extension Charm. Tricky, but I think I've done it okay; anyway, I managed to fit everything we need in here."

— Hermione Granger explaining her use of the charm in 1997

The Undetectable Extension Charm (incantation unknown) is a charm that undetectably makes things bigger on the inside only.

Effect

This sophisticated charm has two main effects, the first of which is to expand an object's internal space whilst retaining its external dimensions. The Charm also magically disguises itself so that no one but the caster (and those the caster confides about the Charm in) can realise that this spell has been cast over an object, even if they are experiencing it.

History of use

Hermione Granger used this charm on her small beaded handbag in 1997 so that she was able to pack all the supplies she believed she would need while hunting down Lord Voldemort's Horcruxes with Harry Potter and Ron Weasley. This included books, changes of clothes, money, Harry's Cloak of Invisibility, and healing supplies. Eventually, Hermione also placed the portrait of Phineas Nigellus Black that had previously been in 12 Grimmauld Place in her bag as well to prevent Severus Snape from spying on them.

Possible uses

• This may be the charm that allows the Flying Ford Anglia to be bigger on the inside than it appears. This is supported by the fact that Mrs. Weasley opines that the odd effect of the charm over the car must be a feature of all Muggle cars, which is odd (since the natural conclusion would be magic). Hence, this is likely a result of the "undetectable" element of this charm.
• The same, or a similar, spell was used to make Ministry of Magic cars, tents at the Quidditch World Cup, the Knight Bus, and Alastor Moody's trunk larger on the inside.
• This was possibly used also with the Cast-a-Spell Kit because it holds a training room.

Behind the scenes

• The effects of this spell is similar to the effects of a Bag of Holding in many role playing game scenarios and the TARDIS in Doctor Who.
• The effects of the spell is also very similar to Mary Poppins's carpet bag.
• This spell appears to be advanced magic; if it were more simple, then one might expect Hogwarts students to be using it on their trunks to fit all their supplies in (however, they do not).
• This charm may be used by wizards in the aversion of Muggle attention.
• This charm appears to have a number of interesting side-effects when cast over an object. For example, when Hermione drops the bag she has cast this charm over in Harry Potter and the Deathly Hallows it makes a suspiciously loud thump (making it seem heavy) yet she carries the bag with ease (despite the number of objects inside). Of course, she may have also cast another enchantment to make the bag easier to carry too.
• This charm may be related to the Fidelius Charm, given its ability to prevent everyone but the caster and their confidants from learning of the Charm's effect over an object.

Appearances

• Harry Potter and the Chamber of Secrets (possibly, in the form of Arthur Weasley's Flying Ford Anglia)
• Harry Potter and the Prisoner of Azkaban (possibly, in the form of the Knight Bus and the Ministry cars)
• Harry Potter and the Goblet of Fire (possibly, in the form of the tents at Quidditch World Cup)
• Harry Potter and the Goblet of Fire (film) (possibly, in the form of the tents at Quidditch World Cup)
• Harry Potter and the Deathly Hallows
• Harry Potter and the Deathly Hallows: Part 1