Muggles' Guide to Harry Potter/Magic/Sneakoscope

Overview
The Sneakoscope is a glass spinning top that stands upright when placed on a surface, and spins, whistles, and lights up when it detects someone untrustworthy.

Extended Description
Harry receives a Pocket Sneakoscope from Ron for his birthday in Harry Potter and the Prisoner of Azkaban. Almost every time we see it in the book, it is lit up and spinning at great speed; and every time, it is dismissed as being unimportant. Ron, at least, should know better than to dismiss it so easily; after he got it, before he shipped it off to Harry, Bill had been telling Ron it was cheap and probably defective because it kept lighting up; but while Bill did not know that Fred and George had put beetles in his soup, Ron did know, and so knew that there really was someone untrustworthy in range. It also went off when he was tying it to Errol's leg for the journey; at the time, it was Ron who was being untrustworthy, because he knew he wasn't meant to use Errol for any long-distance journeys.

Professor Moody also has a Sneakoscope in his study in Harry Potter and the Goblet of Fire; he says he's had to disable it, it's an extra sensitive model, but there are too many untrustworthy people in its 20-mile range. Harry's own Sneakoscope is mentioned only in passing in this book; Harry needs a gift for Dobby, so he presents Dobby with the socks he has been using to cushion his Sneakoscope since he first got it.

There is also mention in Harry Potter and the Order of the Phoenix that there are a number of Sneakoscopes in the Room of Requirement when Dumbledore's Army first meets there.

While it is not directly identified, the old, broken Sneakoscope that Harry cleans out of his trunk at the beginning of Harry Potter and the Deathly Hallows is almost certainly the one Ron had bought him four years before. On his birthday four days later, Hermione presents him with a new one.

It is the new Sneakoscope, we believe, that Hermione packs into her little beaded bag, and carries with the rest of their gear as they leave Bill and Fleur's wedding, and then again later as they abandon the house at Grimmauld Place. The Sneakoscope, usually sitting quiescent on the table, is used extensively as an indicator of possible peril while the Trio are camping in various locations around England.

Analysis
The Sneakoscope only serves a major plot purpose in Harry Potter and the Prisoner of Azkaban. There, it is produced as a way of pointing out that someone in the vicinity is not trustworthy. When it is triggered in the Hogwarts Express, the only people we know about who are present are Harry, Ron, Hermione, Professor Lupin, Scabbers, Hedwig, and Crookshanks, although presumably Draco Malfoy could have been in range. When it is triggered on Christmas Day, only Harry, Ron, Hermione, Scabbers, and Crookshanks are present. Looking at this analytically, we have to assume that it is either Crookshanks or Scabbers that we are meant to distrust; however, most readers will discount them, as they are only animals and thus cannot, presumably, affect the Sneakoscope, and so will conclude that the Sneakoscope is defective, something that Ron has been saying all along.

In later books, Harry Potter and the Goblet of Fire and Harry Potter and the Order of the Phoenix, the Sneakoscopes are no more than set dressing. Because of their function, it is expected that anyone who is involved with battling against Dark Magic will have one; so we see that Moody has one and the DA practice room has several.

We see the Sneakoscope again several times in Harry Potter and the Deathly Hallows; there, it is used to let us know that there are no untrustworthy people about, as it is always standing quiescent, rather than spinning and whistling. For instance, as Ron tries to rejoin Harry and Hermione, we see that the Sneakoscope remains quiescent despite the sounds and indications of someone nearby that both Harry and Hermione notice. This should indicate that whoever is in the vicinity is a friend. The Sneakoscope does report when the Snatchers surround the tent some months later, but the warning is given too late.

Questions

 * 1) Ron knew from the beginning that the Sneakoscope was not defective; why did he dismiss it during the year?
 * 2) The Sneakoscope does not sound at the Dursleys'. Does this indicate that Vernon and Dudley are trustworthy? Could there be another explanation, perhaps to do with their being Muggles?

Greater Picture
Eventually, we learn that Scabbers is actually Peter Pettigrew, a.k.a. Wormtail, and that he is not trustworthy, so the Sneakoscope triggering in his presence would be expected, if it is reliable. Scabbers was in its effective range all along, of course, in the boys' dorm, so the question is, as it clearly is working properly, why was it not always sounding the alert? When it first triggered, in the Hogwarts Express, the motion of the train had dislodged it from the nest of Vernon's socks that Harry had packed it in for travel; at Christmas, Ron, trying to intervene in the sudden battle between Scabbers and Crookshanks, had dislodged it again. We can assume that at all other times, it was securely packed in its little nest and could not sound; the description of it spinning and whistling suggests that, like a child's musical top, it makes its whistling noise by spinning, and if it can't spin, it also can't whistle.

Harry's finding it, cracked, in his trunk and discarding it at the beginning of Harry Potter and the Deathly Hallows, is a useful device to explain away any instances where it should have sounded in the intervening books, and didn't. Harry had given Vernon's old socks to Dobby at Christmas in Harry Potter and the Goblet of Fire, so the packing that would keep it quiet was gone; if Harry's trunk had been approached by anyone untrustworthy after that point, the Sneakoscope should have sounded, but can now be explained away by saying it had previously broken, and Harry had simply never cleaned out his trunk. This also gives Hermione a ready-made idea for a birthday gift for Harry.