Invented during the Magic Wars of centuries ago, when the slinging of spells was common in combat and unrestricted by treaty or convention, these were crude at first, but even crude thaumometers were enough to, for example, prevent someone from being polymorthed into a small purple teapot because they spent too long in a high magic zone.
A thaumometer is a device for measuring the amount of magical energy in a location. Powered by a small piece of tassite , a dial is powered that goes up on a scale of 0 to 100 depending on the amount of magic in a location. When the amount of magic is over 80% a small crystal of quartz starts glowing red and an alarm is sounded, meaning that staying for much longer in that location could be hazardous to your health and indeed your chances of staying as you.
0-Means you are in a null magic zone where no magic can be cast, which is quite rare.
1 to -10-Normal readings of magical radiation in the air. No danger to anybody.
11-20-Means you are close to a magical artifact or a ley line. No danger to anyone unless they are allergic to magic, when they might get a rash at worst.
21-30-Means someone has cast a spell in this location very recently. No danger to anyone unless they are allergic to magic, when they might start feeling sick.
31-40-The normal amount of magic radiation in the average magical battle. Those who are allergic to magic should get out of there before it threatens their lives.
41-50-The amount of magic found in a magical laboratory or witch's coven. At this level. magic starts to build up in the tissues over time unless precautions such as lead lined clothes are taken. For those who know how to use magic, this is not a problem, but otherwise it can cause trouble such as a random magical discharge aimed randomly (which could get the person in trouble if it injures or even kills someone) random objects flying aimlessly around the affected person, or in a few unlucky cases, magical allergy or even the slow growth of cancer. Those who are allergic to magic are in danger of either dying or random polymorthing, depending on the GM. The Sorcery Springs Geyser Basin has this amount of magic within it.
51-60-The amount of magic found in a crater from one of the now banned Balefire spells of long ago, or that an elemental or other highly magical creature gives off at close range. Again, magic users can handle it without problems as their bodies are used to it, much like arsenic eaters who eat tiny amounts of arsenic over a long period of time can slowly eat more and more of it without harm. The allergic to magic have little chance of surviving at this level, whilst non magic users should consider wearing lead suits or other anti-magic devices to stop magic building up in their bodies and either hurting them by causing cancers a few years later or erupting and causing chaos around them for several minutes at a time.
61-70-The amount of magic found in magical testing grounds, or needed to create Undead. At this level, even lesser magic users should limit their exposure to not more then an hour per day, and non magic users should absolutely have some way of fending off or grounding the magic to prevent serious affects/cancers. At this level, random objects can animate if they are not grounded, and partial polymorthing or aggressive cancers of the kind that kill within a year if not healed before then are caused in unprotected non magic users. At this level and higher the allergic to magic die on the spot.
71-80-An Archmage or Master of a magical college will not be affected by this level of magic, but lesser magic users and certainly non magic users need lead plate armor or layers of protective amulets or pentagrams to prevent bodily damage, polymorthing or even limb loss caused by this magic, and/or random deadly spells arcing out from an affected person. The most complicated spells that are generally cast in special testing zones carry this amount of magical charge. This level of magic can, for example, raise large numbers of Undead at once to create Undead armies. Magic at higher levels then this is banned in most countries for everybody's safety.
81-90-At this level, the crystal glows and the alarm goes off meaning that even high ranking magic users should evacuate the area unless clad in lead suits and/or proper protective amulets. Average people who are unprotected are in danger of such horrors as lifelong magical allergy, random curses affecting them, turning into someone-or something-else, or being made vulnerable to possession by ghosts. This is normally caused by a major magical accident in a testing ground or some seriously evil magical villain, and it can take years or even decades for the area to be safely entered again.
91-100-At this level, even the most powerful magic users and full grown Dragons are not safe, and protective amulets and heavy lead suits will fail within a few hours. Unprotected people will either be killed on the spot, be randomly polymorthed into something that might not even be alive, die within months of cancer, or be continuously having random and unwanted spells bursting from their fingers and negatively affecting anyone who gets within twenty feet of them. Only the worst magical accidents or most horrible spells cast by magical villains can create such high levels of magic, and whole towns can be made no-go zones for centuries until the magic finally dies down to safe levels again.