I have also been thinking about this. It's a well thought out (i should know) invention of some scientist witch is capable of strange things.
The SCP in question is the invention of Prof. Rotwiller (Rot-will-er) created in 1895. The idea is complex, but i will describe it's effects and what it does.
(NOTE that it's an idea. I would like to know weather it can be expanded upon and create a descent SCP out of it.)
The contraption in question is a small, simple device. It consists of two cylinders witch are facing each other. The cylinders are held in place by a holder separated into two parts, in each part is one cylinder. The thing is controlled by a remote control with the ON and OFF option. When it is turned on the two cylinders will instantly magnetize and collide. Upon colliding, an explosion erupts from the device. The explosion is like in one of those Hollywood action movies, a large fireball-like explosion. The explosion takes a perfect spherical shape and just continues exploding.
The explosion will not spread further than a certain diameter (say, 10 meters) and will keep it's perfect spherical shape. It will just continue erupting fire and energy from it's center (the device) for an infinite amount of time or until someone decides to click the OFF option, in witch case, the cylinders will de-magnetize and return to their start position. When this happens, the explosion stops. It will cave in on it'self and vanish.
There are multiple other facts and details about this SCP. If the idea is good, I will consider finishing it the Foundation Sandbox and post it on drafts and critiques. There, also, are numerous consequences that follow after turning the device off. It's up to you if you think they are worth finding out.