Hello everyone, I wanted to ask one question. My friend has a physics exam at school were the topic is to come up with a interesting yet simple physical device that would prove a single or multiple physics concepts. Now we were thinking that it would be interesting to experiment with the action-reaction idea. Also we wanted to device to incorporate possibly other means of ideas like relativity. And so I came up with the idea of a rotating toroidal pipe/transformer which would have a coil much line an ordinary transformer but instead of having a laminated iron core it would have an empty core with liquid metal in it. The whole idea should work like this, we have a rotor and slip rings, we supply some DC current through the slip rings to the rotor, the current goes into the toroidal coil wrapped around our empty torus, the torus pipe itself would have to be made of some insulating material and then I assume contacts should be put through so that the liquid metal inside the torus could touch them and conduct electricity, as it would conduct being situated in a uniform magnetic field it would then move and the whole assembly should work similarly to a magnetohydrodynamic pump. The main idea is that (given the DC polarity is set right) the liquid metal (probably mercury) should flow in one direction while the rotor is turned in the other, in other words lets assume the rotor spins with 100rpm, given enough current the liquid mercury would flow in the torus the other way and carefully matching the speeds we could make the mercury stand still inside the torus even though the torus itself would be rotating, right? the question is, is this possible, one factor is that I don't want alot of mercury inside the torus but only a little bit so that it would be visibly clear at which point the mercury is at any given moment, since mercury is liquid I assume it would want to spread out, how would I contain it more or less at one spot , would I need to pressurize the rest of the torus with some noble gas in order for the mercury to stay together? Any help is appreciated, thanks.