云室是个用来侦测游离辐射的粒子侦测器。由英国物理学家查尔斯·威耳逊发明，因此又称为威尔逊云室。最简单的云室，只是一个密封的环境，里面充满过饱和的水蒸气或酒精。当一束带电粒子（α粒子或β粒子）与云室内的混合物相互作用时，会将混合物离子化，造成的离子会扮演云凝结核的角色，使离子的周围产生雾气（因为这些混合物刚好正处于凝结点）。带电荷粒子走过的时候，会产生很多离子，所以就留下了它们走过的轨迹。这些轨迹的形状独特(如α粒子的轨迹较阔，显示出碰撞造成的弯转痕迹，β粒子较细与直)。当施加垂直的均匀磁场于云室时，这些带电粒子会偏转，带正电的偏转向一边，带负电的会偏转向另一边，遵守洛仑兹力定律。

The cloud chamber, also known as the Wilson chamber, is a particle detector used for detecting ionizing radiation.

In its most basic form, a cloud chamber is a sealed environment containing a supersaturated vapor of water or alcohol. When a charged particle (for example, an alpha or beta particle) interacts with the mixture, the fluid is ionized. The resulting ions act as condensation nuclei, around which a mist will form (because the mixture is on the point of condensation). The high energies of alpha and beta particles mean that a trail is left, due to many ions being produced along the path of the charged particle. These tracks have distinctive shapes (for example, an alpha particle's track is broad and shows more evidence of deflection by collisions, while an electron's is thinner and straight). When any uniform magnetic field is applied across the cloud chamber, positively and negatively charged particles will curve in opposite directions, according to the Lorentz force law with two particles of opposite charge.