 Here is a photograph of a particle track in a hydrogen bubble chamber from the Brookhaven National Lab. Given the direction of the applied magnetic field, electrons will curve clockwise. This is a medium speed electron arching through the cloud chamber. Since we know the strength of the magnetic field applied across the bubble chamber, we can calculate the particle's momentum by measuring the radius of its curvature. The straighter the path, the faster the particle is moving. Also, remember that accelerating electrons lose energy by radiating photons. This causes the electrons to slow down and their curvature to increase. At these three points, we see that stationary electrons in our path have been bumped into motion. These are called knock-on electrons. Because they are moving very slowly, they spiral rapidly to a stop. We see a lot of these in bubble chambers. The track ends when the electron is captured by a proton to form an electrically neutral hydrogen atom. In this bubble chamber photograph from CERN, we see a particle's track rotating counterclockwise. This indicates that it is positively charged. The particle's line is also thicker than the others. This indicates that it is a large, slow-moving particle. This is a characteristic of proton tracks.