| Particle Number Density Meter: This meter measures the number density of liquid or solid particles. You must attach the number density meter to an orange hotspot on the Ultrascope (see Ultrascope View (Freezing and Melting)). In the Properties Box of the number density meter, you can choose to measure the numbers of either solid or liquid particles. During the phase transition, the change in number density is deliberately exaggerated to illustrate the direction of the density change. For example, as water freezes or melts, the change in particle number density is about three times the actual change. You must place an number density meter to plot number density using the graph tool.