How do you determine the difference in time between the arrival of the primary (p) wave and the arrival of the secondary (s) wave during an earthquake?

Primary (P) wavesa type of seismic wave that involves alternating compression and expansion of the material through which it passes. 
6 to 13 kilometers per second, these waves arrive before S waves. 1.7 times faster than S waves. These waves are compressional.Secondary (S) wavesa seismic wave that involves oscillation perpendicular to the direction of propogation. 
second to arrive and bigger amplitude than P waves. These waves are shear.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -