There is one in a second, none in a minute, two in one hour, but none in half. What am I?

Bears the load of travellers Loses effectiveness at high ambient temperatures, Subject to high friction when in use Does not roll A simpler alternative to the above can be made from barn woods by the experienced

History and legions, Both comprise my kind, A milestone, so rarely spanned, All this time, entwined

A question, a quandary, a quagmire of confusion. I beckon with promise, then twist the illusion. I speak without speaking, I'm seen yet unclear, You’ll chase me for ages - I won’t disappear. What am I, wrapped in riddle and doubt?

A single word that is composed of all three of these E over m raised to one half Test out of thirty-six Four score and seven years ago our fathers brought forth on this continent, a new acronym, conceived in Liberty to drop its A.