As the title says, I want to know if the horse hairs in a bow all go in the same direction? I know that human hair has a direction which it is smooth and a direction which it is rough. I assume its the same with horse hair, which makes me wonder if all the hair goes in the same direction and therefore there is a difference between bowing one way and the other (and that there is a correct way to put the hair on the bow). Or does the hair go in different directions so you get uniform friction in either bowing direction?
As a bowmaker, I'd like to expand upon Carl's answer. The question of the scales seems to be answered by microphotographs of bow hair, which show all the scales to be gone once the hair is played in, and thus to play no role in grippiness. All I can add is that it is a complex issue. My tendency is to reverse half the hair in violin family instruments, and leave the root (thick) end at the head with gamba (vielle, underhand bow grip) instruments. But that's just me.
That’s a great question. Every hank I have received is knotted at the root end. The tips of the hair are identifiable because they taper to a point (they usually darken towards the tip as well). There are two premises for making a choice of which way to orient the hairs or to jumble them. Horsehair is seen to have a scale structure (like shingles on a roof) going from the root to the tip). Some have thought that this accounts for the “grippiness” of horsehair on the string and would be a good argument for jumbling the direction of the scales so you get similar grip in both bowing directions. However, the scales are so small (.5um) in relation to the hair diameter that they present a flat surface to the hair. It is the chemical nature of the hair protein that attracts and holds the rosin, and it is the stickiness of the rosin that gives the bow hairs grip on the string. The other consideration is strength. As mentioned, the bow hairs taper to the tip, especially over the last couple of inches, so for the strongest hair it is best to trim off (waste) hair more from the tip than from the root. Since players can exert more force on the frog end of the bow than out at the tip of the bow I orient the root of the the hair all at the frog end of the bow. This scientific article by Francoise Rocaboy has an excellent discussion of these points along with some (not very helpful) electron micrographs. Here’s a link to an excellent micrograph of a horse mane hair: /horse-hair-mane. I actually had trouble finding micrographs of horse tail hair, but almost all micrographs of mammal hair seem to show this same scale structure. Finally, I have an instructional DVD on bow rehairing and the presenter divides the hank and reverses half the hairs, so there a probably a fair amount of people doing this if they follow his lead.