I've never really thought about this before, even though I'm a violinist, because you don't normally see such bows. It makes a lot of sense to have a bowl out this when playing the Bach unaccompanied sonatas, which are fiendishly difficult and include a lot of double stopping in some trouble stopping. I wonder if they are easier to play, or indeed what they sound like, when played with a curved Bach era bow? I've seen double bass players using bows of this general kind, but I've never in real life seen or heard such a bow being used on the violin.
That's interesting! Writing about this inspired me to look on Wikipedia, and there's a bit about a violinist named Spivakovsky using a curved bow to perform Bach's sonatas and part eaters for unaccompanied violin, and there is a photograph the great Russian cellist Rostropovich using a bark bow on his cello. Well there you go, you learn something new every day.
The modern bow is considerably different. It has a slight curvature in it, but in the opposite direction to the bark bow, and it is tensioned by and not with the screw and screw thread, so it has to be tensioned appropriately before you start playing, you can't make adjustments intention as you play, although if you're watching a string quartet playing you will from time to time see one of the musicians make a slight adjustment to the bow tension either between movements, or during a bars rest in the music. If you're using natural bow hairs (from a horse's tail), then susceptible to humidity and temperature variations, wearers synthetic bow hairs are more stable, but I think the majority of string quartet players stick to the traditional horsehair. The wooden bow itself will be susceptible to temperature and humidity variations, but as it is varnished all the way round (quite different to the varnishing on the violin, where the outer surface of the sound box is varnished, but the inner surface is bare wood so can absorb or release humidity fairly rapidly) but as the wood that is used, purnambuco, is extremely dense, isn't likely to have too much effect. The modern bow has many advantages, even though it doesn't have this in-flight tension adjustment capability, such as better balance, better springiness, a range of stiffness is and frequency of bouts depending how near the heel of the point about making contract with the string, and so on. This all goes together to permit a huge range of bowing techniques and variation in sound volume and quality, which cannot be achieved with a bark bow.
I think I forgot to mention, that it is the friction/stiction characteristics of the bow hair that causes the vibration as the bow is drawn across the string, quite different to the standing waves in the air that are created when blowing a wind instrument, and are dependent on being excited by a of turbulence and pressure waves (as created when blowing against the edge of the sound hole of the flute), or the pulsating in air pressure created by the vibration of a reed, or in a brass instrument by the preparation of the players lips, which act in some respects like a reed.
That have a look at what happens when you draw the bow across a string that is in high tension between the bridge and the knot of the violin. I haven't mentioned that the bow hair has rosin applied to it (this is a purified and treated tree resin). Is basically the same stuff that is used in powdered form by gymnasts are working on the parallel bars (they put on their hands to give grip), or ballet dancers in their shoes, regained give grip, and also on hand and shoes of free climbers, where the rosin is carried in a small pouch on their way spelt. In this case it's to try and stop them killing themselves, but it's not always successful. Playing a violin is nowhere near as dangerous as this, at least not for the player but is a violinist is bad, it could be similarly dangerous for the audience!
So rosin is applied to the bow hairs to make them more grippy, which means that the coefficient of friction between the rosined bow hair and a violin string is increased. If the vileness applies pressure to the bow and starts to move the bow across the string, at right angles to the string, because of the high friction between the 2, the string is pulled and stretched to one side, much as when you pull the bow string on an archery bow. As the string is pulled to the side, it increases its tension and the force between bow and violin string that is trying to make it slide. Eventually the coefficient of friction is insufficient to prevent sliding, and the string twangs back to its original position. I mentioned earlier a term called stiction. This is an engineering term that is about the variation in sliding friction between something that is static and something that is moving. As soon as the string starts to slide across the bow hair to return to its original position, the coefficient of friction reduces, so it's much easier for the violin string to return back to its neutral position. Because of this, it's travelling so fast when it gets to its neutral position that it overshoots and stretches out in the opposite direction. Having done that, it then tries to return back to it's original resting position. This is what causes the vibration. But the violin bow is continuing to be pulled across the string by the violinist, so at some point or other when the string happens to be moving at the same speed as the bow during its vibrations, suddenly the coefficient of friction increases again (remember this friction/stiction thing of varying coefficient of friction). So the string sticks to the bow and is pulled out to the side again until eventually it let's go, and the whole sequence begins again. Thus if you keep the bow moving steadily, the vibrations will be kept going and you get a continuous load from the instrument.
The coefficient of friction (how hard you have to push something to slide it across surface) increases with the increase of force between the Sioux services. So if you want to play quiet note, you don't press hard on the bow, there is little friction between the bow hair and the violin string, so the vibrations that are set up are quite small. If you press hard on the bow, the coefficient of friction increases so that the string is pulled much further to one side before the forces become so great, and the string twangs back into position. In this situation the amplitude of vibrations are higher, the greater the amplitude of the louder the sound will be, so you play allowed note.
As well as varying the pressure you can vary the velocity of the bow, so that moving the bow slowly, the string tends to break loose sooner, and moving quickly tends to break loose after it has been dragged further to the side. This gives another means of producing soft and loud notes. So the violinist has to ways of altering the volume, the pressure on the bow and the speed of movement of the back. Although they both have an effect in volume, they also have an effect on the tone created by the vibrating string, so violinist is always juggling these 2 factors to achieve the volume and tonality that they wish. And remember that this can all happen within the course of a note that may last a fraction of a 2nd. All very clever stuff, try to get robot to do it!
And then, when you're listening to a performance of the string quartet or watching a ceilidh band or a barn dance band on stage while you're dancing, think of the poor fiddler or in the case of the string quartet, the violin, viola and cello players, all exciting there instrument strings in this way, using the laws of friction and stiction to achieve music.
You might have wondered what happens to the note as the violin string is dragged to one side and the tension is increased. If you increase the tension of the string, the pitch increases, this is the whole point about tuning an instrument and turning the pegs on that instrument. Increase the tension and the pitch goes up, decrease the tension and the pitch goes down. So when you drag a violin bow across a string it does change the tension. If you are very vicious with your playing, as when playing an accented note, the pitch will increase, it will go sharper. You get away with it because normally this sudden vicious accident only lasts a few moments. But even when playing a note of constant volume and pitch, as the string grabs hold of the bone is pulled aside, the pitch will increase, and as the friction is broken and the string twangs back to its natural position, the tension will reduce in the note will go down. Why don't you hear the note going up and down? Well, on a double bass, you can. The bottom note on the double bass is about 46 Hz (Cycles per 2nd). If bass player plays the note very loudly and you listen carefully, you can detect just about this slight wavering of the note, that 46 Hz is quite fast, is the speed more or less at the mains hum you hear on a piece of incorrectly earthed or screened amplification equipment i.e. the low hum. When you get to a violin, then the a string plays at this frequency of 440 Hz, much too fast to hear this warbling of the notices really happening.