 Automated music boxes began as cumbersome sets of bells struck by hammers, but over the course of several hundred years, they've evolved into compact devices like this one. I'm sure you know how it works. Wind it up, and it plays a tune. The melody is programmed on this rotating drum. The drum has protrusions called pins that pluck the teeth of the comb. The comb is a piece of steel with 18 teeth. Each tooth is a note. Longer teeth are lower notes, and shorter teeth are higher notes. The comb works like a multi-pronged tuning fork. In this high-speed video, slowed by 250 times, the drum appears not to move, but you can see the teeth vibrate. The shorter tooth is vibrating faster than this longer one. These vibrations produce the sound. The teeth are like this saw blade. When it's longer, it produces a lower note when plucked, and when shorter, it produces a higher note. Notice when you turn the comb over, the teeth don't have the same thickness. The longer teeth, the lower notes, are weighted more on the ends. This added weight lowers their resonant frequency even farther. Here, I taped a lead weight to the end of the saw blade, and it produces a lower note than without the weight. Because of this weighting, the comb is more compact. For this particular design, if the comb were unweighted, it would have to be roughly 40% longer to produce the same range of frequencies. Another advantage of the weighting is that the combs can be mass manufactured in a single size. You just cut away the proper amount of material to produce a unique set of notes. For example, although each comb has 18 notes, the specific notes vary for a particular song. Here's a music box playing London Bridge with a comb specifically designed for this melody. And now, here it is with the comb cut for a different melody, this old man. The timing is the same, but the notes are different, and it sounds odd. The difference in weighting is so subtle that these two combs are indistinguishable by eye. Inside the casing of the music box is a clock spring. It's a coiled strip of steel that is 40 centimeters long unwound. The outer end of the spring has a t-shape, which affixes to the casing of the music box, and so holds it in place. The inner end of the spring has a slot, this slot hooks into a notch on a metal shaft. This shaft is attached to the winding key. The shaft also has an angled six tooth ratchet gear. This gear fits inside this larger plastic gear. On the inside, there are four flexible poles, so the axle turns independently from the plastic gear. This happens when the music box is wound. When the spring unwinds, the axle turns in the opposite direction, and the six tooth gear catches the poles, which rotates the larger plastic gear with it. This rotation drives the music box. As the spring unwinds, it rotates this bevel gear, which engages a second bevel gear affixed to the drum. But there's a problem with this setup. The spring will unwind quickly, and the music will play too fast. This piece, called the governor, solves this problem. It's connected to the drum by a gear train. The gear train is compactly built into the music box. The rotation of the governor controls the speed, stop the governor, and the drum stops. The governor uses air resistance to control the release of the energy from the spring. Air resistance is proportional to the velocity squared of the object. When started from rest, the governor encounters little resistance and speeds up readily. But when it spins rapidly, over 3,000 revolutions per minute, air resistance swiftly increases, which prevents it from moving much faster. This action limits the speed of the governor, and limits the rotational speed of the drum. To spin the governor so fast, the music box uses a multiplying gear train. It starts with the bevel gear driven by a spring, which engages a smaller gear on the drum. This multiplies the rotational rate by the ratio of the number of teeth on the larger gear to the number of teeth of the smaller gear, here 2.75 times. The drum is also affixed to a larger gear, which engages another smaller gear, this time multiplying the rotational speed by 5.75 times. The larger gear on this piece engages a smaller end of another spur gear, further multiplying the rate by 6.3 times. Lastly, this spur gear engages a worm screw on the shaft of the governor. It moves so fast it's blurred. Here, slowed down by a factor of 30, the movement is visible. The gear that engages the worm screw differs from the other gears. It has curled teeth. The shape of these teeth allows it to better engage the screw. The worm screw turns once for every tooth on the gear, and since there are 24 teeth, it multiplies the rotational rate by 24 times. This means that for every single revolution of the first bevel gear, the governor rotates 2400 times. Since the first gear rotates roughly one and a half times a minute, the governor spends at 3600 revolutions per minute. As I noted, this music box evolved from devices that used bells struck by hammers. The replacement of these bells with a comb was the technical breakthrough that catalyzed a music box industry that blossomed in the 19th century. The compact comb movements were built into snuff boxes, clocks, and large pieces of furniture. As the industry flourished, music boxes grew more complex. Some, for example, sported dual barrels and combs, which played simultaneously to produce rich harmonies. The first music boxes used cylinders, but were superseded by boxes that used discs which could be easily changed. Here, the melodies were punched into a metal disc. With this innovation, music boxes shrunk and their costs declined. For a hundred years, music boxes were the way a family listened to music in the home. But by the turn of the 20th century, the phonograph and radio had displaced them. Music boxes were shoved into attics or, more often, left to rot in junkyards. These modern music boxes, then, are a charming vestige of a past filled with brilliant engineering and craftsmanship. One last thing. If you hold a music box in your hand, it's not very loud. But if you place it on a hollow container, it's much louder and richer. The vibrations of the comb are transferred through the metal base into the container where they resonate. This resonance amplifies the sound. Also, if you rest the music box gently against your teeth, the music will resonate inside your skull. So the next time you listen to a music box, appreciate its sound, but also think of the centuries of innovation and design that led to it. I'm Don Hammack, the engineer guy. The drum has protrusions called pens that pluck the teeth of the comb. The comb? Pins. Pins. Pins, right? Pins. Pins. Pins. Okay, I'll get it. The drum has protrusions called pens. The drum has protrusions called pens. Pins? Pin. Pin. Yeah. Okay. The drum has protrusions called pens that plunk the teeth. Can I get it right? In. In. In. Say in. In. In. Pin. Pin. Okay. The drum... And now I can't say the I in one because when I say it, I say pen. Pen. Pen. Pen. Okay. The drum has protrusions called pens that pluck the teeth of the comb. The comb is a piece of... Did I not get it? Uh-huh. Pen? Pin. Pin. Pin. In. Pin. Pin. Is that right?