What's the difference between quartz and liquid crystal?

How do quartz and liquid crystal make your watch tick?
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When you glance down at your watch, you're probably not thinking about how different quartz and liquid crystal are from one another. And you probably shouldn't -- by the time you're done ticking off their contrasts, you might be late to your next appointment. But while quartz and liquid crystal aren't even in the same state of matter, they have one important commonality: the average digital watch or clock won't work properly without them.

Before we contrast their functions inside of a watch, let's look at some of the properties of each substance. Quartz is one of the most abundant minerals found on the surface of the earth. Known by the chemical name silicon dioxide, quartz is a crystalline solid used as a component in jewelry and sandpaper. Quartz crystals are also a common component in devices like cell phones, television receivers, and, of course, watches and clocks. One of the main reasons quartz is used in so many electronic devices is because it is piezoelectric, meaning it generates an electric charge when pressure is exerted upon it. Quartz also demonstrates a reverse piezoelectric effect: When an electric charge is applied to a quartz crystal, it begins vibrating. As we'll see, these attributes become very important when it comes to keeping time.

Liquid crystals, on the other hand, are not a singular substance. First discovered in the 19th century by scientists trying to determine the molecular weight of cholesterol, liquid crystals are actually a classification of compounds that comprise a fourth state of matter: The rod- or plate-shaped molecules in liquid crystals tend to flow like liquids, but maintain the alignment and order seen in solids [source: Nobelprize.org]. You've probably noticed that LCDs -- also known as liquid crystal displays -- are commonplace in computers, televisions and a host of other pieces of technology. That's because liquid crystal molecules have optic properties that can affect light when it passes through them. The first watch with a liquid crystal display appeared in 1973 [source: The New York Times].

Now that we know about the different properties of quartz and liquid crystal, let's look at how each of them functions inside of a watch.

The Role of Quartz

Let's review the basic components that a watch or clock needs to work correctly:

  • a timekeeping mechanism, which measures the length of time by transmitting a constant frequency
  • a display to convey time in a way we can understand
  • an integrated circuit, which stores the date and time and communicates with the timing mechanism and the display
  • a battery to power these elements
  • a circuit board to house and connect all of the pieces [source: Exploring the Nanoworld]

Remember that we're talking specifically about digital quartz watches and clocks. These are different from the mechanical watches that must be wound regularly and do not usually incorporate quartz, and they're different from the analog watches and clocks that use gears to drive the hands along a face, which typically don't incorporate liquid crystals.

Quartz functions as the timekeeping mechanism. First introduced into watches by Seiko in 1969, quartz crystals have become standard for modern watch manufacturers in the decades since [source: Seiko]. The quartz used in watches vibrates at a very high frequency. This translates to great accuracy: The time displayed on a quartz watch might deviate from real time by just a few seconds in a given month [source: Lombardi]. Additionally, because quartz is piezoelectric, it requires very little power to vibrate, enabling a single alkaline battery to power a quartz watch for years on end.

Quartz works in tandem with the watch's integrated circuit. In short, the battery sends a small electric charge to the integrated circuit, which is attached to a quartz oscillator with a pair of tiny electrodes. The circuit passes the electricity through to the quartz oscillator and it begins vibrating. The quartz oscillators used in watches have been standardized to vibrate 32,768 times per second, or at a frequency of 32.768 kilohertz. The integrated circuit is programmed to count the vibrations of the oscillator and measure the intervals -- seconds, minutes, hours and so on. At each interval, the circuit transmits an electric pulse.

And where does this pulse go? The liquid crystal display. How does liquid crystal display numerals on a digital watch face? Read on to find out.

The Role of Liquid Crystal

LCDs on watches are divided into seven- or 14-segment sections, which display those familiar numerals.
LCDs on watches are divided into seven- or 14-segment sections, which display those familiar numerals.
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While quartz measures the passage of time in micro-moments that are imperceptible to humans, liquid crystal helps depict the passage of time on the display in ways we can easily comprehend.

We mentioned earlier that liquid crystals have optic properties that make them a favored component of LCD screens. Liquid crystal substances are classified on a spectrum between solid and liquid, depending on their individual characteristics. The most common type of display used for LCD screens on a digital watch is called a twisted nematic (TN) display. The molecules that make up the liquid crystal in this display are arranged in a twisted, spiral shape. But the chemicals can be untwisted by applying an outside force -- like an electrical pulse -- which will affect the light passing through them.

An LCD on a watch essentially sandwiches liquid crystal chemicals between two electrically conducting glass plates, with polarization filters attached to each plate and oriented 90 degrees to one another. The liquid crystal molecules naturally run parallel to the polarization filter, allowing the illumination from ambient light to reflect off of a mirror placed underneath the polarizer -- this is what gives the LCD its silver appearance. However, applying an electric charge to the liquid crystals changes their molecules' orientation to run perpendicular to the polarization filter, and the filter absorbs all of the light, leaving black cells on the display.

When the integrated circuit releases those electric pulses after every second, minute, hour and other intervals measured by the quartz oscillator, it communicates with the LCD to turn parts of the LCD on and off. The LCD is divided into a series of seven- or 14-segment sections, and each seven-segment section conveys a numeral zero through nine. The combination of sections can reveal the passage of time from 12:15 to 12:16, for example, or 1:00 to 2:00. (By virtue of having more cells, 14-segment sections can reveal letters as well as numbers.)

As you can see, quartz and liquid crystal are very different substances serving very different purposes when it comes to keeping time. But without both of them working together, we'd probably be running late.

Author's Note

The biggest surprise during my research for this story was the paradox: Liquid crystals and quartz have almost nothing in common, but within the guts of a wristwatch, they depend on one another. I guess I never considered this, because I haven't worn a watch in years (as long as I have my iPhone, I probably never will again). But it's striking to know now that, whenever I encounter someone sporting a quartz watch, time is dictated by thousands of micro-vibrations a second; that we recognize the forward shift because light passes through filters; and that this is all going on completely out-of-sight, yet so close to us in a simple piece of technology that we take for granted. It makes me wonder what's inside my garage door opener.

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Sources

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