Quartz clock
Long before the wristwatch, quartz filled a whole cabinet in a telephone company's laboratory. The engineers didn't want an accurate second, they wanted an accurate frequency. The second came along for free.
- 1880the Curie brothers discover piezoelectricity
- 1927Marrison and Horton build the quartz clock
- 50,000 Hzthe frequency of the quartz bar
The clock nobody ordered
In the 1920s many conversations shared one telephone line, each on its own carrier frequency. Those frequencies had to stand rock still, or the conversations ran into each other.
In 1927 Warren Marrison and Joseph Horton at Bell Labs built a quartz oscillator and, to test it, divided its frequency down and connected a clock. The clock turned out to be more accurate than almost any other in existence.
A stone that answers with a current
In 1880 Pierre and Jacques Curie noticed that squeezing a quartz crystal puts a charge on its faces. And the other way round: a voltage makes the crystal deform very slightly.
A bar of quartz between two electrodes can be set vibrating by a current, and it rings at its own frequency like a tuning fork. That frequency depends only on the bar's size and shape.
Pushing in time
On its own the bar would soon fall silent. The valves amplify the faint current the quartz gives off at each vibration and feed it back to the electrodes exactly in step. It is the same feedback as a pendulum and its anchor, only at 50,000 vibrations a second.
From 50,000 to one
Valve frequency dividers brought 50,000 vibrations a second down to a low frequency that drove a synchronous motor. The motor turns the hands like an ordinary mains electric clock, except that the mains here is a crystal.
The hands in the drawing show your time. The seconds hand is the brass-coloured one.
Kept at fifty degrees
Quartz has a weakness: its frequency drifts with temperature. A bar of that era moved by about 20 parts per million for every degree, which is almost two seconds a day.
So the quartz was kept in an oven at a constant temperature above room temperature, 50 °C in the drawing. We have warmed the room to 30 °C, and the oven's thermometer hasn't moved.
What happens if you switch it off
We have switched the oven off. The bar cools to room temperature, its frequency creeps up, and the clock starts to gain tens of seconds a day. Move the pointer or swipe sideways over the drawing to change the room temperature.
The Earth under suspicion
By the 1930s quartz clocks had overtaken the best pendulum clocks, and in 1939 the Greenwich Observatory switched to quartz. The new clocks were so steady that they revealed the unsteadiness of the Earth itself: it spins a little faster or a little slower depending on the season.
In 1934 engineers learned to cut quartz at a special angle, the so-called AT cut. Its frequency hardly changes at room temperature, and the oven was no longer needed. From there the road led straight to the wristwatch of 1969, where the quartz shrank to a tuning fork the size of a grain of rice.