Review: Arnold & Son Time Pyramid

Given that the lever escapement is over two-and-a-half centuries old, you would think that there’s nothing left to refine. All that can be invented should have been invented, with only the smallest improvements left to make. It’s a sensible thought, but it’s wrong, and the Arnold & Son Time Pyramid can show us why.

The fundamental issue with the mechanical movement is known as constant force—or rather, the lack of it. If you want your time-telling to remain accurate, the power that drives it needs to remain constant, providing an even level of energy the entire time. That’s the theory, but in practice it’s rather different. You know how a torch starts to dim as the battery runs dry? The same is true of the spring inside a mechanical watch. As it unwinds, the force it generates decreases. That’s easily demonstrated with an elastic band—the more you stretch it, the harder it becomes to keep it stretched. It, like the mainspring, puts up less of a fight as the tension it’s under lessens.

And the result of this absence of constant force is surprisingly dramatic, the mainspring producing just half of its torque at half wind, with a dramatic peak at full wind and dip when nearing the last few hours. With less force, the balance spring cannot be spun as far around its centre, decreasing the time between beats and speeding the movement up. It’s a conundrum that’s plagued watchmakers since the very beginning.

There have been a multitude of ways to combat this problem, each as inventive as the last. There’s the bulky fusée and chain, which uses a conical guide with a fine chain wrapped around it to gradually shift the amount of torque required to compensate for the spring. Then there’s the remontoire, an intermediate mechanism that draws power from the mainspring and releases it to the escapement once it has reached its minimum torque level, a bit like an electrical capacitor.

There are simpler ways around the problem too, using a physical block to prohibit use of the two extremes of the mainspring’s wind. IWC's seven-day movement, for example, is actually an eight-day movement with the last day of wind mechanically restricted to maintain the accuracy of the usable power reserve.

But the most common way to at least dampen the effects of decreasing mainspring torque is to run multiple mainsprings at once. A long, single mainspring marks the problem more pronouncedly than multiple smaller springs, which reduce—but don’t remove—the shift in accuracy as the power is drained.

As you can probably tell, it’s a tricky little issue, with no solution simple enough or precise enough to really consider the problem solved. But what if the answer for this age-old mechanical question could be found in modern technology instead?

When you look at the mechanical engine, you’ll find a similar problem to the mechanical movement: lack of consistent torque. Even the most carefully designed sequentially turbocharged engines can’t provide an even level of torque through the gears, resulting in a power delivery that has dips throughout the rev band—typically in the lower RPMs. It’s a by-product of the technology, one that’s been around for almost a century and a half.

The biggest development in fixing this power delivery issue is the hybrid engine. With the addition of a battery powered motor, capable of delivering full torque from zero RPM, the dips in the mechanical engine’s power delivery can be backfilled by the motor in order to generate a completely uniform experience—and it’s this torque filling that Arnold & Son is taking advantage of.

There’s not an electric motor within Arnold & Son’s Time Pyramid, but the calibre A&S1615 does have a secondary power source. Straddling either side of the dial you’ll see two mainsprings, fairly small ones, but this isn’t a system run in parallel like you might normally see. There’s a clue in the addition of two power reserve indicators, which hint at the independent operation of each spring.

Think of the right-hand primary spring as a car engine. When it is at full wind, the watch will receive the full capacity of torque and be running accurately. Of course, as that spring winds down, its torque would decrease—but that’s where the second spring jumps into action. This left-hand spring acts like the electric motor to torque-fill the engine, making sure the primary spring stays fully wound. So, for some forty-five hours, the primary spring will remain completely full, drawing from the secondary one to remain so, keeping the movement accurate.

When the secondary spring, the motor, is fully depleted, the primary spring, the engine, runs on its own for another forty-five hours, just like a hybrid car when the battery runs out. In this mode it will lose torque in the typical fashion, but will at least continue to run for a period—whilst it’s not being worn, for example. Keep the secondary spring topped up, however, and it will remain true and accurate.

As you’d expect with Arnold & Son, this whole array is beautifully presented, stacked in a pyramid shape that gives the watch its name. Often a movement can be seen from the back, and sometimes the front, but here you get an unbridled view from both, leaving you free to explore the ingenuity of the idea and the execution of its construction. Enjoy it—it’s not often you get to view the entire going train like this.

This is one of the things I love most about watchmaking: you think you’ve seen everything—and I’ve seen a lot—and then something like this Arnold & Son Time Pyramid comes along and blows your socks off. Not only is it a fine example of watchmaking, but it’s also the perfect illustration of the ingenuity and creativity of humankind, that ability to take something that already exists and has done for a long time and make something new and impressive with it. Long may it continue.

Looking for a Arnold & Son watch? Click here to shop now