Resonance watches are one of the rarest types of watches, and for an excellent reason: They're extremely difficult to pull off, certainly with any reliability. Beat Haldimann makes a resonance tourbillon (in extremely small numbers) and of course, F. P. Journe is celebrated for his Chronométre à Résonance, but in the modern watchmaking world, which is, as far as complications are concerned, increasingly populated by endless variations on just about every complication imaginable, the resonance watch remains extremely scarce.
A resonance watch is one in which there are two balance wheels, rather than one. There are double balance wheel watches that are not resonance watches, such as the Greubel Forsey Double Balancier and of course the Philippe Dufour Duality; in these watches there is a single going train, with the force from the mainspring barrel and train divided by a differential. The differential averages the rate of the two balances and the basic notion is that due to differences in the configuration of the balance springs and placement of the balances, positional errors should more or less cancel each other out. In resonance watches, however, the idea is to have the two balances oscillate in resonance with one another, which should produce greater rate stability than can be obtained with a single balance.
The OG Resonance Watchmaker: A. L. Breguet

Breguet conducted several experiments with resonance watches, but they remain extremely rare and when one came up for auction in 2012 it was a very big deal indeed. Find out more about no. 2667, and why it went for CHF 4.3 million, right here.

The problem is that the forces mechanically coupling the balance springs are almost homeopathic in strength – the outer attachment point of the balance springs exerts an extremely tiny lateral tug on the movement plate, which is how vibrations get transmitted from one balance to another. It works, but the two balances have to be adjust to a fairly close rate to each other to begin with – Breguet himself was incredulous that his experiments actually worked, writing, "This appears to be absurd, but experiment proves it a thousand times over." (For years after François-Paul Journe debuted his resonance watch I vocally dismissed it as an impossibility, and then finally in 2014 I actually got around to reading the chapter on Breguet's experiments in The Art Of Breguet, and discovered that Journe had very much extended Breguet's work, which led to an online mea culpa.)
There is however another way to achieve a resonance effect, and that's to directly mechanically link the balance springs so that they operate in phase with each other without relying on the tiny forces a balance spring produces on the mainplate. Linking the two balance springs is an approach pioneered, as far as I am aware, by Beat Haldimann, and for the last several years, a variation on this method has also been used by Armin Strom, who debuted it in their first Mirrored Force Resonance watch in late 2016. The Mirrored Force Resonance watches have two separate going trains, each one of which directs its energy to one of the two balances. The balance springs are connected by an extremely fine, quite complex steel spring, which provides a comparatively robust connection and which has the added advantage of being beautiful to look at as well.*
A logical thing to do when you have two balances oscillating in synchrony, is to have two dials, and this most recent version of the Armin Strom resonance watch – the Masterpiece 1 Dual Time Resonance GMT – does exactly that. The mechanism for bringing the two balances and springs into synchrony with each other remains the same, but each of the dials can be independently set, allowing you to read the time in two different time zones. It's a bear of a watch; the case is 59mm x 43.4mm, and 15.90mm thick, which puts it well into pocket watch range and in the same company as other examples of exuberantly over-the-top, highly complex wristwatches intended at least as much for entertainment value as for actual chronometry (I think, for instance, of the Harry Winston Opus 14 from 2015). At the same time however, this titanium-cased mega-watch has some serious horological intentions as well, acting as a sort of on-the-wrist nod to a concept that has fascinated many watchmakers, but which very, very few have been able to implement, even taking the entire five hundred or so years of mechanical horology into account.
The real litmus test for this sort of thing is, of course, whether or not it actually works. Armin Strom says, " ... laboratory testing has revealed gains in precision of 15-20% for two COSC chronometer-level regulated movements placed in resonance." I didn't have a chance to test drive this particular watch, however I did have an opportunity to test drive an earlier Mirrored Force Resonance wristwatch in 2017 over an eight-day period, and in daily use the two balances never became desynchronized and the watch kept the same rate, losing exactly 6.5 seconds per day.*
While that may not sound impressive from an accuracy standpoint, bear in mind that the objective of such an exercise is rate stability, which is not synonymous with accuracy – the goal is to have an oscillator system whose rate varies as little as possible (marine chronometers were considered to be fulfilling their purpose if they did not wander on their rates – a chronometer that is always the same number of seconds fast or slow per day, allows a navigator to obtain the necessary correction for the right time with ease).
The movement, caliber ARF17, is 52.55mm x 39.95mm, and 11.67mm thick, running in 70 jewels at 25,200 vph.
The approach taken by Armin Strom does result, obviously, in an extremely big watch, which the titanium mitigates to some degree, however at nearly 60mm across this is clearly intended as a major conversation piece, not a subtle addition to the history of thin, elegant watchmaking. Journe's resonance watch is quite compact for its complexity; he managed to fit two going trains, and two balances and their springs, into a 40mm case with a 40 hour power reserve. Inherent to the design of the resonance mechanism in the Armin Strom is that it requires more space and as well, the Armin Strom has a much longer power reserve than the Journe, at 110 hours.*
This is a limited edition of five pieces, priced at $169,000 each. It's the sort of thing that if you're disinclined to take an interest in technical challenges in watchmaking more or less as intellectual exercises, is probably not going to exert a lot of appeal; if you are inclined to do so, though, it's a most interesting addition to an extremely tiny collective body of work produced by an extremely small number of watchmakers. As an exercise in a particular kind of extroverted, complicated watchmaking, which has been a small but important part of watchmaking for centuries, I found it a very entertaining watch indeed, with more than enough serious watchmaking depth to make it much more than a mere novelty.
The Armin Strom Masterpiece 1 Dual Time Resonance GMT: five-piece limited edition in titanium; price, $169,000. Case, 59mm x 43.4mm, and 15.90mm thick; movement, caliber ARM17, with two balances connected by Armin Strom's proprietary resonance system; 110 hour power reserve, running in 70 jewels at 25,200 vph. Find out more about resonance watchmaking at Armin Strom right here.