Basic structure and principle of Japanese Wadokei

The History of Wadokei

The wadokei is driven by gravity. Like European clocks, wadokei typically feature two driving weights suspended beneath the mechanism. Both two weights work as power sources for clock moving as well as oscillating a hammer to ring a bell at every one toki.

The internal structure of wadokei is fundamentally the same as that of European clocks manufactured during the 14th and 15th centuries. These foreign timepieces were introduced by priests and were frequently presented as gifts to local daimyo (regional lords).

However, as noted earlier, Japan closed its borders to the outside world in the early 17th century. This isolation resulted in an information cutoff and a cessation of incoming new technologies. Consequently, the country became isolated and was conspicuously left behind in technological development. For instance, Japan was unable to mass-produce steel suitable for applications other than swords. Furthermore, horse-drawn carriages were never produced—largely due to the inability to manufacture high-grade steel springs, but even had such technology been available, they would not have been permitted in commercial service under the strict policies of the Edo shogunate, which sought to maintain regime stability by suppressing major infrastructural and transport developments. 

From the perspective of horological technology, advanced clockwork mechanisms were not introduced, and modern anchor or lever escapements were never adopted. Even toward the end of the Edo period, wadokei still relied on the old-fashioned crown escapement—the exact same mechanism as the old verge escapement invented in the 13th century.

When examining the internal structure of early single-foliot clocks made in Europe—such as the one shown in the picture—the structural lineage is unmistakably clear. Their core mechanism is virtually identical to that of early wadokei.

However, a fundamental problem arises with single-foliot clocks in this context: they were designed for European equinoctial hours (fixed hours). In Japan, as noted earlier, the timekeeping system was based on seasonal, variable units known as toki-temporal hours (or unequal hours). How did clock artisans and operators in the Edo period manage to adapt to these fluctuating hours?

The Single-Foliot Early Wadokei

As noted earlier, early Japanese wadokei are direct copies of European imports. These early model had only one foliot, often called itcho-tempu (一挺天符/一丁天符) in Japanese.

The first European clock was brought to Japan by Francis Xavier, who presented it as a gift from the Viceroy of Portuguese India to Ōuchi Yoshitaka (1507–1551), the daimyo of Suō Province (present-day Yamaguchi Prefecture). Unfortunately, this clock has not survived to the present day, leaving its exact appearance unknown.

One surviving example is preserved at Kunōzan Tōshōgū Shrine, which enshrines Tokugawa Ieyasu following his death in 1616. This clock was presented by King Philip III of Spain in 1611.

While the exact date and creator of the very first Japanese-made clock remain uncertain, Owari-shi—a historical record of Owari Province (present-day Aichi Prefecture) published in 1832—records that "Tsuda Sukezaemon, living in Tokiwa-cho, made a so-called 'tokei'." Sukezaemon was a swordsmith employed by Matsudaira Tadayoshi (1580–1607), the daimyo of Owari and fourth son of Ieyasu. He repaired a clock originally presented by the Emperor of Korea. Drawing upon the experience and knowledge gained from this repair, he successfully manufactured Japan's first mechanical clock, wadokei.  Although the exact year he repaired and built these clocks is unclear, his visit to Sumpu (present-day Shizuoka City) to repair a clock must have occurred before 1598.

In the early stages of adopting any new technology, imitation is entirely natural. From the very beginning of wadokei production, artisans in Japan examined imported European clocks with intense enthusiasm and meticulously copied their design—retaining the single hand and the single-foliot mechanism. Thus, the first single-foliot wadokei came into being.

Although the clocks made by Sukezaemon do not survive today, we are fortunate to have an invaluable technical reference: the Karakuri-Zui (Illustrated Compendium of Clever Mechanisms), published by Hosokawa ’Hanzo' Yorinao in 1796.

The internal structure of the single-foliot clock is clearly illustrated in the book, with the details of each gear meticulously depicted throughout the text.

The fundamental principle of a single-foliot wadokei is essentially the same as that of early European clocks featuring a verge escapement. A cord is wound around a barrel (or drum), with one end secured to the barrel and the other connected to a driving weight, which serves as the power source.

Unlike European clocks, which typically employ both an hour and a minute hand, traditional wadokei feature only a single, central hour hand. They possess neither a second hand nor independent minute tracking.

This single hand is directly linked to the hinoura-guruma (hour wheel) which rotates 360 degrees once every 24 hours. Rather than using a minute hand, the system relies on finely calibrated gauges or notches engraved onto the dial. Each major segment, representing one toki (approximately two hours), is further subdivided into ten equal increments, allowing for temporal readings with precision. This setup means that the single hand advances precisely every 12 minutes to mark the next small increment.

Finally, however, one massive problem still remains: how did they manage to adapt to the fluctuating, seasonal intervals of the Japanese temporal hours system?

How did they manage to adapt the single-foliot clock to the variable Japanese temporal hours?

The answer is deceptively simple: move and adjust the positions of the small weights suspended on the foliot twice a day!.

It sounds ridiculously—almost disappointingly—simple!

Every morning and evening, the operator had to manually adjust the position of the weights on the balance bar. Shifting the weights inward or outward alters the moment of inertia of the foliot, thereby changing its oscillation period and the rotational speed of the escape wheel.

This practice was widely used until the invention of the dual-foliot (nicho) wadokei.

However, imagine how tedious it must have been to adjust those tiny weights every single day—especially at dawn and dusk!

The Solution: The Invention of the Dual-Foliot Mechanism and Technological "Galapagosization"

The ultimate solution was to introduce a second foliot dedicated to nighttime operations. This is so called, nicho-tempu (二挺天符/二丁天符) in Japanese.

The two foliots—one for daytime and one for nighttime—were fitted with different small weight positions, allowing the oscillation period to vary and accurately reflect seasonal changes in hour in daytime and nighttime separately. This was a brilliant conceptual breakthrough. Instead of daily adjustments, the operator only needed to tweak the position of the small weights roughly once every two weeks in accordance with the nijushisekki (the twenty-four solar terms).

However, translating this concept into a working mechanism was far from simple.

At first glance, it might seem logical to link the switching of the two foliots directly to the movement of the hinoura-guruma (hour wheel). 

But wait a moment: the rotation of the hinoura-guruma is remarkably slow—completing only one rotation a day. If the foliot-switching mechanism were directly and slowly driven by this wheel, there would be a prolonged "dead time" during the transition phase, causing the clock to stop instantly.

Clock artisans in the Edo period recognized this as a formidable challenge, yet they successfully engineered a brilliant solution. 

While several methods were devised, one common approach employed a lever and a cam (shaped like a triangular plate) attached to the hinoura-guruma. Though the wheel itself turned sluggishly, the cam and lever mechanism caused the switching action to snap over rapidly when triggered, avoiding any dead time.

This was a remarkable invention by Japanese artisans.

Indeed, Initially, early wadokei merely mimicked European imports. Yet through relentless ingenuity, Japanese artisans remarkably refined and evolved these imported technologies into something entirely new and advanced.

This, I believe, represents one of the earliest examples of the technological "Galapagosization" unique to Japan. Much like cameras, quartz watches, VHS tape recorders, Walkman and the Shinkansen bullet train, the wadokei stands as a supreme epitome of Japan's distinctive path of hyper-specialized technological evolution.

The Basic Structure of Wadokei

The single-foliot wadokei works precisely as same as early European clocks in the 15th century.

The rotation of the barrel, which is driven by weght,  is progressively stepped down, typically through a train of four gears. Finally, through the alternating movement of the oscillating foliot, the fourth and final arbor—fitted with the crown escape wheel and fourth(final) pinion—rotates once every 60 seconds (1/60 of a minute) in European clocks, and approximately once every 120 seconds in Japanese clocks. This mechanism regulates the descent of the weight and maintains the timekeeping accuracy of the clock. Indeed, this is the most critical and vital component of both European and Japanese horology.

However, this initially simple wadokei possessed a fatal structural defect. 

It could not automatically adapt to the Japanese seasonal time system (toki), in which the basic unit of time—a toki—varied in length depending on the season. Consequently, the operator was required to manually adjust the foliot (the balance bar) frequently, specifically twice a day at dawn and dusk.

The Revolution—Invention of dual-foliot system—

This significant limitation was overcome by a revolutionary innovation: the introduction of the dual-foliot system (nicho-tempu). While the exact inventor and year of this development remain uncertain, this ingenious solution employed two separate foliots—one designated for daytimekeeping and the other for nighttimekeeping was astonishing. By mechanically switching between these two specific regulators at the precise border moments of day and night (known as Ake-mutsu and Kure-mutsu), the toki length could be adjusted for daytime and nighttime respectively and instantaneously. Thus, dual-foliot system was completed, resolving the fundamental issue of the seasonal time system.

The Historical Evidence and Development

Until the beginning of the 19th century, all documented literature described single-foliot clocks. A notable account was left by the physician Philipp Franz Balthasar von Siebold (1796–1866) in his famous compilation "Nippon," detailing Japanese culture, nature, and technology during his stay at the Dutch trading post in Dejima from 1823 to 1831. Intrinsically, Siebold described both single-foliot and dual-foliot wadokei in his work.

One of the earliest extant examples of a dual-foliot clock, manufactured in 1773 in Kyoto, was preserved by the Mody Collection, marking it as likely the earliest known wadokei of this type. Additionally, another dual-foliot clock dating to 1796 is preserved in the National Museum of Nature and Science in Tokyo.

Thus, the dual-foliot wadokei was successfully developed around the end of the 18th century. However, single-foliot clocks continued to be produced well into the middle of the 19th century. The precise reason for this overlap is not entirely clear, though it was likely due to the higher craftsmanship and manufacturing costs required for the more complex dual-foliot mechanisms.

The Mechanical Challenge: Preventing Clock Stoppage

The dual-foliot concept was not merely a theoretical idea; realising it required artisans to devise and perfect a sophisticated switching mechanism.

This innovation introduced a critical mechanical challenge: both foliots had to be switched simultaneously. If this transfer was not instantaneous, the escapement would momentarily halt, causing the entire clock to stop.

To resolve this dilemma, one brilliant artisan devised the dedicated switch mechanism.

Bell Striking Mechanism

Amazingly, Japanese wadokei possessed the remarkable capability to automatically ring a bell every single toki (approximately two hours).

It is driven by a mainspring and triggered by the movement of the hour wheel (hinoura-guruma). While the underlying gear train mechanism is quite similar to that of the clockwork itself, it dispenses with a conventional crown wheel escapement, instead incorporating a speed adjuster (kazakiri-guruma) to accurately count and regulate the bell strikes.

Traditionally, the bells used were modeled after Buddhist household altars (orin). In particular, those cast from sahari—a traditional high-tin bronze alloy containing a high proportion of copper and tin—produced an exceptionally sweet, resonant tone. Another material frequently used was brass, an alloy of copper and zinc. For our modern recreations, however, we prefer to use Silzin bronze (a silicon bronze alloy composed of copper, zinc, and silicon), as original sahari is very expensive and difficult to replicate.

As mentioned previously, the driving force for the striking train relies on a traditional, long flat spring.

Epilogue

The dual-foliot wadokei represents the zenith of Japanese horology, successfully achieving both adjustable hours and the automatic transition between daytime and nighttime settings.

However, the government decree abolishing the Japanese seasonal time system in 1872 abruptly rendered these magnificent devices obsolete, forcing traditional artisans to abandon their craft. Yet, their pioneering spirit and sophisticated technologies did not vanish; rather, they endured within our cultural fabric and way of life, ultimately bearing fruit in the legendary precision of modern Japanese manufacturing.