Here we present you with a bit of a mystery: That of the centrifugal mercury clutch. The design was patented in 1950 by Automatic Steel Products Inc. of Delaware, and featured in this 1959 advertisement.

Other centrifugal clutches are used today in a multitude of applications, in everything from power tools to small vehicles. In short, clutch shaft connects to motor; as that motor increases speed, attached weights swing outward and push clutch friction pads into an engaging surface. Then the two parts spin as one. Centrifugal mercury clutches are uncommon, but their operation is similar: Here, the heavy liquid metal swings into a chamber to apply force for consistent torque, even at up to 100 hp. Part of their uniqueness is that the clutches build this torque gradually, because the flow of mercury takes time.

What else do we know? Well, Automatic Steel Products is no longer in business under that name. The company was involved in a 1960 lawsuit that set legal board precedent. In addition, usage of mercury is drastically reduced since the inception of the Environmental Protection Agency 30 years ago, and its increasingly strict regulation of mercury transport, use, and disposal — detailed at

There, the trail ends. How widely were centrifugal mercury clutches used in their heyday? Were they always limited to specialty applications? Hard to say …

Do you remember anything about “MercoTorque” action? Or do you know of an application that still runs or requires a centrifugal mercury clutch? If so, please email us at If we print your insight in a future issue, you'll receive trivia trove and cookbook What Einstein Told His Cook: Kitchen Science Explained by Washington Post columnist Robert L. Wolke.