The illuminated cage

rohit gupta | Updated on January 20, 2018 Published on April 08, 2016

One and the other: The ball for night polo (right), and the gyroscope (Bohenberger, 1813) which redefined the course of warfare.


What connects polo for women at night, World War artillery and smartphones?

The most unusual object in the Textile gallery of Jaipur’s City Palace Museum is a spherical cage of brass, the size of a volleyball. The placard says that it was used to play polo at night by royal ladies, since modesty prevented them from doing so in daylight. A candle placed on the disc inside the ball would illuminate it like a lantern — which could be whipped around the field with a mallet by horse-mounted women.

Whether gender segregation spurred this scientific ingenuity (because necessity is the ‘mother’ of invention or not) is a different question. More importantly, how can a glass-encased candle spinning in three dimensions stay aflame for more than a few seconds? Because access to the instrument and historical record is restricted, and the date of its manufacture unknown (polo has been played in Asia for centuries), we must resort to conjecture.

One mechanism that could keep the candle upright even as the ball was thrashed around, is a gyroscope (see image). The basic idea comes from an ancient children’s toy — the spinning top. Newton’s first law of motion describes inertia as the tendency of a body “to remain at rest or in uniform motion unless subject to external forces”. In the case of an upright spinning top, the force of friction gradually slows it down until it collapses. A more extreme example is the Earth itself, with an axis of rotation tilted by about 23.4 degrees. This axis wobbles like an acorn over millennia, which we call its ‘precession’. As a result the celestial North Pole drifts away from Polaris, pointing at the star Vega instead... and then circles back to Polaris in 26,000 years.

Inside a gyroscope, an Earth-like top is supported by clamping its axis in two more gimbal rings, all of which can move freely and independently. Relatively speaking, now it does not matter whether the central body moves or the outer shells holding it. In the case of the ‘night polo ball’, the outer shells are in motion, while the candle-disc stays ‘at rest’ — independent of all its surroundings. On board a ship in a stormy ocean, such an instrument would act like an inertial compass, “...as if it were a thing apart from the earth. Its own inertia is its only law, but that law is absolute.”

At such times, it would be far more useful than a magnetic compass or the astronomical sextant. Just like the famous Foucault’s pendulum, one could use a ‘gyrocompass’ to detect the rotation of the Earth at any point of latitude on the terrestrial surface. During the 1800s and early 1900s, these instruments became further refined after their parts were miniaturised and the rotation was stimulated by electrical motors.

World War II brought the dormant power of this children’s toy into the frontlines like never before. Until then, the gyro had been employed chiefly as a stabilising and navigation agent — for battleships, torpedoes, and airplanes. Airpower at this point still overwhelmed the navies and infantry, the planes moved simply too fast to be aimed at with accuracy.

The secret lay in precession, or what happens when a gyroscope is moved by an external force, such as a gunman. In 1940, it was realised that the axis of a gyro inside an anti-aircraft gun could track the movement of gunsight as it followed a plane in the sky. And not only that, “The gun is actually pointing ahead of the target by the right amount so that the enemy will fly into the bursting shell,” boasted the magazine Popular Science (in the July 1945 issue) just before the Allied victory. General Patton himself had praised the magical micro-wheel’s contribution to the war. Only a few days later this descendant of a harmless children’s toy would also help pilot B-29 planes and guide the two nuclear bombs dropped over Japan.

Speaking of toys again, who invented the night polo ball for the royal ladies of Jaipur? We can only suspect that it must have been someone familiar with the tradition of royal astronomy started by Sawai Jaisingh II in 1735. The relative motion of miniature astronomical models — celestial globes, telluriums and orreries — is very similar to a gyroscope. Today the large majority of motion sensing for telescopes, tablets, gaming consoles, virtual reality headsets, smartphones, satellites and spacecraft is done by micro-electromechanical great-grandchildren of the gyroscope (sometimes called accelerometers) employed by the female polo players of our story.

Every time that a group of young women position themselves to take a selfie, they look through their eyes like a World War II gunner, but the phone re-orients itself using its sense of inertia. Like the gyroscopic cage, the selfie is making us aware of a phenomenon beyond the Earth. The sudden twisting of faces into duck-pouted lips, and that one raised eyebrow — is it not like an angular shift in some symmetric axis of the mind? Or the mirrored image of some mysterious force which pervades the whole universe?

(Rohit Gupta explores the history of science as Compasswallah)


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Published on April 08, 2016
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