Recreating the Cavendish Gravity Experiment: My First 2 Attempts

Cross-section drawing from Henry Cavendish's 1798 paper published in Philosophical Magazine. Credit: Public Domain

By Andrew Bennett

The Cavendish Gravity Experiment Explained

In the late 1700s, a British scientist named Henry Cavendish developed a device to measure the gravitational force between two small objects. The data from his experiment was used to determine the mass of the Earth, as well as the value for the Universal Gravitational Constant, which appears in Newton's Law of Universal Gravitation.

Nearly everyone is familiar with gravity as being the thing that makes us fall down toward the ground. The revelation that all objects (not just stars, planets, and moons) exert gravitational pulls on each other is fairly shocking for most of us. For small objects, like you and me and a rock, the size of this force is so small that we never notice it. Cavendish's experiment was the first time that anyone was able to measure the size of this tiny force.

The unfortunate side effect of building a device sensitive enough to measure this tiny force is that the device is prone to disruption by even very small disturbances. The air movement created when someone walks near the device is enough to disrupt it for hours sometimes. Getting a highly precise value for the Gravitational Constant (G) has been an ongoing challenge for physicists for more than 200 years, and for much of that time, the Cavendish Experiment was the only way we could make that measurement.

Even today, scientists look for different ways to measure that constant and sometimes update the value slightly to reflect new data.

Why Am I Trying to Recreate the Cavendish Experiment?

Mostly, I just enjoy having people who think the Earth is flat calling me an idiot on YouTube. On a more serious note, this experiment is a fascination for many of us. The idea of being able to observe the effects of the tiny gravitational force between non-planetary objects is captivating for me. This experiment has been done countless times. There are better ways to measure the value of G now. All that aside, though: I'm just fascinated.

My first two attempts (and undocumented, incomplete third attempt with help from my AP Physics students from a year ago) have given me some insight into the complexities of designing and building an instrument that is both simple and extremely sensitive. I think I now have the skills to be able to design and build an apparatus that will be fully functional.

Diagram of the torsion balance used in the Cavendish experiment. Credit: Chris Burks (Chetvorno) [public domain], from Wikimedia Commons

Why Are There No Videos of Professional Scientists Conducting This Experiment?

First and foremost, YouTube is simply not the preferred medium of science. Maybe it's old-fashioned, but science is still largely communicated through written articles in peer-review journals.  Scientists, by and large, don't have any incentive to publish their work in a friendly-to-the-masses format on YouTube. Instead, they gain recognition, prestige, and funding by publishing in journals that are read by only a select group of people who are both interested in that type of research and educated enough in the field to be able to make sense of what is written.

Secondly, the Cavendish Experiment is outdated. Much of what I've seen about recent studies of the Gravitational Constant focus on atom interferometry, rather than using a torsion balance.  

The result of these two factors is a void of professional-level videos showing a well-working Cavendish apparatus being used to calculate an acceptably reliable value for G. The videos that are on YouTube (my own previous attempts included) seem to be from amateurs with varying levels of success and understanding. This gives fuel to the Flat Earth groups who point to YouTube as though it were the repository of all scientific information and say, "See? This experiment is nonsense. Gravity is fake and the world is flat. I can see through the conspiracy."

My goal in this series is to develop a working Cavendish Experiment and to post robust, low-uncertainty data from that experiment showing the gravitational attraction between small objects and calculating a value for the Gravitational Constant in a format that is more accessible to the masses than articles in professional science journals.

Cavendish Gravity Experiment Time Lapse Version 1

My first attempt at recreating this experiment was in fall 2013. Unfortunately, the results were compromised due to environmental problems. The air conditioner in my classroom was turning on and off, and the vent was blowing on the apparatus. Overall, this setup had too much uncertainty to be able to observe the desired changes.

This video had a surprising number of views and feedback (some helpful feedback and some rather angry comments from Flat-Earthers). I kept the helpful comments in mind for my second recreation of the famous gravity experiment.

Cavendish Gravity Experiment Time Lapse Version 2

My second time-lapse gravity demo was recorded in fall 2016. This version of the experiment addresses some of the issues that came up the first time around. However, I'd still consider this the testing phase for developing a better Cavendish apparatus.

Coming Later This Summer: Gravity Demo Version 3.0

Please subscribe to my blog and YouTube channel to stay in the loop on my big summer project: a new and improved recreation of the Cavendish experiment. I'm currently building a large-scale apparatus in my home, so I can better control the environmental conditions. This should allow me to make measurements precise enough to determine the value of the Gravitational Constant.