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Journal Club Week 11 Answers

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Sophia Chen (2018), Kilogram Redefined. The Metric System Overhaul Is Complete, Wired

Below are a selection of the best students summaries for this week, congratulations!

What one thing did you learn from the interview with Sophia Chen?

I thoroughly enjoyed watching the interview with Sophia Chen because it allowed me to gain a perspective from a writer's point of view that I have never accessed before. In the interview, the main piece of knowledge that I learned was the process that science writers undergo to get their work published and the different ways that this is possible. From the initial steps of finding a topic that yourself and the publisher are interested in developing to then actually writing articles and working with the publishers to get this work finalised is really interesting and I would never have known the different ways of publishing work without having watched this interview so I found it very beneficial as it is something new to consider. In particular, I found the question on how Sophia got into science writing intriguing because for Sophia it started off as just an internship at a radio station and then has developed into something much greater than that to having her work published and I just think that it's so amazing how it can go from just being an interest to a full job.


Why do we need to have a ‘standard’ for the kilogram?

A standard is needed so you are able to reliably know the masses of anything. This is important for a huge array of things, all the way from international trade and manufacturing to baking. For science, in particular, knowing the precise weight of different components is crucial because it is vital for ensuring that experiments can be reproduced - and thus that theories can be tested. Without a standard for the kilogram, we would be unable to reliably compare the masses of two objects unless we physically had both of them and a mass balance. Obviously, this is a completely unworkable model that would completely hinder the mass production of goods, especially pharmaceuticals and electronics because of the very small tolerances in mass.


How does a Kibble balance work? And what is its advantage over ‘Le Grand K’?

The Kibble balance is a balance beam system with a motor at one end and a mass pan with a superconducting electromagnet below. It runs on two modes: weighing mode and velocity mode. In weighing mode, the kilogram mass standard is placed on a mass pan, and a current is passed through a superconducting electromagnet to provide a magnetic force to counteract the gravitational force due to the Earth’s gravitational field. The surroundings can have an influence on the value of the gravitational field strength, so a gravimeter is used to measure it precisely. Once the system is in equilibrium (the mass standard in perfect levitation, measured using a laser interferometer) we can equate the two forces to find mass.

However, the magnetic field strength(B) and length of the wire(L) are difficult to measure with the precision needed to improve the definition of the kilogram, so we need to find a way to eliminate these variables. This is where velocity mode comes in. In velocity mode, the coil of wire that constitutes the electromagnet is moved up and down at constant velocity by the motor, and this is adjusted until it reaches the voltage found for equilibrium.

We know that V = BLv, where V is voltage, B is magnetic field strength, L is the length of our coil of where, and v is the velocity. We also know that in the case where the gravitational force of the standard mass is equal to the electromagnetic force --mg = BLI -- where m is mass, g is the gravitational field strength, and I is current. Through algebraic substitution we can eliminate B and L to find m. Voltage is measured very precisely and related to Planck’s constant through a quantum phenomenon known as the Josephson effect. Current is substituted through Ohm’s law as V/R where R is resistance, and resistance is measured precisely and related to Planck’s constant through the quantum Hall effect.

The advantage of this method over ‘Le Grand K’ is that we are no longer basing the unit on a physical prototype whose mass could and has changed slightly over time. By relating mass directly to Planck’s constant, we are no longer reliant on a changing value but rather one which is universal and intrinsic in nature.


Whilst we know now that Team Planck ‘won’ the race to redefine the kilogram, after reading everything this week, are you Team Avogadro or Team Planck? Explain why.

Whilst both teams presented new and ingenious ways of defining the kilogram, there was clearly a fault with team Avogadro’s plan from the start. Even when using pure silicon 28, there was still a fraction of the silicon sphere that contained different isotopes, which as small as it was, would have made a difference to their measurements on the macroscopic scale considering such a large amount of atoms were being used. The existence of these isotopes in the sphere meant that there would be an error margin when counting how many silicon atoms make up exactly one kilogram. However what is most problematic about this approach is that the amounts of different isotopes in the silicon (even in the pure silicon 28 sample) would be different for every sphere, so the team would get a variety of different results depending on which sphere they used. Of course, they could take an average of these results, but with any finite sample size there would still be uncertainty and for that reason, I agree with team Planck.



Remember, reading a paper isn't like reading a piece of fiction or a newspaper article. Don't get frustrated if it doesn't immediately make sense - you might need to do a little research of your own to understand some of the ideas. This article gives you an idea of how scientists read differently.

Each question refers to a specific part of the paper e.g. Page 2, Column 3 is written as (P2, C3).

Next week, we'll publish solutions to the questions and the best submitted summaries from students across the country.


As it's the last week before the 'summer holidays' for most of you, we are hoping to give you something very current and very detailed which might take a bit more thought. We're hopefully going to be looking at recent experiments on neutrino oscillations and how these are vital for our understanding of how the universe came to exist in the first place.