# The kg is dead, long live the kg – by Veritasium

The kilogram, mole, kelvin, and ampere will be redefined by physical constants.  Continue reading

# For Good Measure

Contributed by: Derek Totten

…a good balance of science and fun!

### Big Ideas:

•  Simple machines help objects to move.
• Simple machines and mechanisms make life easier and/or more enjoyable for humans.

# How We’re Redefining the kg

I have been obsessed with measurement for a long time and I’m not sure quite how it happened. The world’s roundest object played a role in this. I guess I’m just fascinated by how difficult it is to pin down a quantity like a kilogram. Continue reading

# Significant Figures in Measurements

Mr. Key outlines how we use numbers in science, with specific regards to precision and significance.

Please note that all questions, comments and critiques are welcome. However, because the primary purpose of these videos is educational for students/educators, the comments are moderated. Any comments that are deemed abusive, inappropriate, or unrelated to the topic matter will not be posted and the user potentially blocked.

# Why the metric system matters

For the majority of recorded human history, units like the weight of a grain or the length of a hand weren’t exact and varied from place to place. Now, consistent measurements are such an integral part of our daily lives that it’s hard to appreciate what a major accomplishment for humanity they’ve been. Matt Anticole traces the wild history of the metric system.

Lesson by Matt Anticole, animation by Globizco.

# World’s Roundest Object!

The world’s roundest object helps solve the longest running problem in measurement — how to define the kilogram.
A kilogram isn’t what it used to be. Literally. The original name for it was the ‘grave’, proposed in 1793 but it fell victim to the French Revolution like its creator, Lavoisier. So begins the tale of the most unusual SI unit. The kilogram is the only base unit with a prefix in its name, and the only one still defined by a physical artifact, the international prototype kilogram or IPK.

But the problem with this definition has long been apparent. The IPK doesn’t seem to maintain its mass compared to 40 similar cylinders minted at the same time. The goal is therefore to eliminate the kilogram’s dependence on a physical object. Two main approaches are being considered to achieve this end: the Avogadro Project and the Watt Balance.

The Avogadro project aims to redefine Avogadro’s constant (currently defined by the kilogram — the number of atoms in 12 g of carbon-12) and reverse the relationship so that the kilogram is precisely specified by Avogadro’s constant. This method required creating the most perfect sphere on Earth. It is made out of a single crystal of silicon 28 atoms. By carefully measuring the diameter, the volume can be precisely specified. Since the atom spacing of silicon is well known, the number of atoms in a sphere can be accurately calculated. This allows for a very precise determination of Avogadro’s constant.

Special thanks to Katie Green, Dr. David Farrant, the CSIRO, and the National Measurment Institute for their help. Thanks also to Nessy Hill for filming and reviewing earlier drafts of this video.

There is debate as to whether this is truly the roundest object ever created. The Gravity Probe-B rotors are also spherical with very low tolerances such that they may in fact be rounder.

Music by Kevin McLeod (incompetech.com) Decision, Danse Macabre, Scissors