A half kilogram standard is photographed at the National Institute of Standards and Technology. Photo: Salwan Georges/The Washington Post via Getty Images

The definition of the exact mass of a kilogram was updated on Friday, as the world shifts to use a scientific formula instead of the metal cylinder used for more than a century.

Why it matters: This change will apply across industries, including computing, climate change, manufacturing and more, the AP reports. The kilo is the last of "the seven main units humanity uses to measure its world," which include the meter, the second, the kelvin, and others. The director of the Bureau of Weights and Measures, Martin Milton, told the AP that updating the kilo is "a tribute to what was done in 1875," as its the last form of measurement to be updated.

  • The original form of measurement known as the Grand K, along with six copies, is kept underground in a vault in France. To open the vault, you need three different keys which are kept in separate locations.
  • Milton said the kilo is "one of the areas where all of the states of the world come together with absolutely the same objective."

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Joe Biden's campaign released his 2019 tax returns on Tuesday, showing that he and his wife, Jill, paid nearly $300,000 in federal taxes last year.

Why it matters: The release, timed just hours before the first presidential debate, comes days after a bombshell New York Times report said that President Trump paid only $750 in federal taxes in 2016 and 2017. Biden's team is hoping to make the tax contrast a sticking point during their showdown.

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New York City's coronavirus positivity rate has ticked up to 3.25%, its highest since June, Mayor Bill de Blasio said at a news conference on Tuesday.

Why it matters: The jump — from 1.93% on Monday — came on the first day that public elementary classrooms reopened in the city after months of closures, but guidelines state that all public schools will have to shut if the citywide seven-day positivity rate stays above 3%.