FEATHERS, LEAD, AND GOLD
When a Pound Is Not a Pound
Which weighs more, a pound of feathers or a pound of lead? When answering this age-old riddle, your instinct may be to go with the lead. After all, in equal amounts, lead is heavier than feathers. But it is actually a trick question: A pound is a unit of weight, so therefore, a pound of anything weighs the same as a pound of another thing.
Or does it?
If you put a pound of feathers on one side of a balance and a pound of lead on the other, the scale would rest with both sides even with one another. But what if you were to swap out the lead for gold? In that case, the balance would end up tilted, showing that the feathers weigh more. It may seem like dark magic, but it has nothing to do with the feathers or the gold: It has to do with how the weight is measured. Whenever you use the term “pound” (when discussing weight at least; it means something else when talking about cakes and British currency), you’re talking about pounds as defined by something called the “avoirdupois system.” It’s the standard, nonmetric system you’re familiar with if you live in the United States. In the avoirdupois system, there are 16 ounces in 1 pound. One avoirdupois ounce is equal to about 28.35 grams. Multiply those 16 ounces per pound by 28.35 grams, and you end up with about 453.6 grams per avoirdupois pound.
Feathers are measured in avoirdupois pounds, and so is just about everything else—from cheese and rubber bands to graphite and, yes, lead. But gold is a special case. It, and other precious metals and gemstones, are measured in something called “troy pounds.” (It’s unclear why that is, as the origin of the troy weight system has been lost over the course of history.) A troy pound is equal to about 12 ounces, and a troy ounce is equal to about 31.1 grams. Multiply the 12 ounces by 31.1 grams, and it comes to about 373.2 grams per troy pound—or about 20 percent less than what a pound of feathers weighs.
So if you want to win a bar bet, put a pound of feathers on one side of your (hypothetical) balance, and a troy pound of gold on the other side. The feathers will weigh more. Of course, testing this in real life may be a little tougher to manage; first, you’d need a balance that can accommodate roughly 50,000 feathers. Second, you’d need to make quite the ATM withdrawal, as a troy pound of gold costs more than $10,000 as of 2018. However, if you did manage to overcome these hurdles, you’d come away victorious.