Wikipedia:Reference desk/Archives/Science/2022 April 24

= April 24 =

handheld scale
I was at an outdoor Chinese market and the lady there weighed vegetables on a handheld scale that looked pretty cool, primitive and yet totally functional. I didn't manage to look at it closely since she didn't seem to like it when I tried. I am wondering what it worked. I think it may have been a steelyard balance but according to the article, those work by having the weighing pan at one end, the pivot in a fixed location some distance from the end, and a sliding weight that you move on the arm to make the arm horizontal.

I am wondering if there is a known variant of this, where the weighing pan is fixed at one end of the arm, the counterweight is fixed at the other end, and you move the pivot until the arm is horizontal. The pivot would be a knife-like edge and the arm would have a bunch of marked notches for different weights. So you'd put the knife edge under whichever slot brought the whole thing into balance.

If that's not a common design, is there a reason it wouldn't work? I'm thinking of making something like it as a very portable weighing scale. Materials would be a wooden chopstick as arm, pocket knife blade for pivot, and water bottle or similar for counterweight. Thanks. 2601:648:8202:350:0:0:0:4671 (talk) 07:50, 24 April 2022 (UTC)


 * Well it would work, but (if I understand correctly) to adjust it you would need lift much of the mechanism plus the load, rather than simply slide a small mass along the bar, so it would be a bit fiddly. And it would only be as accurate as the distance between the notches.--Shantavira|feed me 08:21, 24 April 2022 (UTC)
 * A simpler design is the traditional apothecary’s balance, and our weighing scale article also has some details. Alansplodge (talk) 09:53, 24 April 2022 (UTC)
 * Thanks both. The apothecary balance is a double pan balance that needs a set of weights, so it doesn't seem practical here.  I'm trying to have something very lightweight and simple.  Lifting the balance with the load isn't likely to be a problem since it would only be for weighing light stuff.  I gave the example of a water bottle (500g of 1kg) as counterweight but in practice the loads and counterweights would be a lot lighter, like a few ounces.  2601:648:8202:350:0:0:0:4671 (talk) 10:55, 24 April 2022 (UTC)

Chinese "daching" scales for traders were first mentioned in western literature in the 17th century as "dotchin" scales. They are a type of steelyard. On one side of a horizontal rod hangs the pan and a brass weight hangs on the other side of the rod for counterbalance. The rod is marked with dots as counters for weights. An object is weighed by putting it on the pan and moving the weight along the long arm of the rod. When the rod is exactly horizontal the weight of the object is indicated on the engraved scale on the rod. See this antique example in Singapore. Philvoids (talk) 10:58, 24 April 2022 (UTC)


 * Thanks! Yes I think that is what I was looking for.  Some further searches turned up more examples, including this one on fleabay that has some good photos.  I think the purpose of the multiple cords on top of the arm are to weigh heavier and lighter objects by moving the pivot closer to the load.  I love the levelling needle on the one that you linked.  I think I can concoct something like this based on the pictures.  Wow!  2601:648:8202:350:0:0:0:4671 (talk) 16:11, 24 April 2022 (UTC)

Closely related is the "Pendulum Scale", which has a fixed counterweight. The advantage of this type of scale is that you don't have to fiddle around with the counterweights because they're fixed. I think the disadvantage is that they have fairly narrow ranges of weights they can handle. You mostly see them sold as pocket postal scales, because they're more portable than comparable digital models. ApLundell (talk) 00:14, 28 April 2022 (UTC)

Raindrops on eyelashes
When I see e.g. large green reflections from traffic lights due to raindrops on my eyelashes, is it diffraction, refraction or something else? Thanks. 212.180.235.46 (talk) 15:39, 24 April 2022 (UTC)
 * The droplets are rounded, so they act as lenses. which is refraction, and at that scale you are likely to see some diffraction, so yes. Abductive  (reasoning) 17:26, 24 April 2022 (UTC)