Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.
Yesterday I saw some odd pet walking on a leash in central Warsaw. Neither dog, nor cat, medium-sized, had an elongated snout, orange fur with dark stripes on the spine and tail. Don't have a photo, but if someone suggests an image, I'll probably guess which one. I don't think it's some exotic dog/cat breed. Brandmeistertalk 06:53, 6 May 2018 (UTC)[reply]
Kindly check if my English is correct and if there's anything to add on this brand new article. Thank you! Ericdec85 (talk) 08:48, 6 May 2018 (UTC)[reply]
The English looks fine. I've added a second reference to show that it started last year. Rojomoke (talk) 09:25, 6 May 2018 (UTC)[reply]
Could a DC device be 'agnostic' regarding the polarity of the current it receives?[edit]
A DC device (like a laptop) forces us to plug it with the right polarity. This wouldn't be too useful, but could a DC plug be indifferent to polarity, since it checks it and adapts to the right one? That is, without a fuse blowing or circuit breaker tripping, in the same way it doesn't matter the way you plug your hair-dryer.--Doroletho (talk) 16:33, 6 May 2018 (UTC)[reply]
First of all, wall sockets are AC, so devices are insensitive to which way they are plugged in. However, the wiring might only switch one side (the live/hot), so maintaining polarity is normally a good idea. Back to your question, the easiest way is to use a bridge rectifier to fix the polarity (they also convert AC to DC), at the expense of a 3Volt drop across the rectifier. LongHairedFop (talk) 16:56, 6 May 2018 (UTC)[reply]
Many older electrical devices don't care - a light bulb, for instance. As far as A/C, polarized plugs came in my lifetime. Before that, things could go either way - things like toasters, etc. Bubba73You talkin' to me? 17:29, 6 May 2018 (UTC)[reply]
While some motors that run on DC will reverse direction if the polarity is reversed, other types won't (see universal motor for one such). Subway trains in many cities, including the London Underground, run on DC, and there have been some lines where the polarity has been changed at some time. And on the old City and South London Railway from about 1900 to 1915, the third rail was generally positive on one track and negative on the other, but as the trains approached the terminal stations the polarity reversed![1] This didn't mean the train would no longer be able to drive forward. --69.159.62.113 (talk) 06:49, 7 May 2018 (UTC)[reply]
It's not cost-effective. It's trivially easy to create a plug that cannot be plugged in backwards. It's a lot harder and more expensive to add circuitry to accept either polarity. -Arch dude (talk) 18:46, 6 May 2018 (UTC)[reply]
I have seen countless examples of polarized receptacles which are miswired such that phase and neutral were reversed. That’s why they sell little tester a homeowner or business owner can use with lights to indicate if phase, neutral and ground are on the correct holes. (The testers usually can not determine if it is neutral or ground on the neutral and ground holes. That requires measuring the impedance between neutral and ground). Edison (talk) 15:09, 7 May 2018 (UTC)[reply]
In many US residential settings, where a receptacle is on one half of a split-phase, there may be actually no electrical difference between neutral and ground contacts at a socket, just two separate conductors to the same bus bar in the breaker box. Unless "insulated vs uninsulated in romex" or "insulated inner vs metal conduit outer" is of interest. DMacks (talk) 02:05, 8 May 2018 (UTC)[reply]
There is confusion here. I answered the question as stated, which is about a DC connection. (i.e., from the wall wart to the computer). If the question is about the AC connection to the wall, then almost all wall warts are agnostic and can be plugged in either way. For a US 120 VAC circuit, the plug is NEMA 15, ungrounded and non-polarized. -Arch dude (talk) 14:46, 8 May 2018 (UTC)[reply]
This is commonly done for battery equipment, with a single rectifier diode. If it's plugged in the wrong way round, it safely doesn't work. The user then plugs it in the right way round. Sometimes there's even an LED connected backwards, which will then light up as "Wrong polarity".
The diode bridge rectifier circuit can be used, but rarely. It drops two diode forward voltage drops, so loses 1.2V. For a 12V circuit, that would also dissipate 10% of the power used across the diodes, which is enough to need a large rectifier, and to be wasteful of battery capacity.
Rarely, there's voltage sensing and switching. The connection is made by a DPDT relay, driven by a circuit which reverses the connections when it senses a mismatch. In the 1950s there were all electromechanical polarised relays which did this, where the coil current reversed according to the supply and connected the relay in one direction or the other (normally centre-off). These were also used as contactors - the initial current through the connector being made was just the low current to drive the coil, it was the relay contacts, not the connector, which switched the main current on or off.
Overall, this technique is possible but has drawbacks. So polarised connectors are used instead, or a safety device to stop (but not correct) reversal from mis-connection. Andy Dingley (talk) 10:49, 8 May 2018 (UTC)[reply]
Almost the opposite of what you say is useful. The new USB C connectors will still deliver DC power and have the data signals okay if turned upside down. The old ones are famous for needing four tries at inserting even though there is only two possible ways it looks like they can be inserted. ;-) Dmcq (talk) 14:28, 8 May 2018 (UTC)[reply]
I found this image, which is labelled as Abies canadensis
What is Abies canadensis? A search for this name online found only homeopathy sites. The image is categorized as Picea glauca. If that's what it is, and Abies canadensis is just a defunct alternate name, can we find a reliable source and document that on Wikipedia and Wikispecies? HLHJ (talk) 20:05, 6 May 2018 (UTC)[reply]
_(6025440683).jpg)