Why do some devices, Carrier grade base stations for WISPs and cell towers for example, operate on a -48v system instead of a +48v system?  A few of our higher end devices operate on a -48v system instead of a +48v system. Why would these companies choose to operate these devices on a negative polarity? are there benefits to this? and to power these devices does your power supply need to specify that it can operate this way?
It seems common that PBX and other telephone hardware use a positive-ground power supply, where the "hot" line is at -48v.  What's the reason for that?
What do I need in a camera to take non-blurry photos of a fast-moving child? I'm a mom who is obsessed with taking photos of my now 2 year old daughter.  I currently have a Nikon j1 which I like, but I find the shutter speed is still a bit slow and when you're trying to take photos of a speedy little girl, I end up with a lot of blurry images.  So I'm looking for a camera with a very fast shutter speed that will allow me to take good photos of a very fast moving child.  Also that can take photos in low light. I've been looking at the canon eos rebel but I'm not sure. Would like it to be able to take videos as well. Any advice? How to remove warnings like `Unknown media type ...` Why do close votes expire? I nominated  to be closed as a dup. I see that two other people upvoted my comment, so they might have voted to close it as well. Now, we didn't gather enough votes, and all close votes expired ... and I can't re-vote to close. What is the motivation behind expiring close votes ... especially for close-as-duplicate? Update: Please take into consideration:  Beta Stack Exchange sites have a smaller community, and fewer people with voting privileges Closing as a duplicate is &quot;usually the right thing&quot; IMHO - much less controversial than closing as off topic. After a vote is reset, the original users can't vote again. On a small site, this makes it even harder to get a consensus of five votes. In MySQL, how do I get a list of all foreign key constraints pointing to a particular table? a particular column? This is the same thing as , but for MySQL. I would like to toggle the Do Not Disturb in the notification center with a keyboard shortcut.  I am using BetterTouchTool for some extra keyboard shortcuts but it does not support enabling/disabling the notifications in the default options.  It has an option for executing a terminal command so I'm asking here how to enable/disable Do Not Disturb from the terminal?  I found  and I tried to run the commands but it didn't seem to work. WordPress twenty eleven sidebar on Pages I am using 2011 theme, any how I managed to get sidebar on single.php pages by installing child theme,  But now I want sidebar on pages,  How do I get it? Copy Physics To Multiple Objects Let $f$ be a measurable function on a measure space $X$ and suppose that $fg \in L^1$ for all $g\in L^q$. Must $f$ be in $L^p$, for $p$ the conjugate of $q$? If we assume that $\|fg\|_1 \leq C\|g\|_q$ for some constant $C$, this follows from the Riesz Representation theorem. But what if we aren't given that such a $C$ exists? Lesser-known integration tricks I am currently studying for the GRE math subject test, which heavily tests calculus. I've reviewed most of the basic calculus techniques (integration by parts, trig substitutions, etc.) I am now looking for a list or reference for some lesser-known tricks or clever substitutions that are useful in integration. For example, I learned of this trick   $$\int_a^b f(x) \, dx = \int_a^b f(a + b -x) \, dx$$  in the question    I am especially interested in tricks that can be used without an excessive amount of computation, as I believe (or hope?) that these will be what  is useful for the GRE. Index entries duplicated for captions In document class book, and package caption, creating index with makeidx. Index entries for the main text and for footnotes collate properly; but those for captions create their own entries.  Example: for the tag \index{New York!Custom House!\textit{Cartouche}}, text and footnotes collate properly; but used in a caption, a new entry is created at the third level, and is also out-of-alphabetic-sequence. Note: the top level, and first sub are correct; only the second sub is duplicated.  How to get the caption index entries under the same entry? I am using an old laptop and doing heavy processing that needs high CPU usage for a long time (~30mins - 2hours). When the process puts heavy load on CPU for a long time, CPU overheats and computer shuts down. I don't want to use a cooler/stand because I am carrying this laptop around and working outside home.  Is there a way/application that I can limit CPU usage of a process in Windows?  Note: Setting the priority of the process from task manager does not work.  Note 2: Fans are clean. Is the following word order acceptable? I need your opinion about the word order in the following sentence: “In her garden grows a cherry-tree.“? Is it grammatically acceptable? Strange difference between pwd and /bin/pwd I added a symlink to the current directory with ln -s . aa. If I execute cd aa, and after that I executed pwd, the response is /home/sim/aa.  But if I execute /bin/pwd it prints /home/sim (the current directory hasn't changed).  Where does this difference come from? Maximum number of edges in a bipartite graph Prove that for a bipartite graph $G$ on $n$ vertices the number of edges in $G$ is at most $\frac{n^2}{4}$.     I used induction on $n$.    Induction hypothesis: Suppose for a bipartite graph with less than $n$ vertices the result holds true.    Now take a bipartite graph on $n$ vertices.Let $x,y$ be two vertices in $G$ where an edge exist between $x$ and $y$. Now remove these two vertices from $G$ and consider this graph $G'$. $G'$ has at most ${(n-2)^2}\over4$. Add these two vertices back.  Then the number of edges $G$ can have is at most    $$|E(G')|+d(x)+d(y)-1$$   My question is in my proof I took $d(x) + d(y) \le n$, where $d(x)$ denotes the degree of vertex $x$.  Can I consider $d(x)+d(y) \leq n$? I thought the maximum number of edges is obtained at the situation $K_{\frac n 2,\frac n 2}$ How to make sure that Qt5.4.2 is installed properly I installed qt5.4 from  in Ubuntu 14.04, but when I run:   qmake -v   I always get the old version Qt5.2.1, although I think it is installed here Qt version 5.4.2 in /opt/qt54/lib. Can anyone tell me how to make sure that Qt5.4.2 is installed properly?   Thanks How can you quickly get the complete path to a file for use in terminal? Why do we say "It's time we ate" and not "It's time we eat"? Why do we use the simple past but not the present or future in the following expressions:  Don't you think it's time we went a little further Don't you think it's time we ate Don't you think it's time we went home I have read that watchdog timer pushes microprocessor/controller into reset state whenever it experiences any software anomaly like executing in infinite loop. and after some time a very senior prof told us that after reset the processor resumes operation from the next instruction to the erroneous one.  when watchdog timer resets the processor, from where does the processor start its next execution?  I just wanted to know what is correct or if some other concept is employed? When should I use a struct rather than a class in C#? When should you use struct and not class in C#? My conceptual model is that structs are used in times when the item is merely a collection of value types.  A way to logically hold them all together into a cohesive whole.  I came across these rules :   A struct should represent a single value. A struct should have a memory footprint less than 16 bytes. A struct should not be changed after creation.   Do these rules work? What does a struct mean semantically? How do I challenge an audit failure? I've run through Both SO and Meta searches for an answer, as well as reviewing the , and I cannot find a canonical process to challenge an audit failure.  I can find discussions of the single-choice "I Understand" topic that appear unresolved, in that no solution presently exists.  The particular question in point is .  Despite the number of up-votes, I feel that this quite clearly falls into a common closure category: "Here's my problem; how do I write the program?"  Compare this to   I also failed review on the first link I posted above, .  This looks like a case of code review, rather than SO.  I don't want to get rid of audits; I've learned good things from some of my failures, especially at the start.  However, in the past few weeks, a majority of my failures have been highly questionable, and I've drawn more than one suspension from those that are outright wrong by current standards.  If we have a challenge mechanism, please point me in the right direction.  Otherwise, it's simply not worth the annoyance any more, and I'll drop back to merely comments and answers.