「中」 is one of those essential kanji that anybody who knows any kanji will more than likely already know. Beginners will probably learn it first as 「なか」 and in compounds such as 「中国」. However, in this intermediate post, I’d like to discuss two other usages that I’ve had to figure out on my own. Now you don’t have to.
Using 「ちゅう」 instead of 「～している」
「中」 can be attached to a noun, in order to indicate that the noun is currently taking place. This essentially takes the place of 「している」 and means pretty much the same thing. In this usage, 「中」 is always read as 「ちゅう」. This may be obvious but the noun must be an actionable item such as “search” or “investigate”. Basically, it’s any noun that can be followed by 「する」 such as 「仕事」 or 「勉強」. You can’t say for example 「体中」（からだちゅう） because 「体する」 or “doing body” makes no sense.
検索中 – searching
勉強中 – studying
考え中 – thinking
「考え中」 is an interesting example because it is a noun that came originally from a verb. But this is not commonly done universally. For example, nobody really says 「思い中」 or 「売り中」. I would consider 「考え中」 as an expression of it’s own.
１) 今考え中だから、ちょっと静かにしてくんない？ – I’m thinking now so can you be a little quiet?
This usage of 「中」 is simply a more concise way to say [noun]をしている. You will often see it used as simple status updates such as computer wait screens (or my current Twitter status).
Using 「じゅう」 as throughout or all over
Another usage is to attach 「中」 to a noun to talk about the noun throughout or all over. In this usage, the reading is 「じゅう」 and you can tell the difference from the previous usage because the noun is not actionable. Instead, the noun must have some kind of length whether physical or in time. Unlike the previous example, 「体中」（からだじゅう） is a perfectly correct example. In this case, it means “all over the body” and not “doing body”.
１） 事故のせいで、体中が傷だらけだ。 – Due to fault of the accident, body is full of injuries all over.
Another common usage is with periods of time such as 「今日」 or 「一日中」 to indicate throughout the entire time period. One interesting thing to note is that 「今日中」 means “within today” while 「一日中」 means “the whole day”.
１） 今日中にやらなければならない。- I have to do it by today.
２） 一日中やっても終わらない。 – Won’t end even if you do it all day.
３） 一晩中カラオケで遊んでいた。 – Was playing all night at Karaoke.
Overall, this usage tend to be more established expressions so I wouldn’t arbitrarily attach it to time spans without seeing some usage samples. But at least now you’ll know how to read it properly and know what it means should you encounter it.
Also, with time spans, 「ちゅう」 tends to be used to mean “within” while 「じゅう」 is used to mean “throughout”. Both are not necessarily always usable for a given time span. It’s pretty arbitrary.
More details and examples can be found here: http://www7a.biglobe.ne.jp/~nifongo/conv/chyu.html