My 12 year old son was asking maths questions the other day and we got onto the root of minus 1.  I remembered it as being "j", however it is infact "i".

Here is an extract from good article explaining "i"

What is the square root of minus one? In other words, is there a number which when multiplied by itself, produces an answer "- 1"?

In arithmetic, minus multiplied by minus gives plus. (Taking a debt away from someone is really like crediting a positive sum of money - and in a similar way, a sentence containing two negatives becomes positive). Thus, because (- 1) squared equals +1, the answer to Ö (-1) cannot be "minus one" (nor can the result be plus one).

As far as ordinary, everyday numbers go, the square root of minus one does not exist. However, mathematicians find it useful to invent a completely new kind of number to fulfil that role. They decided to assume that such a root really did exist, and wrote 'i' to represent it. ('i' was chosen because it stands for "imaginary").

We can now go on and find the square root of minus four: the answer is '2i', because

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