Wish Online Shopping

jane-e 

This is a DTI.  As the arm pivots, the dial needle moves.  In this case, each division = 0.01mm (0.0004").  some of the best examples come from Switzerland, which is be the workings are not unlike a clock movement, but Japan makes excellent versions nowadays.  China does knock-offs, but they do the job.   They are mostly used to measure 'comparative' movement.  Checking how concentric a work piece runs in a lathe chuck for instance. You'd also use them to square a vice to a milling machine slide.  The important thing is that you are just trying to make the needle move as little as possible.  I'll explain why in a moment.

This is a Dial Indicator.  It does a similar thing, but in this case you can use it to 'measure' linear movement.  The important thing is that the movement is linear.  The DTI above moves the lever through an arc, which makes linear measurements have what is called 'cosine error'.

It is not a problem when measuring very small distances, and the arm is flat (they are generally designed to be accurate when the arm is at 12° to horizontal).  But if the arm moves through more than a very few degrees, then the vertical movement is no longer directly proportional to the indicated movement on the dial.  I found this wee diagram that demonstrates it.  Hey, you are an ace with math, so you'll be totally on top of the trig.

Those prices are NZD.  I think I paid NZ$10 for a Chinese DTI.  The items all now show as shipped, and have tracking numbers, so it might even work out.

Graham, I found some more information comparing AliExpress with Wish.  AliExpress is slightly more expensive, but more reliable.  You definitely take a chance with Wish.  I guess if I decide to do any more China direct bargain hunting, I'll use AliExpress.