Ohms is the measurement used for the amount of resistance of a resistor.
Step 1: The Formula
Is the formula which is Ohm's law. According to this formula, V=IR, I=V/R, and R=V/I.
V means voltage, I means current, and R means resistance. So, Voltage equals Current times Resistance. To remember this, here is Ohm's Pyramid
Step 2: To Make It Easier
You want to find the amount of ohms of resistance you need to power a device that uses
40mA (Milliamps) with a nine volt battery.
All you have to do is divide voltage(9) by Amps (in this case, milliamps(40(in amps it's 0.04))) So, it would be like this:
Step 3: Bonus
You can make your own resistor just by drawing a rectangle on paper!
This is true, and you can do it too!(lol nice slogan huh?)
Just draw a really, REALLY dark rectangle on a piece of paper about two centimetres long(with a HB2 pencil). Then take a nine volt batery with a battery clip, hook the red wire(+) to the longer lead of an led(+) and touch the shorter lead of the led(-) to one end of the rectangle, and the black wire of the clip(-) to the other end of the rectangle.
The led should light up. Also, what's even cooler is, the closer the black wire and the led, the brighter it is!
***Do NOT touch the battery directly to the led. It will burn out, and possibly explode.
For people making synthesizers and piano thing with 500k pots, a dark hairline(~0.5mm thick) ~4.5cm long is 500k