How do i determine the resistors needed to reduce a 9 volt battery to 2.5 milliamps?

I'm looking to make my own Transcranial Direct Current Stimulation device. 

mrmerino1 year ago
With Division!

In all honest, if you can't understand Ohm's Law, maybe you should take a step back.
iceng1 year ago
You can not reduce voltage to current ! !

You can limit the output current flow of a battery
by putting a resistor in series with the battery.   R=V/I=9/.0025=3600 ohms

BUT as the battery discharges your resister will also lower the cranial
milliamps you so desperately need to think you are evolving. .

You should use a superior constant current regulator like
an LM317 three pin IC.

This will deliver a 2.5ma cranial current down to a 5  volt dead battery.