.. 2.4

Measure resistance by comparison
================================

**Objective**

Learn to apply Ohm’s law to find the value of an unknown resistance by
comparing it with a known one. Voltage across a resistor is given by
V = IR . If same amount of current is flowing through two different
resistors, the ratio of voltages will be the same as the ratio of
resistances, :math:`I = U_{A1}/R_2 = (U_{PV1} − U_{A1})/R_1`.

.. image:: schematics/res-compare.svg
	   :width: 300px

**Procedure**

-  Connect the unknown resistor R from PV1 to A1.
-  Connect :math:`1~k\Omega` (:math:`R_2`) from A1 to Ground.
-  Set PV1 to 4 volts.
-  Measure voltage at A1. Calculate the current :math:`I = U_{A1}/R_2`.
   Value of :math:`R_1 = (U_{PV1} − U_{A1})/I`.
-  Select Electrical->Plot I-V curve from the menu to get an I-V plot

**Discussion**

What is the limitation of this method ? How do we choose the reference
resistor ? suppose the unknown value is in :math:`M\Omega`, what will be the
voltage drop across a :math:`1~k\Omega` reference resistor ? Our voltage
measurement is having a resolution of :math:`1/4096`.

We will use this method later to measure the resistance of solutions,
using AC.
