If you want to know the resistance of low resistance components such as wires, switches, and coils, this milliohmmeter can be used. It is very simple and inexpensive to make. It even fits in your pocket. Most ohmmeters are accurate down to 1 ohm but this one is sensitive to low resistance in the range of milliohms or even microohms.
Step 1: Materials
R1: ~220 ohm resistor
R2: Unknown resistance
2x thin wires (eg. mobile charger cords)
Rectangular shaped plastic box
5V source (eg. USB port, mobile chargers)
2x alligator clips
DC jack and connector (optional)
Multimeter with ohms and millivolts ranges (the lower the voltage range, the more sensitive the milliohmmeter)
Step 2: Drill Holes on Case
Drill holes for the wires and leads.
Step 3: Soldering
Soldering can be done boardless. Just hot glue the parts to the box. If your power supply is bullky and you want it detachable, include the DC jack and connector.
Step 4: Using the Milliohmmeter
Before testing the unknown resistance, measure the resistance of R1. It should be close to 220 ohms.
To measure the unknown resistance (R2), attach it to the milliohmmeter's test leads. Measure the voltage across R1 and R2. When measuring R2's voltage, measure it across R2 directly. Do not measure the voltage across the alligator clips because the contact resistance will add up the voltage drop and overestimate the resistance.
Based on Ohm's law, we know that R1 and R2 has equal current flowing through them. Because of this, we can use V2 and the current to calculate the unknown resistance.
R2 can be calculated as follows:
V1=Voltage across R1
V2=Voltage across unknown resistor
R1=Measured value of R1 (~220 ohms)
In the second picture, I used an ammeter as an example.
This link has more details about the low resistance tester..
Step 5: Measurements of Low Resistance Parts
Based on the calculations and expected values, this milliohmmeter was reasonably accurate.
Since the voltmeter has a range down to 0.1 mV, it can measure down to 0.01 ohm. To increase the sensitivity, you can purchase a more sensitive voltmeter or use a lower resistor value. Because resistors are sensitive to temperature changes, the power rating needs to be higher.