History of Resistance

Georg Ohm and Resistance Units

The unit for resistance is an Ohm (Ω), named after Georg Ohm. Ohm was a German physicist and mathematician. While working as a school teacher, Ohm began his research with the new electrochemical cell, the Voltaic Pile, invented by Italian scientist Alessandro Volta. During Ohm's time, the convention of resistance was thought to be dependent on the distance current had to travel. Through experiments using equipment he invented, Ohm found that the resistance between two points of a conductor was constant based not only on distance but also the material. For this discovery and others, we named the value of resistance after him. The name of electrical resistance is an Ohm and annotated with the Greek letter Omega. 

What is Resistance?

Resistance is defined as the opposition to current flow. All materials except for superconductors have resistance. The value of this resistance is determined by two factors: 

Usually, when describing the flow of electricity a common analogy is a water flowing out of a tank. So in terms of things effecting resistance the length would tie directly to the length of the pipe that the water would need to travel through. The longer the pipe the more resistance on the system. 


The material of the conductor can be thought of like a clog in the pipe representing resistance. Imagine these two scenarios. One, there is a sponge in the pipe and the other there is a rock and they are both the same size. 

The pipe with a sponge blocking the flow of the water will still allow some water to pass through because sponges are porous. The pipe with a rock will barely allow water to flow and may even block the flow completely.  These clogs resist the flow differently while still in the same size pipe. 

The cross-sectional area is analogous to the diameter of the pipe. A wide-open large diameter pipe will allow for a large volume of water to pass through. Unlike If there is a pinch in the pipe making the diameter of the pipe smaller, then the volume of water that can flow through that point would be less. The narrow pipe resists the flow of water through it even though the water is at the same pressure as the tank with the wider pipe. 

This is resistance. In electrical terms, this is represented by two circuits with equal voltages and gauge, or thickness of the wires, with the same resistances. Assuming the the conductors are made out of the exact same material, we can see how diameter of the conductor physically limits the flow of electrons (current.) In the example using the pipes, a bigger pipe can literally fit and allow more electrons to flow at a time than the smaller pipe. This is the same for a conductor- the bigger the cross-section of the material the more electrons can fit and flow through a certain point. 

Measuring Resistance

We measure resistance using an Ohm-meter or a multimeter. These tools will pass a steady voltage through the test leads and read the current internally. This allows for the meter to calculate and display a resistance using Ohm's Law. We can remember that different materials have a unique conductivity. The more conductive something is, the less resistance that material has. You can see that the resistance of the metal nail is lower than the plastic bottle because metal is more conductive than plastic. 

Adding Resistance

When measuring resistance it is also good to have a basic understanding of how resistances can add up. However, there are different rules for adding resistances depending on what type of circuit you are actually measuring. For more detail on this see our pages on Series and Parallel Circuits.

When measuring a single load or resistor, the measurement is simply the value of that resistor. This is a 470 Ohm through-hole resistor and you can see it measures the same. 

In this example there are two 470 Ohm through-hole resistors in Series with each other- one feeds directly into the other. This would be the same as doubling the length of the pipe in our water analogy. You can see the resistance measured is doubled.

In this example there are two 470 Ohm through-hole resistors in Parallel with each other- both ends of one are connected to both ends of the other. Since there are two paths the voltage outputted from the multimeter can go, the total resistance is less-this is called diminishing resistance. This would be like adding a second pipe to our water analogy. You can see the resistance measured is less.

Experiment with Resistance! Measuring Resistance of Pencil Graphite and your Skin!

Experiment #1: The Resistance of Pencil Graphite

The Graphite in your pencil is made from a mixture of carbon and clay. The proportion of those two is actually how they label the softness of each pencil- #2 Pencil. The ratio of carbon and clay will change the resistance of the material. This is actually the same material that is used in some resistors- an electronic component designed to specifically resist current. If you scratch a dark, thick line on paper you can test the resistance with a Multimeter. Change the thickness and length of the line for different resistances. This is the same as changing the length or diameter of our pipe in the water analogy. 

Experiment #2: The Resistance of your Skin

All Materials have some sort of resistance. A fun activity you can do as well is use a multimeter on your skin. Believe it or not, your skin is very resistive but variables like how much you're sweating can change its resistance. This is how novelty lie Detectors work by measuring the resistance of your skin from how sweaty you are from being nervous.