The passive two terminal electrical components that resist the flow of electrical current in an electrical network can be termed as Resistor. Resistors can be used for controlling or reducing the voltage levels within a circuit, and the voltage across an element in a circuit can be determined by the voltage drop across the resistor in parallel to that element. Resistors are of fixed or variable resistances, viz. photoresistor, humistors, varistors, trimmers, thermistors and potentiometers. Resistors are classified into different types based on different properties.
We know that there are different types of resistors. However, are you aware of something amazing like this: the pencil that we use in our day-to-day activities related to marking and drawings can also be used as a resistor in the electricity networks?
What is a Pencil Resistor?
Yes, the pencil that we use in our day-to-day activities can be used as a resistor in electrical networks. As compared to other types of resistors this resistor also offers resistance to the flow of electricity through it. In fact the lead inside the pencil consists of properties of an electrical resistor and thus it acts as a resistor. It can be considered as a variable resistor with the following description of this article gives a better understanding about these types of resistors. It can be called as home made resistor or self made resistor.
The pencils are made of wood, glue, metal, rubber and pure graphite. Every pencil has a lead in it, which is the main part of the pencil used for writing/marking purpose. The lead used inside the pencil is made up of a material, namely graphite – an allotrope of the carbon.
Graphite has some very useful features like the following:
- Graphite is a semi metal and it is an electrical conductor.
- It is non-inductive, and has a negative temperature coefficient.
- Graphite is easily obtainable and can be recycled for reuse.
- Under standard conditions, the graphite is in the most stable form of the carbon.
- Resistance of graphite varies with its grades used in lead manufacturing, and the grades of pencil or lead are shown in the figure below.
Resistivity of a Pencil Resistor
The resistivity of a pure graphite is given by 0.0000138 Ohm/meter – and, as discussed above – there are different grades of graphite leads used for manufacturing pencils, and the resistance of these different grades of the graphite leads can be given as follows: H=25 Ohms, 2H=20 Ohms, B=7 Ohms, 2B=6 Ohms, and HB= 19 Ohms.
Pencil Resistors’ Working
The resistance of this type of resistors can be varied by varying the grade of the graphite lead of the pencil. Even the line or mark drawn by a pencil on a paper consists of resistance; and, by dragging the wires across this line or mark, the variation in the resistance can be observed.
Consider the below example showing in which this resistor is used as a variable resistor.
Variable Pencil Resistor
Take a piece of white paper and draw a straight line with a pencil on it. Similarly draw another curved line with each line having two ends. Measure the resistance of the straight line and curved line by placing the multimeter terminals at both end points of each line. Note the readings while the terminals of multimeter are placed at the end points of each line. Then drag the one terminal of the multimeter towards the other terminal fixed at one end point of the line and note the readings of resistance at different points on the line with different lengths.
In the above figure, if we observe the multimeter is set to Ohm and showing some resistance value as an example, while measuring the full length of straight line marked by pencil.
Similarly, draw a bold line or darker line with bigger width than the above lines as shown in the above figure and then measure the resistance of the line by placing multimeter terminals at the end points, note readings. And for a second time note the readings at different points with different lengths of line.
If we observe above figure, the multimeter is set to read resistance in KOhms and the resistance measured is showing in some KOhms while the line drawn is darker with bigger width.
Thus, we can say that the pencil can be used as a variable resistor.
Pencil Resistor Project
Consider a pencil resistor do it yourself project circuit as shown in the below figure. It consists of a pencil fixed on a cardboard, a free wire end to slide along the pencil’s lead graphite, an electric bulb, a battery and copper wires – all are used for connecting the circuit as shown in the below figure.
After connecting the pencil resistor circuit as shown above, provide a power supply to the circuit from a 9v battery, and then observe the intensity of the bulb connected to the circuit through the free end adjustable copper wire connected to the opposite end (with respect to the green wire) of the graphite of the pencil such that the full length of the graphite must be connected or take active part in the network. In this condition, if we observe the bulb’s intensity, as shown in the figure, it is very low, which means, the bulb glows with a very dim intensity. This is due to the fact that the entire graphite of the pencil’s resistor is connected in the circuit or full resistance of the graphite is offered to the circuit, which is reducing the current flow through the bulb.
Similarly, drag the sliding wire (red wire) connected to the graphite of the pencil towards the green wire’s end and observes the variation in the intensity of the bulb at different positions of the sliding wire with respect to the graphite of the pencil. We can detect that the intensity of the bulb increases as the length of the graphite part connected in the circuit decreases. This is due to the fact that as the length of the pencil resistor (graphite) decreases the resistance also decreases in the circuit causing more current to flow in the circuit which makes the bulb to glow brighter.
Thus, the variation in the resistance with the variation of length of the pencil’s resistor (graphite) causes the circuit’s bulb to glow brighter and dimmer accordingly. Even the mark or line made by a pencil on a white paper can also be used as a pencil resistor in which the length and the thickness of the mark or line change the order of the resistance values.
Do you know any specific applications in which Pencil resistors are being used and does these resistors have similar properties like other resistors, when these are connected in series and parallel? Post your ideas with your comments below so that other readers get to know more about this.