Resistance: Methods, Steps and Formula

Key Concepts

• ohmmeter
• multimeter

Introduction: 

Electrician who comes to your place uses a single device to measure the voltage, current, to check diode, to measure capacitance, frequency, resistance. In this section, you are going to learn about this device known as a multimeter. You will also read about a few other ways to calculate resistance.  

Explanation: 

Calculating resistance: 

There are multiple ways to calculate resistance. One of the ways is through the voltage current graph which you have done already. The slope of such a graph gives the resistance. The other methods are: 

1. Using ammeter and voltmeter 

2. Using ohmmeter 

3. Using multimeter 

Let us learn about each one of them.

1.Using ammeter and voltmeter: 

Voltmeter: 

The device which is used to measure this potential difference is known as voltmeter. It is connected in parallel across the two points where the potential difference needs to be measured. The reading obtained is in Volts (V). 

The symbol used to denote this voltage in an electric circuit is.

Potential difference is also known as Voltage. It is measured in Volts (V); Voltage is the work done in moving a unit test charge.  

Mathematically,  

otential difference is also known as Voltage. It is measured in Volts (V); Voltage is the work done in moving a unit test charge.  

Mathematically,  

Voltage , V= Work done, WCharge, qVoltage , V= Work done, WCharge, q

Steps to connect a voltmeter in a circuit: 

• Plug the probes into the meter. 
• Note the zero error of the voltmeter, if any. 
• Choose the electrical component along which voltage needs to be measured. 
• Hold the probes by the insulated handles and touch the red probe to the positive side of the circuit and the black probe to the negative side of the circuit. 
• Your voltmeter is parallel to the component. 

• In case of a resistor, there is no positive or negative side, but rather it is taken as per the flow of current.  
• Check the units displayed on the voltmeter. Note the readings in those unit. 

Ammeter: 

Ammeter is a device that is used to measure the current in a circuit. It is connected in a series a circuit. A multimeter also measures the current in the circuit when connected in series in A, mA  
and μA. The symbol of an ammeter when connected in an electric circuit is 

It has both positive and negative terminals. The red one is positive, and black one is negative. If both terminals are black, the one with 0A is positive, and the range terminal is negative. 

After the French physicist André-Marie Ampère, SI unit of electric current is Ampere (A).  

A few prefixes of Ampere are used as multiples, and submultiples of SI unit are microampere, milliampere, Ampere and kiloampere.  

When these two are connected in an electric circuit, we can get both the readings of voltage as well as current in the circuit.  

Steps to connect an ammeter in a circuit: 

• Plug the probes into the meter. 

• Note the zero error of the ammeter, if any. 
• Make the ammeter part of the electric circuit. 
• Connect the negative probe with negative end of any of the electrical component of the circuit and positive probe with the positive end. 
• Your ammeter is in series to the component. 
• Ammeter has very less resistance and introduce no new resistance in the circuit. 

• Check the units displayed on the ammeter. Note the readings in those unit. 

Ohm’s law: 

Ohm’s law states that, in an electrical circuit, the current passing through most materials is directly proportional to the potential difference applied across them.

Mathematically,

I ∝VI ∝V

→ → 

IR = V 

where R is the constant of proportionality known as resistance. 

There are three forms of Ohm’s Law:    I = V/R  

 V = IR

R = V/I

where:   

I = Current

V = Voltage

R = Resistance

In practical units, this law can be written as 

Voltage, V = Current, A × Resistance, Ω 

Using this formula, we can easily calculate the resistance.

1. Ohmmeter 

It is a device that is used to measure the resistance in an electric circuit. It can  
be joined in series or parallel in a circuit depending on the type of ohmmeter.

There are three types of meters depending on the value they measure.

1. Micro-ohmmeter

2. Mega-ohmmeter 

3. Milli-ohmmeter, Symbol of ohmmeter is

B. Multimeter

• It is also known as volt-ohm-milliammeter (VOM).  

• These abbreviations and symbols are often found on  
  multimeters. They vary depending on the make and  
  model: 
• Vm: DC voltage 
• V~: AC voltage  
• VΩmA: AC voltage 
• mA~: AC current 

• Ω: Resistance 
• V–: DC voltage 

Steps to connect a multimeter in a circuit: 

Questions and answers 

Question 1: What are the ways to calculate the resistance? 

Answer: 

Few of the ways to calculate resistance are: 

1. By taking the readings through voltmeter and ammeter and then applying Ohm’s law. 

2. By using ohmmeter 

3. By using multimeter.  

Question 2: Samanta made the following connections using a multimeter. What is she measuring? 

Answer: 

Voltage, as the knob points towards DCV. 

Summary

• Voltmeter is used to measure voltage. It is connected in parallel.
• Ammeter is used to measure current. It is connected in series.
• Ohm-meter is used to measure resistance. It is connected in series or parallel depending on the model taken.
• Multimeter is used to measure either voltage, current or resistance.
• Red probe is connected to the positive end and black probe is connected to the negative end.