Sending sound electronically

Activity for students aged 11-16 that takes about 60 minutes of class time


Type: Main
Time: 60mins
Subjects: Science, Engineering, Design & Technology

Key words: resistance; sound; electric circuits; conduction; vibrations; microphones; sound transmission; design

In this activity students get a chance to see the historic demonstration of Alexander Graham Bell’s liquid transmitter and discuss how the setup works, reviewing their knowledge of resistance. Students then get to build a working microphone from pencil leads and consider how to improve their microphone design.

Projector or interactive white board
‘lead’ from pencils
crocodile clips
match box or small boxes
9V battery
Old/cheap headphones

Media Resources
Student worksheet
PowerPoint instructions



Starter (5 mins)

Challenge students to identify the biggest differences between modern smartphones and the first telephones.

Main (50 mins)

Watch video of Alexander Graham Bell’s liquid transmitter from the Christmas Lectures.

Use the worksheet to label the elements used in the demo and give a one or two sentence explanation of what they are for (10 mins)

Discuss the idea of resistance in a circuit and how resistance varies with length. (5 mins)

Explain will now have a go building own model of a microphone

Go through PowerPoint slides and in pairs or small groups students have a go building a microphone (15 mins)

Students demonstrate their microphones and as decide which microphone is working best and why. (10 mins)

Students work together in their group to create an explanation of how they think that this microphone works. (10mins)

Plenary (5 mins)

Ask students to outline the potential disadvantages to this design of microphone and to Alexander Graham Bell’s liquid transmitter (5 mins)

Additional notes

Removing the graphite from inside pencils can be difficult. If you find this too difficult you can be useful to use refills for mechanical pencils.

It can be interesting to put the design of Alexander Graham Bell’s (AGB’s) liquid crystal transmitter in perspective.

  • It was built by AGB in 1876 at this time electric lighting wasn’t yet available in people’s homes.             
  • The incandescent light bulb wasn’t yet invented and radio was not yet invented.
  • There is controversy over whether AGB stole the design for the transmitter from his another inventor named Elisha Gray.

Discussion and explanation of resistance can be varied according to the ability of the group and their previous knowledge.

Reiterate that the pin in the AGB model is dipping in and out of the liquid thus more or less of the pin is in contact with the acid at any particular time - which in turn increases and decreases the resistance.

Let students explore and develop optimal methods for constructing their microphones.

Allow students time to explore and test their creations and also to see how other groups microphones compare to theirs

Discussion of how microphones work can be led to a greater or lesser extent by the teacher. Draw the students attention to the materials involved in the construction, how the materials are arranged and where the battery and earphones are connected. What element of the construction is varying or changeable with the sound or vibrations from your voice.

When considering disadvantages of each microphone designs draw students’ attention to the idea of the sensitivity of the materials and their responsiveness to vibration. Reiterate why the membrane was used.

Depending on time the final aspect of the lesson could be completed as homework.

Students are given an image of the liquid transmitter and work in pairs to explain the function of each part of the equipment.


If students are struggling can be useful to give them the worksheet first and let them identify the parts while watching the video. Figuring out function of each part can be completed as a group discussion.


Challenge students to sketch a diagram of their pencil microphone and to write a short paragraph describing how it works.

Curriculum links

UK Science

  • Experimental skills and investigations
    2b make predictions using scientific knowledge and understanding
  • Analysis and evaluation
    3d present reasoned explanations, including explaining data in relation to predictions and hypotheses
  • Sound Waves
    31c sound produced by vibrations of objects, in loud speakers, detected by their effects on microphone diaphragm and the ear drum
  • Current electricity
    35b potential difference, measured in volts, battery and bulb ratings; resistance, measured in ohms, as the ratio of potentidal difference(p.d.) to current


UK Design & Technology

  • Design
    1b identify and solve their own design problems and understand how to reformulate problems given to them

    1c develop specifications to inform the design of innovative, functional, appealing products that respond to needs in a variety of situations
  • Evaluate
    3b evaluate their ideas and products against their own design criteria and consider the view of others to improve their work
  • Technical knowledge
    4a understand and use the properties of materials and the performance of structural elements to achieve functioning solutions 

Supported by the Institution of Engineering & Technology

Watch the full lectures

View the full CHRISTMAS LECTURES, Sparks will fly: How to hack your home, along with behind the scenes footage and related content on the Ri Channel