How to make a Simple Telephone

Items Required:

1. Two plastic / paper cups
2. Non-elastic string / thread (2 m to 10 m)
3. Plastic straw
4. Needle / pin / pointed object to make a hole
5. Scissors

Instructions:

1. First cut the thread to your required length (between 2 m to 10 m).

2. Then take the cups and make a hole in the bottom center using a needle.

3. Now take one end of the thread and insert it through the bottom hole of one cup and pull it out to sufficient length.

4. Then cut two pieces of straw to about 2 cm - 3 cm each and tie one piece of straw with the ends of the string parallel to the first cups bottom.

5. Now take the other end of the thread and insert it to the second cup's bottom and tie the other straw piece parallel to the second cup's bottom.

6. Finally it’s done and ready to use. We need two people to check how it works.

Testing:

1. Ask each person to take a cup and walk opposite to each other until the thread is tight.
2. Now the person who is going to speak has to speak into the cup and the other should keep the cup in his/her ear to hear.

Note:-  

1. Make sure that the string is tight between the two cups to hear a clear voice. In other words while communicating the string should be stretched enough to hear a clear voice.
2. Constraint: the distance is always fixed to the string's length and the two people have to be connected to each other by a straight line without any obstacles touching in between.

Observation:

When one person speaks from one end, the other can hear the voice with a negligible delay according to the distance or length of the thread.

Conclusion:

When one person speaks, the sound wave makes a vibration in the thread and travels through it to the other end. There will be a negligible delay in hearing according to the distance or length between the two persons which shows the speed of sound through the thread or string.

In this, we use a non elastic thread because the sound we generate travels through the string in a vibration form which could be absorbed by the elastic property.