Required Material: Two rotary telephones, two 9 Volts batteries and their connectors, 20 meters pair wires, two  potentiometers.

Abstract: In this activity, a simple intercom system using two telephones and two batteries is tested and role of the batteries in biasing the telephones and doing as short circuits for voice signals is demonstrated.

Description of the activity: Each of the two participants A and B receives a telephone, a potentiometer, and 10 meters wires. Having the volunteer’s help, A and B separately make a series connection of telephone, battery and potentiometer (See figure 1).

A makes a series connection of telephone, battery and potentiometer.

B makes a series connection of telephone, battery and potentiometer

Figure 1) A and make their series circuits

A talks through his/her telephone set and hear his/her own voice. B talks through his/her telephone set and hear his/her own voice.

The potentiometer can be used to adjust the volume of the heard voice.

A cannot hear B and B cannot hear A.

Parallel connection of the two circuits (See figure 2.) does not change the situation (i.e. they cannot hear each other.).

Figure 2) Parallel connection of the two circuits

But, when the circuits are serially connected (See figure 3.), A and B hear the voice of each other.

Figure 3) serial connection of the two circuits

The volunteer then explains that batteries behave as a short circuit for time varying signals (voice signals). Hence, when batteries are connected in parallel, voice signals of one circuit cannot travel to the other circuit. However, in series connection both of the two voice signals travel through the whole circuit. As a result, both parties here the voice of each other via the telephone set.