The little carbon button plays an important part in the telephone.
You will see from the sketch of the transmitter that the current of
electricity will flow through the carbon button to the contact point
and through the wire to the primary of the induction-coil.
Now, carbon has a peculiarity, which is this, that if we press this
carbon button, ever so slightly, against the platinum contact, there
would be less resistance to the flow of the electricity through the
wire to the primary, and the more we press it the less the resistance
becomes. The consequence of this would be that more current would go to
the primary, and the secondary would become correspondingly stronger.
If the carbon button were left untouched, and nothing pressed against
it, the flow of current through it would be perfectly even.
Having examined the inside of the transmitter and receiver, and
understanding the effect of pressure on the carbon button, let us now
see