This device is one of the first type of microphones. It was invented by David Edward Hughes (1831-1900). A bar of carbon is placed loosely on two terminals. The microphone is connected in a circuit with a battery and a speaker. If a clock or another source of sound is placed on the wooden base of the device then the vibrations move the carbon bar changing the resistance consequently the intensity of the current. As a result the speaker reproduces the initial sound.

The carbon bar in this device was horizontal and it is missing.

A very interesting article about the inventor (with information about the microphone as well) can be found at:
http://davidedwardhughes.com/David_Edward_Hughes.pdf
p.123-124