At the top of a mahogany box is stretched a diaphragm of bladder that is set in vibration by sounds from a brass mouthpiece stained black. The vibrations cause an intermittent contact between a small platinum disk at the center of the diaphragm, and a light brass lever in contact with an electromagnet. The brass contact key at the side of the instrument is to activate the call bell. The receiver (missing) consisted of a needle placed inside a coil mounted in a wooden box acting as a resonator. The modulated current in the telephone line caused the needle to vibrate at the same rate as the diaphragm. There was considerable debate whether this system, invented by Johann Phillipp Reis, was capable of reproducing speech. However, the Reis transmitter, reproduced by many makers, remained essentially a demonstration instrument without significant practical applications.