The discovery of the microphone by Professor Hughes has enabled us to understand the reason of this failure. The transmitter of Reis was based on the plan of interrupting the current, and the spring was intended to close the contact after it had been opened by the shock of a vibration. So long as the sound was a musical tone it proved efficient, for a musical tone is a regular succession of vibrations. But the vibrations of speech are irregular and complicated, and in order to transmit them the current has to be varied in strength without being altogether broken. The waves excited in the air by the voice should merely produce corresponding waves in the current. In short, the current ought to undulate in sympathy with the oscillations of the air. It appears from the report of Herr Von Legat, inspector of the Royal Prussian Telegraphs, on the Reis telephone, published in 1862, that the inventor was quite aware of this principle, but his instrument was not well adapted to apply it. No doubt the platinum contacts he employed in the transmitter behaved to some extent as a crude metal microphone, and hence a few words, especially familiar or expected ones, could be transmitted and distinguished at the other end of the line. But Reis does not seem to have realised the importance of not entirely breaking the circuit of the current; at all events, his metal spring is not in practice an effective provision against this, for it allows the metal contacts to jolt too far apart, and thus interrupt the current. Had he lived to modify the spring and the form or material of his contacts so as to keep the current continuous—as he might have done, for example, by using carbon for platinum—he would have forestalled alike Bell, Edison, and Hughes in the production of a good speaking telephone. Reis in fact was trembling on the verge of a great discovery, which was, however, reserved for others.
