“Suppose that a man speaks near a movable disk sufficiently flexible to lose none of the vibrations of the voice; that this disk alternately makes and breaks the connection from a battery; you may have at a distance another disk which will simultaneously execute the same vibrations.”  The idea so expressed is weak in only one particular.  This particular is shown by the words italicized by ourselves.  It is impossible to transmit a complex series of waves by any simple series of makes and breaks.  Philipp Reis, a German, devised the arrangement shown in Fig. 4 for the transmission of sound, letting the make and break of the contact between the diaphragm 1 and the point 2 interrupt the line circuit.  His receiver took several forms, all electromagnetic.  His success was limited to the transmission of musical sounds, and it is not believed that articulate speech ever was transmitted by such an arrangement.

It must be remembered that the art of telegraphy, particularly in America, was well established long before the invention of the telephone, and that an arrangement of keys, relays, and a battery, as shown in Fig. 5, was well known to a great many persons.  Attaching the armatures of the relays of such a line to diaphragms, as in Fig. 6, at any time after 1838, would have produced the telephone.  “The hardihood of invention” to conceive such a change was the quality required.

[Illustration:  Fig. 5.  Typical Telegraph Line]

Limitations of Magneto Transmitter.  For reasons not finally established, the ability of the magneto telephone to produce large currents from large sounds is not equal to its ability to produce large sounds from large currents.  As a receiving device, it is unexcelled, and but slight improvement has been made since its first invention.  It is inadequate as a transmitter, and as early as 1876, Professor Bell exhibited other means than electromagnetic action for producing the varying currents as a consequence of diaphragm motion.  Much other inventive effort was addressed to this problem, the aim of all being to send out more robust voice currents.

[Illustration:  Fig. 6.  Telegraph Equipment Converted into Telephone Equipment]

Other Methods of Producing Voice Currents.  Some of these means are the variation of resistance in the path of direct current, variation in the pressure of the source of that current, and variation in the electrostatic capacity of some part of the circuit.

Electrostatic Telephone. The latter method is principally that of Dolbear and Edison.  Dolbear’s thought is illustrated in Fig. 7.  Two conducting plates are brought close together.  One is free to vibrate as a diaphragm, while the other is fixed.  The element 1 in Fig. 7 is merely a stud to hold rigid the plate it bears against.  Each of two instruments connected by a line contains such a pair of plates, and a battery in the line keeps them charged to its potential.  The two diaphragms of each instrument are kept drawn towards each other because their unlike charges attract each other.  The vibration of one of the diaphragms changes the potential of the other pair; the degree of attraction thus is varied, so that vibration of the diaphragm and sound waves result.