Mr. Bell’s study of electric telephones really dates from this time, and he passed from complex to simple instruments, making a careful study of the different modes of vibration which arise from different modes of electric action. The following is an abstract, with the omission of more technical details, of the paper read by Mr. Bell to the Society of Telegraphic Engineers, London, October 31, 1877. If the intensity of an electric current is represented by the ordinates of a curve, and the duration of breaks in the current by the abscissæ, the given curve may represent the waves of the positive or negative current respectively, above and below the line of X, and these waves will be more or less accentuated, just as the transmitted currents are more or less instantaneous. If the currents which are interrupted to produce a sound are quite instantaneous in their manifestation, the curve represents a series of isolated indentations, as we see in fig. 7; and if the interruptions are so made as only to produce differences of intensity, the curve is presented under the form of fig. 8. Finally, if the emissions of current are so ordered that their intensity alternately increases and diminishes, the curve takes the form represented in fig. 9. In the first case, the currents are _intermittent_; in the second, _pulsatory_; in the third case, they are _undulatory_. [Illustration: FIG. 7.] [Illustration: FIG. 8.] [Illustration: FIG. 9.] These currents are necessarily positive or negative, according to their position above or below the line _x_, and if they are alternately reversed, the curves present the form given in fig. 10, curves which essentially differ from the first, not merely in the different form of the indentations, but especially in the suppression of the extra current, which is always found in the pulsatory and undulatory currents. [Illustration: FIG. 10.] The two former systems of currents have long been in use for the electric transmission of musical sounds, of which we have an interesting example in Reiss’s telephone already described. But Mr. Bell claims to have been the first to employ the undulatory currents, which made it possible to solve the problem of transmitting speech.[3] In order to estimate the importance of this discovery, it will be enough to analyse the effects produced with these different systems of currents when several notes of varying pitch are to be combined.