The first of these principles is embodied in the “knitting-needle” receiver described above and depicted in figures 22 & 23 on page 33. This receiver differs wholly from the later instruments of Bell, and others, and depended for its action upon the phenomenon of magnetic expansion discovered by Page and investigated by Joule. It was well known before Reis’s time that when a needle or bar of iron was magnetised it grew longer, and when demagnetised it grew shorter. Page detected the fact by the “tick” emitted by the bar during the act of magnetisation or demagnetisation. Joule measured the amount of expansion and contraction. To these discoveries Reis added two new facts; _first_, that if the degree of magnetisation be varied with rapid fluctuations corresponding to those of the sound waves impressed on the transmitter, the expansion and contraction of the rod followed these fluctuations faithfully, and therefore emitted at the receiving end sounds similar to those uttered at the transmitter. _Secondly_, by employing a needle of _steel_ instead of the bar of iron used by Page, Reis obtained an instrument which once used could never become completely demagnetised on the cessation of the current; it was thenceforth a _permanent magnet_, and all that the fluctuating currents could do was to vary its degree of magnetisation. Reis carefully explained in his memoir “On Telephony,” how the frequency of such fluctuations in the magnetising current could act in reproducing the pitch, and further, how the amplitude of the fluctuations set up vibrations of corresponding amplitude in the rod: he added with significance, that the quality of the reproduced note depended upon a number of variations of amplitude occurring in a given time. His theory of these actions was that the atoms (or perhaps our modern word _molecules_ would more correctly represent what Reis spoke of as atoms) of the rod or needle were pushed asunder from one another in the act of magnetisation, and that on the cessation of the magnetising influence of the current, these same atoms strove to return to their previous position of equilibrium, and thus the oscillations of the atoms led to the vibration of the needle as a whole. Whether all Reis’s speculations as to the behaviour of the atoms under varying degrees of magnetising force are justified in the present aspect of science or not, is, however, not of any great importance; the important point is, that, whether his theory be right or wrong, the instrument he devised will perform the function he assigned to it: it will reproduce speech, not loudly, but in reality far more articulately than many of the telephonic receivers in use under the names of Bell, Gower-Bell, &c.