A NEW TELEPHONE.

Globe, Volume XXIII, Issue 2233, 30 May 1881, Page 4

A NEW TELEPHONE.

A paper “ On the Conversion of Radiant Energy into Sonorous Vibrations ” was read before the Royal Society by Mr W. H. Preece, the electrician to the Post Office. The remarkable discovery of Messrs Graham Bell and Sumner Tainter that the rapid intermittent i cidenco of rays of light on discs of hard substances produces sonorous vibrations has attracted very much attention and has excited much physical work to solve an unexpected problem. The advocates of the emission theory of light have striven for 200 years to obtain such a proof of their theory and have failed. Why have Bell and Tainter succeeded, or have they succeeded at all ? May not their phenomena be due to some other cause than to the incidence of light ? It was suspected by many that it was a heat effect, and not a light one at all. M. Mercadier, in Paris (Oomptes Rendus, December 6tb, 1880), and Professor Tyndall (Proc. R.S., January 3rd, 1.881) have placed this beyond the region of doubt, and now Mr Preece has completed the chain of evidence by a careful and elaborate inquiry into the cause of the phenomena. In tho first part of his paper he has shown that ebonite and indiarubber, though opaque to the light rays, are remarkably diathermanous or transparent to the heat rays, and therefore that radiant heat can act through screens of those materials. Indeed, ebouite is shown to be almost perfectly transparent to radiant heat, whilst it is absolutely impervious to light. He next shows by experiments made on very delicate apparatus that no more vibrations than six per second can possibly be produced by the direct impact of heat waves causing expansion of the mass of the 'disc, and therefore that the Bell-Tainter effect it not duo to the absorption of heat changing the volume of the hard' substance experimented upon. He next inquires whether the effect observed is due to a molecular pressure similar to that which produces the rotation of the radiometer, for this being a mere surface action, the element of time is eliminated. Many experiments are described which were made with discs of various kinds in different ways, but the results were so unsatisfactory and variable that the question was raised whether the discs vibrated at all. By the aid of microphones and specially constructed chambers, it is proved clearly that the undulations are those of the contained air, and not of the discs. In fact, the sounds wore intensified fcj removing the

disc*. Moreover, the effects were materially assisted by coating the sides of the containing vessel with [a substance highly absorbent of heat, such os the carbon deposited by burning camphor. It is next shown that the effects are dependent on the number of heat trays that pass through the discs, and not on those that are incident on thorn, and that_ the greater the absorbent character of the air or vapour contained in the case, the more intense the sounds emitted. All these results are repeated and shown with ordinary firsts lampblacked on their exterior and interior. Finally it is shown that there is a time element introduced, and that the loudness of the note emitted depends not only on the rapidity with which the contained air absorbs the radiant energy, but also on the rapidity with which it gives up its heat to the sides of the case and the exits open to it. It. varies also with the form of the closed space and with the character of the contained vapour, and with the diathermancy only of the discs. The effect being thus due to radiant heat and its absorption by suitable surfaces, it was next shown that if a spiral of wire be completely enclosed in a lampblacked case, sounds were emitted when currents of electricity wore rapidly and intermittently transmitted through the wire ; and, moreover, that when these currents were produced by a proper microphone transmitter articulate speech was reproduced. Hence these phenomena are simply effects of radiant heat, and they are duo to the changes of volume in absorbent gases and vapors produced by the absorption of thermometrio heat in a confined space. All the varied and novel experiments which Mr Graham Bell performed when he was recently in Europe with solids, liquids, and gases, with tubes, flasks, and discs, are thus brought within one simple explanation, and are due to a remarkable influence of degraded heat rays on absorbent vapors. The final result of this inquiry has been, not only to unravel an exceedingly interesting scientific problem, but to produce another form of telephone based on a new principle.