1887.] A Thermal Telephone Transmitter. 141 



prominent in extra- tropical cyclones, where the rotational energy is 

 moderate, and the translational velocity great. 



The first set of characteristics may conveniently be classed together 

 as the rotational ; the second set as the translational phenomena of a 

 cyclone. 



Tropical and extra-tropical cyclones are identical in general cha- 

 racter, but differ in certain details, due to latitude, surrounding 

 pressure, and to the relative intensity of rotation or translation. 



III. " A Thermal Telephone Transmitter." By Prof. George 

 Forbes. Communicated by Lord Rayleigh, D.C.L., Sec. 

 E.S. Eeceived February 12, 1887. 



We have had so much evidence of the sensitiveness of the Bell 

 telephone receiver to the minutest changes of current, that we 

 have ceased to be surprised at any transmitter which responds to 

 the sounds of articulate speech. But, in the instrument now shown, 

 it was so extremely unlikely that sensible variations of current 

 could be produced with sufficient rapidity, that even now there is 

 perhaps some interest attached to the experiment. A wooden 

 cylinder was used closed at one end. A saw cut was made across 

 the diameter of the closed end, making a fine slit. In the slit 

 was stretched a platinum wire, O'OOl inch diameter and 2 inches 

 long, with its ends connected by copper wires through the primary 

 of an induction coil to a battery sufficiently powerful to make the 

 platinum wire red hot. On connecting the secondary circuit with 

 a receiving telephone in a distant room and speaking into the 

 wooden cylinder, the words are reproduced and heard in the tele- 

 phone. Each vibration of air in the slit cools the platinum wire, 

 diminishing its electrical resistance, and increasing the electric 

 current. The words transmitted are not quite perfect, the higher 

 harmonics being wanting. It requires some attention to make out 

 all the words of a sentence. A brass cylinder instead of the wooden 

 one, and a Wollaston platinum wire of excessive fineness have been 

 used without materially altering the clearness of the articulation. 

 Platinum foil has hitherto given no sound of the voice. The slit in 

 the brass instrument is made of glass to prevent the short-circuiting 

 and destruction of the platinum wire. 



Wires from one to three inches in length have been used. The 

 longest ones are best. No distinct articulation is heard if the wire 

 be not red hot. The hotter the wire the better is the articulation. 

 An adjustible slit was tried and the narrow slit gave the best 

 results. Mr. Preece some years ago used the expansion and con- 

 traction of a fine platinum wire to act on a diaphragm, and so serve 



