410 On some Thermal Effects of Electric Currents. [May 27, 



each metal, was obtained, further tightening up did not vary the 

 clearness or loudness of articulation. 



Gold would scarcely bear the tension required to reproduce sonorous 

 vibrations, hence its low position. 



5. Very thin carbon pencil, '0625 inch diameter, was tried under 

 compression and under tension, but no effect whatever was experienced 

 unless a bad joint was made, when at once a faint microphonic effect 

 was apparent. 



6. No sibilant sounds whatever could be reproduced. 



7. That the effect was due to heating and cooling was shown by the 

 fact that it was possible to increase the current to such a strength as 

 to render the temperature of the wire sensible to the touch, and then 

 to make its elongation and contraction by low sounds evident to the 



eye. 



It therefore appears from these experiments that wires conveying 

 those currents of electricity which are required for telephonic purposes 

 expand and contract as they are heated and cooled, and as the varia- 

 tions in the strength of the current are small compared with the 

 strength of the current itself, the expansion and contraction vary in 

 the same ratio as the condensation and rarefaction of the air particles 

 conveying the sonorous vibrations which produced these vibrations. 



The mechanical changes, or molecular vibrations in the wire, due 

 directly or indirectly to telephonic currents, which result in the 

 reproduction of sound, bear a close analogy to the mechanical changes 

 due to the direct transmission of sound, but with this important 

 difference, that while the vibrations due to sound are progressive 

 along the wire, and their velocity is low and easily measured, those 

 due to thermal effects are practically instantaneous, and therefore affect 

 simultaneously the whole length of the wire. 



(Received May 27, 1880.) 



Note.— De la Rive, in 1843 (vide " Electricity," vol. i, p. 304), ob- 

 served that an iron wire emitted sounds when rapid discontinuous 

 currents were passed through it, but he attributed the effect to mag- 

 netism, for he failed to obtain the same effect in non-magnetic wires 

 like platinum or silver. 



Graham Bell found, in 1874, that a simple helix without an iron core 

 emitted sounds, and (in 1876) that very distinct sounds proceed from 

 straight pieces of iron, steel, retort carbon, and plumbago when con- 

 veying currents. 



Professor Hughes showed that his microphone was reversible, that 

 is, that it could receive as well as transmit sonorous vibrations. 



Mr. Weisendanger (" Telegraphic Journal," Oct. 1, 1878) repro- 

 duced sounds on a. microphonic receiver, which he called a thermo- 



