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XVIII. On the Fall in Pitch of Strained Wires through which a 

 Galvanic .Current is passing. By Dr. W. H. Stone*. 



THE object of this paper was to apply the vibrations of sound 

 to the measurement of electrical currents, and to distin- 

 guish what was due to heating-effects from those caused by 

 alteration of elasticity. 



Strings of brass and steel, such as are used for pianofortes 

 (No. 16 gauge), w T ere stretched, by means of w 7 rest-pins, across 

 a resonant box, over bridges surmounted by brass bearings, and 

 tuned to unison. On passing a current from two or more 

 Grove's batteries through them, a very marked fall in pitch was 

 obtained. The vibrating string being 24 inches long, and tuned 

 to two-foot C, the tone sank above a fourth in steel and a major 

 third in brass. 



This result being a compound of actual lengthening by heat 

 and of other causes, it was, in a second experiment, endeavoured 

 to eliminate the former element by straining similar strings 

 between the same bridges by means of a weight. This was 

 attached to the arm of a bent lever, to the short end of which 

 the string was made fast. By shifting the position of a four- 

 pound weight along the arm, very accurate unison, or definite 

 periodicity of beats could be obtained. When the current from 

 the battery was passed through this string, free to expand by 

 the falling of the weight, and therefore at a constant tension, a 

 fall of pitch was still noticed. There was also a very marked 

 loss of tone, which, on approaching a red heat, amounted to 

 total extinction of sound. 



A third experiment exhibited the changes of electrical resis- 

 tance in a wire subjected to variations of strain. The wire was 

 accurately balanced against another resistance in a Wheatstone's 

 bridge, and the spot of light from a mirror-galvanometer join- 

 ing the two circuits thrown on the screen. On suddenly in* 

 creasiug the tension and raising the musical pitch of the string, 

 the galvanometer was visibly deflected. This was not an effect 

 of heat (since the balance had been brought about during the 

 passage of the current), and must be due to altered molecular 

 state caused by the strain. 



It was incidentally noticed that, when beats were produced 

 by two strings on the same sonometer, they continued to be 

 sensible to the touch by laying the hand on the instrument long 

 after, from diminution of amplitude in the vibration, or from 

 slowness in the beats themselves, they had ceased to be audible. 

 This afforded a good demonstration of the continuity of sensa- 

 tion in touch and hearing. 



* Read before the Physical Society, May 9, 187-1. Communicated by 

 the Society. 



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