OP ARTS AND SCIENCES. lf>^ 



VIII. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XIIL — ACOUSTIC PHENOMENON NOTICED IN A 

 CROOKES' TUBE. 



By Professor Chas. R. Cross. 



Presented Nov. 10, 1880. 



A SHORT time since, while experimenting with a Crookes' tube, 

 I noticed a phenomenon which was quite striking, and so evident that 

 it hardly seems possible that it has not been observed before, though 

 I have never seen any notice of the fact. 



In working with the tube, in which a piece of sheet platinum is 

 rendered incandescent by the concentration upon it of electrified parti- 

 cles repelled from a concave mirror, I noticed that when the mirror 

 was made the negative electrode, so that this concentration took place, 

 a clear and quite musical note issued from the tube. I thought at 

 first that the pitch of this note would coincide with that produced by 

 the circuit-breaker of the coil, which made about 120 breaks per 

 second, but this did not prove to be the case, for very great changes 

 in the rate of the circuit-breaker did not affect the note given by the 

 tube. The effect seemed to be produced by the vibration of the sheet 

 platinum under the influence of the molecular impact, which vibra- 

 tions were communicated to the glass walls of the tube by the en- 

 amelled rod to which the platinum was attached. This gave rise 

 to a sound somewhat resembling that caused by the pattering of rain 

 against a window-pane, but higher in pitch and more musical. The 

 sound changed its character very greatly when the direction of the 

 current was reversed, only a feeble murmur being heard. I obtained 

 a similar musical note, though far less loud, with the " mean free- 

 |)ath " tube, best when the middle plate was positive. With a tube 

 containing phosphorescent sulphide of calcium the note was very dull 



