Connection ivith Experiments on Torsional Vibrations. 415 



generally, which I venture to bring before the notice of the 

 Physical Society in the hope that by so doing I may 

 perchance save others who may pursue similar investigations 

 from the pitfalls into which I have fallen, and from which I 

 have managed to extricate myself only after a considerable 

 expenditure of time and labour. 



In my earlier experiments a wire about 600 centims. in 

 length and 1 millim. in diameter was suspended vertically, 

 having its upper extremity clamped to a rigid support and its 

 lower one clamped or soldered to the centre of a horizontal 

 bar of brass, from which were suspended, by threads or fine 

 ivires, two cylinders of equal dimensions and mass, and placed 

 at equal distances from the wire. The torsional oscillations 

 of the wire were observed by means of the usual arrangement 

 of mirror, scale, and lamp, the main object of the inquiry 

 being to determine as accurately as possible the logarithmic 

 decrement of the amplitude of swing and the vibration-period. 

 After one set of observations had been completed, the moment 

 of inertia was altered by sliding the cylinders along the bar 

 further away from or nearer to the wire, so that the latter 

 might now vibrate in a different period. If t be the period of 

 vibration, k the moment of inertia, and /the torsional couple, 



f f ' 



The moment of inertia of the bar together with that of 

 the suspended cylinders could be calculated with very fair 

 accuracy; the value of t was also calculated to a nicety; so 

 that it was reasonable to expect that the values of/ obtained 

 with different moments of inertia should be very fairly in 

 accordance with each other. For some time this proved to 

 be the case, and several wires of different metals had already 

 been examined when a curious phenomenon presented itself. 

 A change of moment of inertia had just been made, and the 

 wire was then set in torsional oscillation; but instead of 

 the amplitude of swing diminishing by slow degrees as in the 

 previous experiments, the spot of light was seen to make five 

 or six oscillations of very rapidly diminishing amplitude, and 

 finally come nearly to rest. Soon, however, the amplitude 

 began to increase until in a few vibrations it extended over 

 some two hundred divisions of the scale, when once more 

 diminution and, finally, rest nearly ensued. I will not trouble 

 the Society with my conjectures at the time as to the cause 

 of the phenomenon ; suffice it that at length I discovered, what 

 perhaps I ought to have discovered at once, that the, at first 

 sight, startling apparition was due to a very natural cause, 



