324? Mr. A. Trevelyan on the Vibration ofhe 4 ated Metals. 



When the brass bar at a high temperature is placed on the 

 cool lead, the vibration seldom fails taking place instantly 

 and spontaneously. 



A bar 5 inches long, 2 inches wide, and gths of an inch 

 thick, vibrated when supporting a weight of 12 pounds. 



However near the bar and block may be brought to each 

 other, no apparent phenomenon takes place until they are in 

 actual contact. 



By balancing the bar in a horizontal position on a narrow 

 lead block (fig. 1. p. 332), rounded on the part on which the 

 bar rests, the vibration is well exhibited, the bar moving ver- 

 tically and laterally at the same time. A rod 10 or more 

 inches long, flattened in the centre to prevent its slipping, with 

 a ball on each end (fig. 5), when placed across a heated vi- 

 brating bar (fig. 6), increases the arc of motion, rendering the 

 movements much more conspicuous. 



A thick ring of copper, 5 inches in diameter, when heated, 

 and hung on a lead bar, vibrates backwards and forwards; 

 and when laid orj a narrow lead block, upwards and down- 

 wards. 



The heated bar vibrated on a piece of thin sheet lead, 

 either placed loosely or soldered on brass, and on a lead block 

 burnished with gold-leaf. 



A heated copper bar vibrated on the bottom of a glass 

 tumbler. This is an experiment which the late Professor Leslie 

 tried with me, but it is very uncertain and difficult to obtain. 



The bars vibrate best when placed on blocks of lead with 

 the surface somewhat rough: both metals also should be kept 

 clean, and free from oxidation, which impairs the vibration. 



A bar of heated copper was placed on an iron block 

 rounded on the surface, and being nicely balanced on the 

 centre of the rounded part, showed the vertical motion. 



The shape or size of the bars and blocks is of little conse- 

 quence, except for the more delicate experiments with the 

 hard metals: on lead the hard metal almost of any form will 

 vibrate when heated. 



Mr. K. T. Kemp, the skilful electrician and chemist, in- 

 formed me that after casting some bismuth, whilst it was cool- 

 ing, after being taken out of the mould, he heard a sound pro- 

 ceed from it ; when cool he again heated it, but could not re- 

 produce the sound. 



The bars I use to show the experiments are of different 

 sizes: a bar about 5 inches long, l£ inch wide, and fths of 

 an inch thick, will produce considerable tone; a rod of small 

 wire 6 inches long, must be attached at one end, to serve as a 

 handle. A ridge is formed along the centre of one side of the 

 broad part, by its being bevelled off towards each edge; the 

 other side is hollowed out down the centre so as to throw the 



