Note 68 — Chapter VI

This curious phenomenon was first observed by Mr. Trevelyan, whose Notice regarding some Experiments on the Vibration of Heated Metals will be found in the Transactions of the Royal Society of Edinburgh, vol. xii., 1837. In a Memoir in the same volume, entitled Experimental Researches regarding certain vibrations which take place between metallic masses having different temperatures, Professor Forbes draws the following conclusions:—

1. “The vibrations never take place between substances of the same nature.

2. “Both substances must be metallic. (This is now proved not to be necessary.)

3. “The vibrations take place with an intensity proportional (within certain limits) to the difference of the conducting powers of the metals for heat or electricity; the metal having the least conducting power being necessarily the coldest.

4. “The time of contact of two points of the metals must be longer than that of the intermediate portions.

5. “The impulse is received by a distinct and separate process at each contact of the bar and block, and in no case is the metallic connection of the bearing points in the bar, or those of the block, in any way essential.

6. “The intensity of the vibration is (under certain exceptions) proportional to the difference of temperature of the metals.”—Transactions of the Royal Society of Edinburgh, vol. xii.