Metals undergo when in contact with cold Material. 305 



smaller amount than the temperatures of the general masses of 

 the two metals. A difference then of 50° or 100° in the tempe- 

 ratures of the two metals will probably be quite sufficient to pro- 

 duce the vibrations in question. 



It has already been seen that the larger the areas in contact, 

 the greater the proportion of heat available for raising the block. 

 For this reason large areas in contact are advantageous for the 

 production of vibrations. But, on the other hand, the larger 

 the areas in contact, the more slowly will the two metals tend to 

 regain their original difference of temperature during the intervals 

 when they are not in contact; and for this reason large areas in 

 contact are disadvantageous for the production of vibrations. 

 There will therefore be a certain magnitude for the area in con- 

 tact which will give stronger vibrations than either larger or 

 smaller areas. This conclusion is borne out by experiment. 



I have pointed out that when the hot and cold metals are of 

 the same material there will in general be no vibration, because 

 the contraction of the hot metal will exactly compensate for ex- 

 pansion of the cold metal. This, as a result of his experiments, 

 Forbes has stated to be a general law. There is, however, an 

 exception to this law, first noticed and explained by Professor 

 Tyndall, to which I should refer. He found that when the hot 

 rocker rests upon the edges of thin sheets or upon points of the 

 same metal as itself, vibrations will often occur. The explana- 

 tion is to be found in the fact that, while a lateral flow of heat 

 out of the hot metal takes place, the lateral flow of heat into 

 the cold metal is partially prevented. 



Another general result which Forbes deduced from his expe- 

 riments is, that " the vibrations take place with an intensity pro- 

 portional (within certain limits) to the difference of the conduct- 

 ing-powers of the metals for heat, the metal having the least 

 conducting-power being necessarily the coldest." Now, if the 

 various metals differed from one another only as regards their 

 conductivities this law would be strictly true ; for in this case 

 the possibility of vibration could only arise from the lateral flow 

 of heat out of the hot metal being greater than the lateral flow 

 into the cold metal, and this would only be the case when the hot 

 metal was the better conductor of heat. But in any case the 

 lateral flow of heat has an important influence upon the vibra- 

 tions ; and consequently the above law will be approximately true. 



If, however, the lateral flow of heat into the cold metal be pre- 

 vented by making the rocker rest upon thin sheets or points of 

 cold metal, this law will altogether fail. The experiments of 

 Tyndall prove that this is the case. 



Leeds Grammar School, 

 March 6, 18/3. 



Phil. Mag. S. 4. Vol. 45. No. 300. April 1873. X 



