metaUk Conductors and their Resistance to Electric Currents. 23 



thing higher, probably from the smaller quantity of light requiring a greater 

 intensity to be visible. 



I will conclude with a summary of the principal facts which I have endea- 

 voured to estabUsh in this memoir : — 



1. When a wire of platinum is heated by a voltaic current, its resistance 

 to the passage of that current increases without limit to the verge of its fusion. 



2. That increase of resistance is not caused by the mere increase of the 

 current. 



3. It is not caused by the increased distance of the molecules. 



4. It is not caused by the employment of the molecular forces in generatimj 

 heat. 



5. It is exactly proportional to the increase of temperature of the wire. 



6. The same is the case with copper till the oxidation of the metal inter- 

 rupts the experiment. 



7. In both those metals a given elevation of temperature produces the 

 same proportionate change of resistance. 



8. This change of resistance must always be attended to in rheometry; and 

 the neglect of this precaution may explain some objections that have recently 

 been made to the theory of Ohm. 



9. The heat evolved by a current passing through a wire is as the square of 

 the current, and as the actual resistance of the wire (that increased by the heat). 



10. The highest temperature attained by any part of it is, however, as the 

 current x square root of resistance. 



11. The loss of heat by the air is as the difference of the temperatures of 

 the air and wire. 



12. That by radiation is in this pyrometer as the square of that difference 

 nearly. 



13. Thermic equation of the wire ; from which it follows, that the tem- 

 perature rises very rapidly in receding from the lower end of the wire, till at a 

 small distance it becomes constant. 



14. This constant temperature exceeds that given by the pyrometer by an 

 amount varying from a seventh to a tenth. 



15. The following is the table of the quantities involved in this equation: 



