290 Mr. Ritchie's Experiments on Heated Iron. 



rious to observe the effects produced by moving the knob along 

 the whole length of the poker. The sparks were strong at 

 the cold knob, diminished in size towards the middle, when 

 they entirely ceased, and again increased in size towards the 

 conductor. 



I endeavoured, in vain, to discover the cause of this striking 

 difference between the hot and cold iron, when I found I was 

 searching for the cause of an effect which did not take place, 

 as the electric fluid was silently drawn away by the smooth 

 ball, exactly as if it had been a sharp point. This is clearly 

 proved by the following experiments : 



Experiment 6. Hold a fine polished brass ball opposite the 

 part of the poker brought to a white heat, and not a single 

 spark can be taken from the cold knob, though the electric fluid 

 do not pass in sparks from the heated portion to the brass ball. 



Experiment 7. Raise the knob of the poker to a wliite heat, 

 and it will silently draw off" the electric fluid from the prime 

 conductor without the appearance of a single spark. As the 

 ball cools, sparks begin to strike off in rapid succession, as 

 before. The same effects will take place when the experiments 

 are performed with reference to the negative conductor. 



Experiment 8. Charge a Leyden jar, heat the knob of the 

 poker to a whiteness, and use it as a discharging rod, and a 

 strong spark will strike off between the knob of the jar and the 

 heated end of the poker, exactly as if it were cold. 



From this experiment it appears that, when the tension of the 

 electric fluid is very great, the striking effects of heat in the 

 preceding experiments, entirely disappear. 



I was now anxious to try by more accurate experiments whe* 

 ther there existed any difference in the conducting powers of cold 

 iron, and iron raised to a white heat. This I accomplished by 

 the following experiment : 



Experiment 9. I made a conductor of strong iron wire in 

 the form of BAG, having two brass balls 

 at BC. I then raised a few inches of 

 the rod between A and C to a white 

 heat, fixed it in the conductor, and ap- 

 plied a small brass knob between B and 



