1839.] Memorandum of Experiments, fyc. 853 



Igniting effect produced on platinum or iron wire ; how influenced by 

 distance and thickness of conductors, and length of platinum wire. 



I abstain from all explanation as to the theory or mode of action of 

 this battery, wishing to confine myself here to its effects in the ignition 

 of metallic wires. 



To produce this effect, twist a copper bell- wire ten feet long to the 

 wire of the last zinc plate, and connect a similar wire with the mer- 

 cury tube of the first copper cell. If the free ends of these wires be 

 joined by a fine platinum or iron wire, say two inches in length, the 

 moment the junction is completed the platinum or iron becomes white 

 hot, and if the battery be in full action, generally melts into numerous 

 globules. The ready destructibility of iron by oxidation renders it 

 inferior for the purpose now in view to platinum, which was according- 

 ly used in all the subsequent experiments. 



But if the copper wires touch each other in any part between the bat- 

 tery and the platinum no heating is produced, because the electrical 

 action does not extend beyond the first metallic junction. This most 

 important fact is made use of in a self-acting apparatus which I employ 

 for the explosion of mines at a certain fixed time after the experimen- 

 talist has retired to a safe distance. 



It also shews, that when we wish to produce ignition of pla- 

 tinum wire at a distance we must take some means for preventing 

 the conductors from touching each other. But before describing 

 how this may best be accomplished, it is necessary to examine the in- 

 fluence of two important circumstances over the ignition of the plati- 

 num wire, viz. the thickness of the conductors, and the distance of the 

 platinum wire from the battery. A few experiments will render this 

 quite intelligible. 



A constant battery of twelve cells was employed, and a platinum 

 wire two inches long and l-30th of an inch in diameter. 



1st Experiment. — Using copper bell- wire 1-1 2th of an inch in 

 diameter, this battery caused the platinum wire to become so hot as to 

 kindle saltpetre match-paper at a distance of 130 feet. 



2nd Experiment. — Each conductor was formed of two strands of 

 bell- wire. The platinum was now heated to the same degree to exactly 

 double the distance of the first experiment. 



