ARC IN AIR BETWEEN CARBON ELECTRODES 47 



cool by running water, and concluded that cooling the 

 electrode decreased the amount of voltage required. It 

 has somewhat recently been shown by myself 1 that this 

 decrease was caused by vapor from the mercury passing 

 into the arc and that when this did not occur the voltage 

 increased. Indeed several experiments which were sup- 

 posed to show that cooling the electrode lowered the volt- 

 age were repeated by myself, and when proper care was 

 taken it was found that in every case cooling the electrodes 

 raised the required voltage. 



This is also what one would expect from a consideration 

 of the phenomena bearing indirectly on the subject. For 

 example, if the terminals of the arc are separated for some 

 distance when it is first started, the arc is more apt to go 

 out than it is, if the terminals are allowed to become hot 

 before being separated. If then a longer arc can be 

 maintained when the carbons are hot, one would conclude 

 that heating the terminals allows the current to flow 

 with a smaller potential difference. With an alternating- 

 current arc it requires a higher voltage for a given current 

 when the current is increasing than when it is decreasing. 

 When it is increasing, the arc has been cooled and has not 

 yet attained its maximum temperature. When it is de- 

 creasing, it has been hot and is cooling, so that with the 

 same current the electrodes are cooler in the first place 

 than in the second. 



Again it is much more difficult to maintain an arc in 

 hydrogen than in air, and this has generally been ex- 

 plained as being due to the fact that hydrogen cools the 

 electrodes more rapidly than air, as if cooling the electrodes 

 made necessary a greater potential difference. 



1 Phys. Rev., 30, 315; 1910. 



