34 Professor J. A. Fleming [Feb. 14 



WEEKLY EVENING MEETING, 



Friday, February 14, 1890. 



William Crookes, Esq. F.E.S. Vice-President, in tbe Chair. 



Professor J. A. Fleming, M.A. D.Sc. M.B.I. 



Problems in the Physics of an Electric Lamp, 



More tban eigbty years ago Sir Humphry Davy provided the 

 terminal wires of his great battery of 2000 pairs of plates with rods 

 of carbon, and, bringing their extremities in contact, obtained for the 

 first time a brilliant display of the electric arc* The years that 

 have fled away since that time have seen all the marvellous develop- 

 ments of electro-magnetic engineering, have placed in our possession 

 the electric glow-lamp, and brought the art of electrical illumination 

 to a condition in which it progresses each year with giant strides. 

 In addition to the importance attaching to their ever-increasing 

 industrial use, there are many questions of purely scientific interest 

 which present themselves to our minds when we proceed to examine 

 the actions that take place when a carbon conductor is rendered 

 incandescent in a high vacuum, or when an electric arc is formed 

 between two carbon poles. It is to a very few of these physical 

 problems that I desire to direct your attention to-night, but more 

 especially to one which is particularly interesting from the bearing 

 which it has on the general nature of electric discharge. 



We know as a very familiar fact that if we attempt to raise the 

 temperature of a carbon conductor enclosed in a vacuum beyond a 

 certain limit, not far removed from the melting point of platinum, the 

 carbon begins to volatilise with great rapidity. If an electric glow- 

 lamp has passed through its carbon more than a certain strength of 

 current, the glass bulb speedily becomes darkened by a deposit of this 

 volatilised carbon condensed upon it ; and experience shows us that 

 we cannot raise the temperature of that carbon beyond a definite 

 point without causing this waste of the conductor to become very 

 rapid. In the highly rarefied atmosphere within the bulb of a glow- 

 lamp, the carbon, when at its normal incandescence, must be con- 



* Sir Humpliry Davy laid a request before the managers of the Royal Insti- 

 tution on July lith, 1808, that they would set on foot a subscription for the 

 purchase of a large galvanic battery. The result of this suggestion was that a 

 galvanic battery of 2000 pairs of copper and zinc plates were sot up in the Royal 

 Institution, and one of the earliest experiments performed with it was the pro- 

 duction of the electric arc between carbon poles, on a large scale. It is probable, 

 however, tliat Davy had produced the light on a small scale some six years 

 before and, according to Quctelet, Curtet observed the arc between carbon points 

 in 1802. See Dr. Paris' ' Life of Sir H. Davy.' 



