1890.] on Prohle^ns in the Physics of an Electric Lamp. 35 



sidered to be projecting off molecules of carbon in all directions, 

 partly in virtue of purely thermal actions, but probably also in con- 

 sequence of certain electrical effects to be presently discussed. This 

 scattering of the material of the carbon conductor takes place with 

 disadvantageous rapidity from an industrial point of view at and 

 beyond a certain temperature,* but it exists as well at much lower 

 temperatures than that which is found to determine the practical 

 limit of durability. A curious appearance is found in many incan- 

 descent lamps which have been " over-run," which shows us that this 

 projection of carbon molecules from the hot conductor is not, perhaps, 

 best described by calling it a vaporisation of its substance, but that 

 the surface molecules are shot off in straight lines, and that they 

 reach the glass envelope without being hindered to any great extent 

 by the molecules of the residual air. 



If an electric current is passed through an otherwise uniform 

 carbon conductor, which possesses at any one place a specific resist- 

 ance higher than that of the remaining portion, the current, in 

 accordance with a well-known law, there develops a higher tempera- 

 ture, and the molecular scattering at that spot may in consequence 

 be greatly exaggerated. It may be that the detrition of the con- 

 ductor at that locality will be so great as to cut it through after a 

 very short time. When the carbon has the form of a simple horse- 

 shoe loop, and when this molecular scattering takes place from some 

 point in the middle of one branch, the molecular projection makes 

 itself evident by producing a " molecular shadow " of the other leg 

 upon the interior of the glass. I will project upon the screen an 

 image of the carbon horseshoe loop taken from an old glow-lamp, and 

 you will be able to see that the filament has been cut through at one 

 place. At that position some minute congenital defect caused the 

 carbon to have a higher resistance, the temperature at that point when 

 it was in use became excessive, and an intensified molecular scattering 

 took place from that locality. On examining the glass bulb from 

 which it was taken, we find that the glass has been everywhere 

 darkened by a deposit of the scattered carbon except along one 

 narrow line (see Fig. 1), and that line is in the plane of the carbon 

 loop and on the side opposite to the point of rupture of the filament.f 



I may illustrate to you by a very simple experiment the way in 

 which that "shadow" has been formed.- Here is a p| -shaped rod: 

 this shall represent the carbon conductor in the lamp ; this sheet of 

 cardboard placed behind it, the side of the glass receiver. I have 

 affixed a little spray-producer to one side of the loop, and from that 



* When the rate of expenditure of energy in the carbon conductor is raised 

 until it reaches a value of about 500 watts, or 360 foot-pounds per second per 

 square inch of radiative surface, a limit of useful temperature has been reached 

 for economical working, under the usual present conditions of steam-engine- 

 driven dynamos and modern glow-lamps. 



t The writer desires to express bis indebtedness to the Editor of the 

 'Electrician ' for the loan of the blocks illustrating tliis abstract. 



D 2 



