Any one who begins a study of the problem will be apt to think that what is wanted in a lamp with an electrode is a very high degree of incandescence of the electrode.  There he will be mistaken.  The high incandescence of the button is a necessary evil, but what is really wanted is the high incandescence of the gas surrounding the button.  In other words, the problem in such a lamp is to bring a mass of gas to the highest possible incandescence.  The higher the incandescence, the quicker the mean vibration, the greater is the economy of the light production.  But to maintain a mass of gas at a high degree of incandescence in a glass vessel, it will always be necessary to keep the incandescent mass away from the glass; that is, to confine it as much as possible to the central portion of the globe.

In one of the experiments this evening a brush was produced at the end of a wire.  This brush was a flame, a source of heat and light.  It did not emit much perceptible heat, nor did it glow with an intense light; but is it the less a flame because it does not scorch my hand?  Is it the less a flame because it does not hurt my eye by its brilliancy?  The problem is precisely to produce in the bulb such a flame, much smaller in size, but incomparably more powerful.  Were there means at hand for producing electric impulses of a sufficiently high frequency, and for transmitting them, the bulb could be done away with, unless it were used to protect the electrode, or to economize the energy by confining the heat.  But as such means are not at disposal, it becomes necessary to place the terminal in a bulb and rarefy the air in the same.  This is done merely to enable the apparatus to perform the work which it is not capable of performing at ordinary air pressure.  In the bulb we are able to intensify the action to any degree—­so far that the brush emits a powerful light.

The intensity of the light emitted depends principally on the frequency and potential of the impulses, and on the electric density of the surface of the electrode.  It is of the greatest importance to employ the smallest possible button, in order to push the density very far.  Under the violent impact of the molecules of the gas surrounding it, the small electrode is of course brought to an extremely high temperature, but around it is a mass of highly incandescent gas, a flame photosphere, many hundred times the volume of the electrode.  With a diamond, carborundum or zirconia button the photosphere can be as much as one thousand times the volume of the button.  Without much reflecting one would think that in pushing so far the incandescence of the electrode it would be instantly volatilized.  But after a careful consideration he would find that, theoretically, it should not occur, and in this fact—­which, however, is experimentally demonstrated—­lies principally the future value of such a lamp.