of the copper loop, and the to-and-fro currents are not compelled to confine themselves to a thin film of copper constituting the surface of the copper. When steady currents are passed through the loop, they are not confined to the outer layer of the copper, and find an easier passage through the section of the copper loop than through the human body. With steady currents it is impossible to light the lamp in the above

manner through the human body. The effect of increasing the frequency of to-and-fro currents of electricity is thus to drive them to the surface of metallic conductors. When the frequency or rapidity of vibration is enormous, a rod of copper may not afford any better passage than a rod of glass. Hertz, in one of his papers, points out that our present nomenclature is limited, and only applies to the special cases of steady currents. With enormously rapid to-and-fro currents, a piece of copper acts like an insulator and prevents any to-and-fro currents from passing through it, whereas a piece of glass transmits them unimpaired. A thick disk of copper properly placed between the two coils

of a step-up transformer can completely stop the electrical oscillations from reaching a lamp connected with the secondary coil of the transformer, whereas a plate of glass allows thein to pass on unimpeded. Glass, therefore, is a better conductor for electrical oscillations of high frequency than copper. We are thus approaching the behaviour of light to these two substances.

Is it not possible, therefore, by enormously increasing the frequency of electrical oscillations, to drive them completely off metallic conductors and compel them to be propagated through the ether of space? If we could do this, and if our oscillation should meet metallic conductors, their energy would decay or be absorbed in such conductors, just as light waves are absorbed by nonconductors of light.

The latter supposition leads us again to Prof. Poynting's view of the decay of electro-magnetic radiations which, proceeding from the sun, pervade all space about us. This decay produces the phenomenon of the electric current.

With a sufficiently powerful electro-motive force we can produce such a stress in the medium in an ordinary room, or, in other words, we can polarize the medium and make it such a storehouse of electric energy, that we can light a little electric lamp anywhere in the room without wires. Carrying this lamp in our hand, it will light up when we enter the room and be extinguished when we leave it. Tesla has shown the possibility of this by making the room a great Leyden jar, of which the walls form the opposite coatings and the air takes the place of the glass. The charge upon the coatings of such a jar, or, in other words, the walls of the room, is made to alternate with great rapidity, and electric waves fill the air, giving periodic to-and-fro movements to it.

The molecules in a little rarefied bulb are thus set in very rapid motion, and by their impact on suitable substances can raise the latter to incandescence, and by their mutual collisions can also fill the rarefied tube with a luminosity.

Tesla, in his remarkable lectures on the effects produced by currents of high frequency, shows that a suitably constructed lamp can be made to glow in any space by being connected merely with one terminal of a transformer. A little motor can also be made to revolve by being attached to one wire; the ordinary electric motor requiring two wires, one connected to the positive pole of the dynamo and the other to the negative pole. In considering these experiments we must remember, however, that the electric circuit in both cases of the light and the motor is completed through the medium from the positive pole of the exciter to the negative pole. In other words, lines of electrostatic force extend through the medium, through the walls of the room, back to the negative pole of the generator. We can not isolate an electric effect at one spot, or consider that our lines of stress stop at this spot. The apparatus by means of which Tesla produced remarkable luminous effects is similar to that of Thomson, and can be characterized as a step-up transformer of the second order. Instead, however, of using an alternating machine of comparatively low rate of alternation, Tesla employed one giving about fifty thousand alternations per second. With the very high potentials obtained in his second step-up transformer he was able to excite the molecules of rarefied gases to a rapid rate of movement, and by their impact on suitable matter to produce vivid light.

Prof. Crookes's experiments on radiant matter in highly exhausted tubes may be said to have first drawn

inventors' minds to the possibility of obtaining light by other means than by employing steady currents of electricity to raise matter to incandescence. In one form of the Crookes tubes one terminal of a Ruhmkorff coil ends in a concave mirror (Fig. 28) inside an exhausted globe. At the focus of this globe (b) is placed a bit of platinum on a glass stem. The energy streaming from a is reflected to the focus b, and raises the bit of platinum to incandescence. This lamp may be said to be the forerunner of the later attempts of Tesla to produce light by high electro-motive force. The tube is of importance also in X-ray photography. The commercial employment of strong to-and-fro currents enables one to greatly magnify the results obtained by Crookes.

The experiments of Tesla to obtain a light with a small

FIG. 28.

amount of expenditure of energy are extremely suggestive. At present, however, in order to produce the luminescence of such economical lamps we are compelled to employ powerful dynamos and transformers. We need an engine of at least ten horse power to produce the conditions for such an economical lamp of a few candle power. The problem is, to produce the conditions economically. I can illustrate this problem by

a species of argumentum ad hominem. An electric light of feeble power can be produced by shaking a small amount of mercury in a glass tube which has been partially exhausted of air. The friction of the mercury against the glass walls of the tube produces an electrification which in turn leads to a heightened oscillation or movement of the molecules of air still left in the tube. No heat can be detected in this light. We have produced considerable light by electrical excitation with very little heat, but the amount of heat supplied to the human body in the shape of food is very great, and we have to go through an expensive transformation to produce our little light.

It will be well at this stage of our study of transformations of energy accomplished by the inventions of man to examine into the degree of perfection which has been obtained. The efficiency of any engine is the amount of work it can perform compared with the amount of energy given to it in the shape of fuel. "The steam engine has an efficiency of about ten per cent; the efficiency of the best dynamo machines is ninety per cent; therefore only nine per cent of the energy in the coal is transformed into electrical energy. In the conversion of this electrical energy into light about ten per cent is lost in the conductors, and we have consequently in the lamps only 0-081 of the energy in the coal. Of this energy in the lamp about ninety per cent is expended in producing heat, ten per cent only being useful for the production of light. Thus, the efficiency of the electric lamp is only 0.0081, or about one per cent." *

This result is well calculated to repress any feeling of exultation we may have in contemplating the

* Palaz, Industrial Photometry, p. 265.