34 SCIENCE. 



though large working models show the feasibility of not only run- 

 ning trains upon such a track by any of the motive powers men- 



[Vol. XIII. No. 311 



tioned, but also of running them safely around short curves and at 

 high speeds. 



ELECTRICAL NEWS. 



Light without Heat. 



Professor BraCKETT of Princeton College delivered an inter- 

 esting address before the New York Electric Club some weeks 

 ago. The part of his lecture which treated of the production of 

 light without heat gave an admirable summary of Dr. Hertz's ex- 

 periments on electro-magnetic waves, and afterwards Professor 

 Brackett indicated the lines on which he thought it would be best 

 to experiment to obtain practical results. Briefly the state of 

 the case is this : light consists of electro-magnetic vibrations 

 of a definite and very short period. In our ordinary methods 

 of artificial illumination we produce vibrations of a period that will 

 affect the eye, by heating particles to incandescence, the resultant 

 vibrations being of a great number of periods, only a few of them 

 being of use for illumination. In fact, a great deal of energy is 

 wasted, only a very small proportion of the total appearing in a 

 form that is useful to us. Now, the problem that is presented is 

 to produce vibrations of the period we want, and no other ; and 

 the problem is a very difficult one to solve. In nature there are 

 phosphorescent substances and certain insects — glow-worms, fire- 

 flies, etc, — which are very efficient illuminants, the light produced 



being accompanied by very little heat : so the problem is not im- 

 possible, and we may regard it hopefully. The most serious diffi- 

 culty lies in the extreme rapidity of the oscillations required, 

 billions of them a second, — a rapidity so great that it seems im- 

 possible to attain it by any mechanical means. Nor would it be 

 possible to economically distribute the vibrations when they were 

 produced. 



Professor Brackett proposes to solve the problem by working it 

 backwards, to take a beam of light, polarize it so that all of its vi- 

 brations are in one plane, and " harness that to a wire, so that it 

 will make a current vibrate and also make the magnetic field about 

 the wire vibrate : in other words, if you cannot do the sum, take 

 the answer and work backwards. That is what I intend to do, 

 and I will hint to you exactly how I propose to do it. It cannot 

 be done with the ordinary materials employed for conductors, if it 

 has to heat the wire. . . . We must get something that is not 

 a conductor in the ordinary sense. I remind you that the amount 

 of energy expended in the movement in the high vacuum tube, in 

 the ordinary tube, where you have the most beautiful illuminations, 

 is, in matter of fact, very small. ... I point out to you next 

 that there is one substance in which we have the properties of both 

 the conductor and the non-conductor present, and there are some 

 very hopeful indications in that. A selenium cell, which is semi- 

 transparent, when it is joined up and a battery current is put 

 through it, is found to have its resistance diminished immediately 

 a flash of light falls on it. . . . Suppose we take a polarizing 

 apparatus by which we can polarize a long web of light. It will 

 consist of vibrations all sorted out in parallel planes by themselves. 

 . . . Now let this polarized web of light be passed through a 

 narrow slit so as to pass directly upon or near the conductor in 

 which we wish to set up an alternating electric current. If the proper 

 conductor can be found, it should have the current set up in it, and 

 this should produce a magnetic field about it. . . . What we want 

 is an alternating current or discharge of some sort or other, which 

 shall enable us to produce the alternations with such frequency 

 that the so-called conductors will break out and shine directly. 

 ... A dynamometer ought to be constructed which would be 

 capable of measuring the effect. But with the ordinary opaque 

 conductor, such frequency means confusion among the molecules, 

 which brings about a difficulty. That is what we must get rid 

 of." 



In a subject like this, which is certain to command the attention 

 of investigators and inventors, which gives so much promise, and 

 in which there seems no impossibility, it is well to keep abreast of 

 even the suggested solutions. Professor Brackett's plan seems to 

 be to transmit light from places where the sun is shining, to other 

 places where it is not. But it is hard to see how this plan can 

 work successfully. Vibrations of such rapidity as those of light- 

 waves cannot be transmitted along a conductor, for conductors 

 would offer a practically infinite resistance to currents of such a 

 period. They are transmitted through a dielectric simply as light ; 

 and the fact that we believe light to be an electro-magnetic disturb- 

 ance does not help us to solve the problem. The transmission of 

 light as such stands just where it did before Hertz's experiments 

 or Maxwell's theory were published. The efficiency of transmis- 

 sion of the waves, however they are transmitted, is much greater 

 as the period is increased. A wave of a period some millions of 

 times less than that of light might be transmitted from China here 

 without much loss, provided we did it properly, while the energy of 

 a light-wave would be dissipated before it had gone a mile. If we 

 wish to transmit light, we must reduce the vibrations to a greater 

 period at the sending station, and raise them again at the receiving 

 end ; and this will be difficult. If we wish to produce light, we 

 have little encouragement in the line of producing an electric wave 

 of the required period by mechanical means. The period of vibra- 

 tion of a charge of electricity, displaced on the surface of a sphere 

 of a centimetre radius, might be a thousand million a second ; but 

 to reach the millions of millions necessary for light, the size of the 

 sphere would have to be decreased until it had reached molecular 

 proportions. We have in nature, however, instances of the kind of 

 action we wish. In glow-worms and fire-flies the results we are 

 attempting to attain are reached, and we need not despair of a prob- 

 lem the solution of which is called to our attention on almost any 



