June 8, 1893] 



NATURE 



^39 



acquired, and to add some interest to this demonstration, 'I 

 shall dwell here upon a feature which I consider of great im- 

 !poitance, inasmuch as it promises, besides, to throw a better light 

 upon the nature of most of the phenomena produced by high 

 frequency electric currents. I have on other occasions pointed 

 ■out the great importance of the presence of the rarefied gas, or 

 atomic medium in general, around the conductor through which 

 alternate currents of high frequency are passed, as regards the 

 |heating of the conductor by the currents. My experiments 

 (described some time ago have shown that the higher the fre- 

 quency and potential difference of the currents, the more im- 

 Iporiant becomes the rarefied gas in which the conductor is 

 jimmersed, as a factor of the heating. The potential difference, 

 jhoAever, is, as I then pointed out, a more important element 

 (than the frequency. When both of these are sufficiently high, 

 ;the heating mav be almost entirely due to the presence of the 

 (rarefied ga«. [Experiments were performed showing the im- 

 'portance of the rarefied gas, or generally of gas at ordinary or 

 'other pressure as regards the incandescence or other luminous 

 effects produced by currents of this kind.] 



I Incandescent Lamps, 



1 Disregarding now the modifying effect of convection, there are 

 'two distinct causes which determine the incandescence of a wire 

 or filament with varying currents, that is, conduction current 

 and bombardment. With steady currents we have to deal only 

 iwith the former of these two causes, and the heating effect is a 

 (minimum, since the resistance is least to steady flow. When the 

 current is a varying one the resistance is greater, and hence the 

 heating effect is increased. Thus if the rate of change of the 

 current is very great, the resistance may increase to such an ex- 

 tent that the filament is brought to incandescence with inappre- 

 cialile currents, and we are able to take a short and thick block 

 lof carbon or other material and bring it to bright incandescence 

 with a current incomparably smaller than that required to bring 

 to the same degree of incandescence an ordinary thin lamp 

 'filament with a steady or low frequency current. This result is 

 important, and illustrates how rapidly our views on these subjects 

 are changing, and how quickly our field of knowledge is extend- 

 ing. In the art of incandescent lighting, to view this result in 

 one aspect only, it has been commonly considered as an essential 

 Requirement for practical success, that the lamp filament should 

 should be thin and of high resistance. But now we know 

 (that the resistance to the steady flow of the filament does 

 mot mean anything ; the filament might as well be short and 

 thick ; for if it be immersed in rarefied gas it will 

 become incandescent by the passage of a small current. It all 

 depends on the frequency and potential of the currents. We 

 may conclude from this, that it would be of advantage, so far 

 as the lamp is considered, to employ high frequencies for light- 

 ing, as they allow the use of sho]t and thick filaments and 

 smaller currents. 



If a wire or filament be immersed in a homogeneous medium, 

 ill the heating is dne to true conduction current, but if it be 

 nclosed in an exhausted vessel the conditions are entirely 

 lillerent. Here the gas begins to act and the heating effect of 

 he conduction current, as is shown in many experiments, may 

 l)e very small compared with that of the bombardment. This 

 i especially the case if the circuit is not closed and the 

 ulials of course very high. Suppose a fine filament enclosed 

 1 exhausted vessel be connected with one of its ends to the 

 .ermiiial of a high tension coil and with its other end to 

 I large insulated plate. Though the circuit is not closed, the 

 iTlament, as I have before shown, is brought to incan- 

 Uescence. If the frequency and potential be comparatively 

 ow, the filament is heated by the current passing //;;-(7«^/i it. If 

 !;he frequency and potential, and principally the latter, be in- 

 creased, the insulated plate need be but very small, or may be 

 done away with entirely ; still the filament will become in- 

 ':andescent, practically all the heating being then due to the 

 Sombardment. ... It should not be thought that only 

 'arefied gas is an important factor in the heating of a con- 

 tluctor by varying currents, but gas at ordinary pressure 

 'Jiay become important, if the potential difference and frequency 

 jf the currents is excessive. On this subject I have already 

 ^stated, that when a conductor is fused by a stroke of lightning, 

 '.he current through it may be exceedingly small, not even 

 sufficient to heat the conductor perceptibly, were the latter im- 

 nersed in a homogeneous medium. 



NO. 1232, VOL. 48] 



From the preceding it is clear that when a conductor of high 

 resistance is connected to the terminals of a source of high fre- 

 quency currents of high potential, there may occur considerable 

 dissipation of energy, principally on the ends of the conductor, in 

 consequence of the action of the gas surrounding the conductor. 

 Owing to this, the current through a section of the conductor at a 

 point midway between its ends may be much smaller than through 

 a section near the ends. Furthermore, the current passes princi- 

 pally through the outer portions of the conductor, but this effect 

 is to be distinguished from the skin effect as ordinarily inter- 

 preted, for the latter would or should occur also in a continuous 

 incompressible medium. If a great many incandescent lamps are 

 connected in series to a source of such currents, the lamps at the 

 ends may burn brightly, whereas those in the middle may re- 

 main entirely dark. This is due principally to bombardment, 

 as before stated. But even if the currents be steady, provided 

 the difference of potential is very great, the lamps at the ends 

 may burn more brightly than those in the middle. In such case 

 there is no rhythmical bombardment, and the result is produced 

 entirely by leakage. This leakage or dissipation into space, 

 when the tension is high, is considerable when incandescent 

 lamps are used, and still more considerable with arcs, for the 

 latter act like flames. Generally, of course, the dissipation is 

 much smaller with steady than with varying currents. 



Incandescence of Gases. 



Coming now to the incandescence or phosphorescence of gases 

 at low pressures or at the ordinary pressure of the atmosphere, 

 we must seek the explanation of these phenomena in shocks or 

 impacts of the atoms. Just as molecules or atoms beating upon 

 a solid body excite phosphorescence in the same or render it in- 

 candescent, so when colliding among themselves they produce 

 similar phenomena. But this is a very insufficient explanation, 

 and concerns only the crude mechanism. Light is produced 

 by vibrations which go on at a rate almost inconceivable. 

 If we compute, from the energy contained in the form of known 

 radiations in a definite space the force which is necessary to set 

 up such rapid vibrations, we find, that though the density of the 

 ether be incomparably smaller than that of any body we know, 

 even hydrogen, the force is something surpassing comprehension. 

 What is this force, which in mechanical measure, may amount to 

 thousands of tons per square inch ? It is electrostatic force in the 

 light of modern views. It is impossible to conceive how a bod 

 of measurable dimensions could be charged to sd high a poten- 

 tial that the force would be sufficient to produce these 

 vibrations. Long before any such charge could be imparted 

 to the body it would be shattered into atoms. The sun 

 emits light and heat, and so does an ordinary flame or in- 

 candescent filament, but in neither of these can the 

 force be accounted for if it be assumed that it is associated 

 with the body as a whole. Only in one way may we account 

 for it, namely, by identifying it with the atom. An atom 

 is so small, that if it be charged by coming in contact with an 

 electrified body and the charge be assumed to follow the same 

 law as in the case of bodies of measurable dimensions, it must 

 retain a quantity of electricity which is fully capable of account- 

 ing for these forces and tremendous rates of vibration. But the 

 atom behaves singularly in this respect, it always takes the same 

 " charge." 



It is very likely that resonant vibration plays a most impor- 

 tant part in all manifestations of energy in nature. Throughout 

 space all ma'ter is vibrating, and all rates of vibration are re- 

 presented, from the lowest musical note to the highest pitch of 

 the chemical rays, hence an atom, or complex of atoms, no 

 matter what its period, must find vibration with which it is in 

 resonance. When we consider the enormous rapidity of the 

 light vibrations, we realise the impossibility of producing such 

 vibrations directly with any apparatus of measurable dimensions 

 and we are driven to the only possible means of attaining the 

 object of setting up waves of light by electrical means and 

 economically, that is, to affect the molecules or atoms of a gas, 

 to cause them to collide and vibrate. 



Much would remain to be said about the luminous effects pro- 

 duced in gasesat low or ordinary pressures.' With the present ex- 

 periences before us we cannot say I hat the essential nature of these 

 charming phenomena is sufficiently known. But investigations in 

 this direction are being pushed with exceptional ardour. Every 

 line of scientific pursuit has its fascinations, but electrical investi- 

 gation appears topossess a peculiar attraction, for there is no experi- 



