HIGH FREQ UENCY AND HIGH POTENTIAL CURRENTS. 355

the heating effect is increased. Thus if the rate of change ofthe current is very great, the resistance may increase to suchan extent that the filament is brought to incandescence with in-appreciable currents, and we are able to take a short and thickblock of carbon or other material and bring it to bright incan-descence with a current incomparably smaller than that requiredto bring to the same degree of incandescence an ordinary thinlamp filament with a steady or low frequency current. This resultis important, and illustrates how rapidly our views on these sub-jects are changing, and how quickly our field of knowledge is ex-

Fig. 187.

Fig. 188.

tending. In the art of incandescent lighting, to view this resultin one aspect only, it has been commonly considered as an essen-tial requirement for practical success, that the lamp filamentshould be thin and of high resistance. But now we know thatthe resistance of the filament to the steady flow does not meananything; the filament might as well be short and thick; for if itbe immersed in rarefied gas it will become incandescent by thepassage of a small current. It all depends on the frequency andpotential of the currents. We may conclude from this, that it