In 1902, Lee DeForest was trying to develop a sensitive radio detector which didn't infringe on anyone else's patents. He briefly hoped that the flickering of a burning flame might lead to something useful. However, as recounted in this write-up, he eventually determined that the flickering was merely a reaction to the loud sounds made by his spark transmitter, and not to the radio waves it emitted. But four years later, further investigations on the possibilities of ionized gas detectors would help lead DeForest to the development of a three-element vacuum tube detector, which he would call an "Audion".
Electrical World and Engineer, April 12, 1902, pages 652-653:
An Interesting Sensitive-Flame Experiment.
BY L. DE FOREST.
WHILE conducting experiments with Hertzian waves at the Armour Institute, Chicago, some time since, a rather new and striking sensitive-flame effect was accidentally brought to my notice. At the time I was carrying on the work at night, by the light of a Wellsbach gas-burner. An ordinary induction coil was used, giving at the time a 1-16-inch spark, and located twenty feet from the gas-burner.
The operation of this coil was frequently accompanied by a decided increase in the light in the room; and by altering the adjustment of the air-intake of the Welsbach burner, this increase of light produced by the spark could amount to several candle-power.
For this condition of maximum sensitiveness the mantel was put at considerably less than its maximum brilliancy, portions of it being at but red heat. The flame within, as in the case of the ordinary sensitive-flame, appeared to fan outwardly, at the sound of the spark, extend downward, and play upon the red portions of the mantel, causing these to incandesce. Hence the increase of light. There was a slight lag in the return to normal brilliancy after cessation of the sound.
When the coil was closeted, or placed behind a projecting wall, the effect on the light was naturally diminished; and when the door of the closet was almost but not quite closed, the effect was completely cut off, although the source was in plain view from the lamp.
At first sight this decided light effect from a coil spark might be readily mistaken for a new response to electric oscillations; but as a simple acoustic phenomenon the Welsbach mantel properly illuminated seems to afford an unusually sensitive sound-responsive device. The relative effect of different qualities of sound is very marked; it being, as is the case with the ordinary sensitive-flame for such weak sounds, practically unresponsive to ordinary sound or musical notes, but influenced chiefmost by the sharp crack peculiar to the electric spark. That from a small electric bell held not too far away also showed its effect.
As with the ordinary flame this is a pressure phenomenon. The relative proportions of gas and air have all to do with it. An excess of gas is necessary for the effect described, but by increasing the amount of air the reverse effect may be obtained, a diminution of the light by the sound. Between these two effects is a point of neutral regulation, near that of maximum illumination, at which the flame appears entirely deaf to any such sound. I have been able to obtain a falling off in the light of the mantel to one-half or one-third its normal brilliancy (which latter required to be made very low for this extreme sensitiveness, with a small pressure). In this condition the light responded even to the exceedingly weak sound of the primary spark of coil at the distance named.
When the gas is led to the lamp through a long rubber tube, offering considerable friction to its passage, the response is accompanied by a distinct jingling sound in the burner; and sometimes, as a secondary effect on the flame's motion, the whole mantel is seen to rock to one side when the spark is operated.