IV. On the dark plane which is formed over a heated wire in dusty air

Abstract

In the course of his examination of atmospheric dust as rendered evident by a convergent beam from the electric arc, Professor Tyndall noticed the formation of streams of dust-free air rising from the summits of moderately heated solid bodies. “To study this effect a platinum wire was stretched across the beam, the two ends of the wire being connected with the two poles of a galvanic battery. To regulate the strength of the current a rheostat was placed in the circuit. Beginning with a feeble current, the temperature of the wire was gradually augmented; but before it reached the heat of ignition, a flat stream of air rose from it, which, when looked at edgeways, appeared darker and sharper than one of the blackest lines of Fraun­hofer in the solar spectrum. Right and left of this dark vertical band the floating matter rose upwards, bounding definitely the non-luminous stream of air.”.... “When the wire is white hot, it sends up a band of intense darkness. This, I say, is due to the destruction of the floating matter. But even when its temperature does not exceed that of boiling water, the wire produces a dark ascending current. This, I say, is due to the distribution of the floating matter. Imagine the wire clasped by the mote-filled air. My idea is that it heats the air and lightens it, without in the same degree lightening the floating matter The tendency, therefore, is to start a current of clean air through the mote-filled air. Figure the motion of the air all round the wire. Looking at its trans­verse section, we should see the air at the bottom of the wire bending found it right and left in two branch currents, ascending its sides, and turning to fill the partial vacuum created above the wire. Now as .each new supply of air, filled with its motes comes in contact with the hot wire, the clean air, as just stated, is first started through the inert motes. They are dragged after it, but there is a fringe of cleansed air in advance of the motes. The two purified fringes of the two branch currents unite above the wire, and, keeping the motes a once belonged to them right and left, they form by their union the dark band observed in the experiment. This process is incessant. Always the moment the mote-filled air touches the wire, the distribu­tion is effected, a permanent dark band being thus produced. Could the air and the particles under the wire pass through its mass, we should have a vertical current of particles, but no dark band. or here, though the motes would be left behind at starting, they would hotly follow the ascending current, and thus abolish the darkness.