The London, Edinburgh and Dublin philosophical magazine and journal of science . e of the con-denser is connected to the middle plate and the other to thepositive electrode of the lamp. § 24. In considering the behaviour of the heated carbonelectrode and the cool metal plate in their respective powersof discharging the positive or negative charge of the con-denser, it seemed that the fundamental fact was the power ofthe heated surface to discharge negative electricity out ofitself. Hence arose the question, how iar the observed factswould be modified if the middle metal plate itself could beal
The London, Edinburgh and Dublin philosophical magazine and journal of science . e of the con-denser is connected to the middle plate and the other to thepositive electrode of the lamp. § 24. In considering the behaviour of the heated carbonelectrode and the cool metal plate in their respective powersof discharging the positive or negative charge of the con-denser, it seemed that the fundamental fact was the power ofthe heated surface to discharge negative electricity out ofitself. Hence arose the question, how iar the observed factswould be modified if the middle metal plate itself could bealso heated. One way by which this might have been donewould have been to have rendered this plate incandescent byheating it by radiant heat concentrated by means of a powerfulmirror or lens. Some experiments tried in this way werenot satisfactory, and consequently a method was adopted inwhich a middle plate of carbon could be rendered incandescentelectrically. Experiment 21.—A vacuum tube was provided with twocarbon conductors (see fig. 19), one the ordinary carbon Fig. filament L of a 50 volt lamp, and the other the small carbonS of a 4 volt lamp. The smaller carbon was sealed in theusual way through the glass and placed so as to standsymmetrically between the legs of the laiger carbon loop. The smaller carbon could be rendered incandescent by aninsulated battery of fifteen secondary cells, appropriate 94 Prof. J. A. Fleming on the resistance being introduced. The larger carbon also couldbe rendered incandescent by the proper electromotive the smaller carbon was kept cold and employed simply asa third electrode or middle plate, all the phenomena previouslydescribed as happening with metal middle plates of aluminiumor platinum took place. If the small (cold) carbon loop isconnected through a galvanometer with the positive electrodeof the larger carbon loop when this last is rendered incan-descent by a current, we find as usual a current of a fewmilliamperes pass
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