25. Morgan’s Experiment. No discharge in High Vacua. Wiedemann, vol. 2. Phil. Trans., 1875, vol. 75.—He was led to believe by an experiment, that when the vacuum is sufficiently perfect, no electromotive force could drive the spark from one terminal to the other, however close together they may be. [§ 18]. Details of Morgan’s Experiments were as follows, given roughly in his own words:—A mercurial gauge about fifteen inches long, carefully and accurately boiled till every particle of air was expelled from the inside, was coated with tinfoil five inches down from its sealed end, and being inverted into mercury through a perforation in the brass cap which covered the mouth of the cistern, the whole was cemented together and the air was exhausted from the inside of the cistern, through a valve in the brass cap, which, producing a perfect vacuum in the gauge, formed an instrument peculiarly well adapted for experiments of this kind. Things being thus adjusted (a small wire having been previously fixed on the inside of the cistern, to form a communication between the brass cap and the mercury, into which the gauge was inverted), the coated end was applied to the conductor of an electrical machine, and notwithstanding every effort, neither the smallest ray of light nor the slightest charge could ever be procured in this exhausted gauge.
26. De La Rue and Müller’s Experiment. Constant Potential at the Terminals of a Discharge Tube. Phil. Trans., part 1, vol. 169, p. 55 and 155.—The apparatus consisted of an exhausted bulb, a chloride battery of 2400 cells and a large resistance adapted to be varied between very wide limits. The result was a constant potential at the electrodes of the bulb, during all the variations of the resistance. They concluded, therefore, that the discharge in highly rarefied gases is disruptive, the same as in air at ordinary pressure.
26a. Klingenberg’s Calculations. Direction of Discharge Tube Current in Secondary of Ruhmkorff Coil. Translated from the German, by Ludwig Gutmann. Extract of paper read by G. Klingenberg before the Electro-technischer Verein. It would naturally be inferred that an induction coil, the primary current of which is intermitted, and of one direction, would produce an alternating current in the secondary coil. The fact of the matter is, however, that a good induction coil will produce the sparking only in but one direction. [§ 41]. The reason is the following: If the coil had no self-induction nor capacity, then the current impulses would be represented by a rectangle a, Fig. [1]. On closing, the current would suddenly reach its maximum, which is determined by the terminal pressure and circuit resistance, and this current strength would be maintained as long as the circuit remained closed. On the opening of the circuit, the current would decrease just as suddenly; if not, the arc on opening of the circuit would oppose such sudden fall, therefore the corner will be slightly rounded at a, Fig. [2]. The influence of self-induction, which we find in any coil, is the force that will tend to oppose any change in the current strength. Therefore, the self-induction will be the cause of a retardation of the minimum current. On the other hand, it increases the size of the spark on opening. Next a condenser is enclosed in the main circuit, so that the spool is closed through it at the moment the current is intercepted. If we assume, for simplicity sake, that the magnetization of the iron is proportional to the current strength, then the primary current curve represents at the same time, the curve of the rate of change of line of force in the magnetic field. The secondary E. M. F. is determined by e = n(dw/dt)t t; the rise then will have a smaller E. M. F. than at the fall, like Fig. [3], except that the curve representing the fall should be shown as more nearly perpendicular to the abscissa.
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27. Kinnersley, Harris and Riess’s Experiments. Spark. Pressure Produced by. Ganot, § 790, et al. Encyclo. Brit. Art. Elect.—These experimenters passed a spark through air contained over mercury, so that if the pressure of the air were increased, the mercury would move along through a capillary tube, having a scale so that the amount could be represented to the eye, as in the cut. (Fig. [V.]) The experiments proved that when a spark passes through the air, the pressure is increased, and it was concluded in view of several experiments, that the spark being the source of an intense, but small amount of heat, expanded the air, thereby causing the pressure in a secondary manner, through the agency of heat. A spark as short as 2 mm. will produce a considerable pressure of the mercury. Riess performed an experiment also in causing the spark to pass through cardboard, and also through mica located within the air chamber. [§ 12]. Other things being equal, the increase of temperature was less by using the solid material like mica or cards, than without. This illustrated that a part of the energy of the spark was converted into heat and a part into mechanical force, and explained why sound, [§ 24], is produced by a spark and by lightning.