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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. ssss1. 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. ssss1. 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. ssss1. 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. ssss1, except that the curve representing the fall should be shown as more nearly perpendicular to the abscissa.