interesting point is that under normal conditions no current flows between the two anodes E and C, even when they are only inch apart. If the tube is to work as a rectifier, the distances EB and BC are made small. If the arc is to be used as a source of light, the lamp must be long enough to consume all the voltage applied; and in that case the tube has the form similar to that of a direct-current lamp. First the lower part of the lamp is started, and as the ionized vapour fills the main vertical tube the arc is partly transferred to the two anodes at the top of the lamp; and this transfer can be made complete by opening the switches leading to the auxiliary anodes at the bottom of the lamp. As in the case of the directcurrent arc, a carbon filament helps the propagation of the arc. The details, however, must be omitted, as they would lead too far into the practical side of the subject. In case of a three-phase alternating current a similar rectifier can be used. Three anodes, connected to the three wires of the system, are used. The three anodes are connected by reactances to the mass of mercury at the bottom of the lamp, which is to serve as the common cathode. In the wire leading from the common cathode to the three reactances both half-waves of all three phases are rectified and superposed. The practical importance of the rectifier for alternating current is so self-evident that it need not be emphasized. It would, however, lead too far to enter into a description of the most practical form of such rectifiers, their use for large currents and for high voltages, the operating of many of them in multiple and in series, &c. The description given above was only intended to make known the main principle, and to point out those parts which are of scientific interest. Summary of Main Results. The main results of this investigation may be summarized as follows: 1. By a series of experiments it was shown that in the process of starting an are the cathode plays an important rôle, so that a certain change must take place on its surface before the arc can start. The anode receives the current without any previous excitation. a new method has been devised for an instantaneous starting 2. Starting from the recognition of this rôle of the cathode, of the passage of a moderate voltage-current through the space separating the electrodes, and this no matter how long that space is. 3. The properties of the mercury arc have been studied, and a number of differences in the behaviour of the cathode and anode, besides the one mentioned above, stated. 4. The behaviour of amalgams, as well as pure alkali metals, has been investigated, and the complete analogy between the behaviour of the arc in their vapours and that of the mercury-are shown. 5. Different ways have been found to cause an alternating current to pass through mercury vapour in form of an arc. 6. On the basis of this a theoretically almost perfect rectifier for conversion of alternating current into steady direct current was developed. In conclusion, I wish to acknowledge the great help I have obtained from Dr. Kruh, who assisted me in carrying out the work on the alternating current, and take pleasure in expressing my thanks to Dr. C. P. Steinmetz and to Dr. W. R. Whitney, Director of this Laboratory, for their interest in this work and many valuable suggestions. Schenectady, N.Y., July 17th, 1903. XIII. The Variation of Potential along the Transmitting Antenna in Wireless Telegraphy. By C. A. CHANT *. [Plates XII. & XIII.] I. Introduction. N a former paper † illustrations were given of the manner when the electrical disturbance is produced by electrostatic induction from a Hertzian oscillator at the other end of the wire. The present communication contains a somewhat detailed account of an examination of the aerial wire used to radiate the waves in wireless telegraphy; and, in a section at the end, a brief account of a continuation of the former experiments. The problem of the electrical oscillations about a free-ending wire has been treated from a rigid theoretical basis by Abraham ‡, who determined the electric and magnetic forces at any point in the field by directly integrating the Maxwellian equations. For the purposes of analysis the wire was considered to have the form of a very elongated paraboloid of *Communicated by Prof. Trowbridge. + C. A. Chant, "The Variation of Potential along a Wire Transmitting Electric Waves." Am. Jour. Sci. xv. p. 54 (1903); Phil. Mag. ser. 6, v. p. 331 (1903). M. Abraham, Ann. der Physik, ii. p. 32 (1900). revolution, and the field to vary in such a way that the electric lines of force ended perpendicular to its surface. Sarasin and de la Rive* and others had compared the oscillations about a wire to those in an open pipe; but, as Abraham remarks, though the relations are essentially similar, the analogy must not be pushed too far. In the pipe the radiation is from within outwards, and is greatest in the direction of the axis; while in the electromagnetic case the radiation is from without inwards, being limited by the surface of the wire, and on account of the transversality of the vibrations there is no radiation along the axis. Moreover, in the air-vibrations there is a displacement of the entire system of nodes and loops towards the open end, while, with the electrical oscillations, to a first approximation, there is no such displacement. On a closer examination, however, there is found to be a displacement of this kind, variable with the frequency. The phase of the advancing waves alters in a discontinuous manner, somewhat as in the vibrations of a plucked string †. When two wires are used, as in Lecher's arrangement, the radiation in the direction of the axis does not vanish, and the analogy to the open pipe is more marked. There is then a decided displacement of the nodes and loops, well exhibited in an investigation by de Forest ‡. The best acoustical analogy to a wire connected at one end to earth or to a large capacity and free at the other, seems to be a closed pipe, gas-pressure in the pipe corresponding to potential or charge in the case of the wire. Here there is a displacement of the nodes and loops, but it is very small, and only the odd harmonics are present in the two cases. Of course a rod clamped at one end is similar to the closed pipe. Birkeland and Sarasin § in their investigation of the field about a free-ending wire explored with a circular resonator and found the first node distant from the end by one-half the circumference of the resonator (a result similar to that obtained by Sarasin and de la Rive in their investigation on two parallel wires, and ascribed by them to the geometrical form of the resonator), and other nodes regularly spaced along the wire at intervals equal to twice the diameter of the resonator. The form of the nodal surfaces in the space * E. Sarasin and L. de la Rive, Archives des Sciences Physiques et Naturelles, Genève, xxiii. p. 113 (1890). + Helmholtz, 'Sensations of Tone', p. 54; Rayleigh, 'Theory of Sound,' art. 146. L. de Forest, Am. Jour. Sci. viii. p. 58 (1899). K. Birkeland and E. Sarasin, Comptes Rendus, cxvii. p. 618 (1893). about the wire obtained by them agrees with that deduced by Abraham. Slaby's theoretical treatment of the problem is much simpler than Abraham's, and from his results he was led to his method of syntonic telegraphy. He takes the so-called "telegraphic equation,” where i is the current strength at any time at a place a on the antenna, and R1, L1, C1, are the resistance, self-induction, and capacity per unit length of the wire. A solution † is The where T=4LC, I is the length of wire, A is a constant, and R, L, C relate to the whole length of the wire. frequency is 1/T and λ=4l. From this solution it should follow that the disturbance varies according to the simple harmonic law, and that the free end of the wire is a potential loop, the lower end a potential node. II. Experimental Arrangements and Results. In the present investigation all the wires explored were of bare copper and were stretched horizontally on the tops of wooden poles, about 15 m. high and 16 m. from the wall of the room in which the experiments were made. This room was a large hall on the first flat, about 22 ms. long, 12 ms. wide, and with a ceiling 13 ms. high. The manner of examining the wire at various points in its length was precisely similar to that in the former research. The induction-coil and interrupter, the magnetometer, and the method of taking readings were identical with those used earlier and need not be described again here. In most of the work the detector was the one used before, but during the course of the experiments it was broken, and another, similar to it and indistinguishable from it in its behaviour, was constructed. The manner of applying the detector to the wire was slightly different. Before, the detector was laid on the top of a carriage moving on ways along the wires, with the little wing (w, fig. 1) in a little pocket by the wire; now, a *A. Slaby, Lond. Electrician, vol. xlvi. Jan. 18, 1901; also vol. xlix. April 25, 1902. † See Webster, Electricity and Magnetism, arts. 255, 256. |