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we possess no knowledge why carbon, with its low atomic weight, should be a solid, while nitrogen is a gas, except in so far as we ascribe molecular complexity to the former and comparative molecular simplicity to the latter. Argon, with its comparatively low density and its molecular simplicity, might well be expected to rank among the gases. And its inertness, which has suggested its name, sufficiently explains why it has not previously been discovered as a constituent of compound bodies.
We would suggest for this element, assuming provisionally that it is not a mixture, the symbol A.
We have to record our thanks to Messrs. Gordon, Kellas, and Matthews, who have materially assisted us in the prosecution of this research.
ART. XXV.- The Velocity of Electric Waves ; John TROW
BRIDGE and WILLIAM DUANE.
SOMETIME since the following method of measuring the velocity of electric waves suggested itself to us; increase the size, and if necessary change the shape of an ordinary Hertz vibrator until the period of oscillation is sufficiently long to be determined by photographing the spark; measure the length of the waves induced in a secondary circuit tuned to resonance with the vibrator ; and the quotient of the wave length by the time of a complete oscillation will be the required velocity.
The first apparatus experimented with was that used by Mr. St. John in investigating the peculiarities of waves along iron wires. For a detailed description of this apparatus see this Jonrnal for October, 1891. It has been assumed by certain writers that the reaction between the circuits, arranged in the manner employed by Mr. St. John, is to a large extent, what would be called in the older theory of electricity, electro-magnetic rather than electrostatic. Certain phenomena, however, that appeared in the early part of our investigation seemed to point to the view that the greater portion of the action was electrostatic; and we therefore concluded to arrange the apparatus so that the reaction should be wholly electrostatic, thinking by this means to obtain a more powerful oscillation and a more regular wave than with the apparatus described and used so successfully by Blondlot. The first attempt arranged on the electrostatic principle proved a complete failure. A second trial abont a fortnight later, however, proved so successful that we fully determined to adopt the electrostatic method.
Av. JOUR. SCI.—THIRD SERIES, Vol. XLIX, No. 292.- APRIL, 1895.
The first point in the course of the investigation worthy of detailed notice is the production of electric waves along parallel wires in such a manner that they are actually visible to the eye. The arrangement of the apparatus to accomplish this was as follows:
L A primary condenser A B (Fig. 1) was held with its plates in vertical planes by means of suitable wooden supports (not represented in the figure), and was joined in a circuit B C consisting of two wires about 75cm long placed 4cm apart. In reality this circuit B C should be represented as perpendicular to the plane of the paper, (which is taken as the horizontal plane passing through the center of the apparatus). The plates of the condenser A B were of sheets of tin foil 101 x 40cm glued to hard rubber sheets and the dielectric between them consisted of other similar sheets of hard rubber sufficient in number and thickness to make the distance between the condenser plates 4.2cm. Outside the primary condenser plates and separated from them by hard rubber plates, (total thickness :6cm) were two secondary plates E and F each 40cm square. To these plates was attached the secondary circuit E GJHF, the form of which is represented in fig. 1. This latter circuit consisted of copper wire, diameter •13cm and its total length from E to F was 4200cm. A spark gap with spherical terminals 2.5cm in diameter was placed at C in the primary circuit, and another spark gap with pointed terminals was sometimes inserted at J in the secondary circuit, although this latter spark gap had no effect upon the phenomena to be described. The primary condenser was charged by means of a large Ruhmkorf coil excited by five storage cells with a total voltage of ten volts. The current from these cells was made and broken by the automatic interrupter used by Mr. St. John and described by him in his paper above referred to. Every time the primary condenser was charged, a spark passed at C causing an oscillatory discharge. These oscillations induced charges on the plates E and F which were rapidly reversed in sign, and which traveled out along the wires E G and F H. The dimensions of the primary circuit were altered, until by trial it was found that a charge of, let us say, positive electricity starting from E
would meet at the point G one of negative electricity that had previously started from F and traveled around F H JG, another at the point J, and still another at the point H. Stationary vibrations in the circuit E GJH F were thus set up with nodes at the points G, J and H, and ventral seginents halfway between them at K and L. The method of discovering when the circuits were in tune, and of investigating the shape of the waves will be described later. The point to be noticed here is that the vibrations were sufficiently powerful to cause a luminous discharge on the surface of the wire at points where the accumulation of electricity was a maximum, i. e. at K and L, while at the nodal points G, J and H the wire was entirely dark. Still further the wave formation could be made apparent to the sense of hearing as well as that of sight; for placing the ear within a few centimeters of the wire and walking along it, a distinct crackling sound could be heard at the points K and L whereas no such sound could be heard at G, J and H. By placing bits of glass tubing on the wire the sound was much intensified at the points K and L, and the phenomena made more striking. It might be supposed that by decreasing the capacity of the primary condenser, and therefore the period of its oscillation, the secondary circuit could be broken up into a new set of shorter stationary waves, with nodes at J and at points somewhere near K, L, G and H, and ventral segments between them. This we tried with perfect success except that it was not possible to cause the light at K and L to actually disappear. There was decidedly less light at these points however than on either side of them. The light of course is simply that which always appears around wires carrying very high potential currents, the interesting point being that it appears in some places on the circuit and not in others. The experiment showing how the circuit breaks up in several different ways would form a most beautiful lecture experiment.
As a ineans of ascertaining when the circuits were in resonance, and of investigating the form of the wave in the secondary circuit a bolometer similar to that designed by Paalzow and Rubens* was used. The bolometer was used in this present investigation to detect electrical disturbances in a conductor and to measure their magnitudes; but it did not indicate either the direction or sign of the quantities measured. Its use for this purpose is described in the paper of Mr. St. John already referred to.
The conductors that were electrically connected to the arm of the bridge, and that were brought near the circuit consisted
* Anwendung des bolometrischen Princips auf Electrische Messungen, Wied. Ann., xxxvii, 529.
of two pieces of wire insulated with rubber, bent into circles of about 2cin radius, and fastened to a bit of pine wood by means of a heavy coating of paraffine. The two wires of the secondary circuit passed through holes in this bit of wood in such a manner as to pass through the centers of the two circles. In the early part of the investigation the bolometer and galvanoscope were placed at a sufficient distance from the oscillating circuits to prevent any direct action of one on the other, and the leads running from the circular conductors to the bolometer consisted of long tine wires. Later when longer circuits and longer waves were experimented with, great inconvenience was experienced from the long leads since their relative position had considerable effect upon the galvanoscope deflections. In order to obviate this difficulty short leads of heavily insulated wire were used and the bolometer was placed on wheels and moved along from place to place. A bolometric study of the circuit just described showed the character of the oscillation to be that mentioned, namely, nodes at the points G, J and H, and maximum accumulations at the points K and L. A careful run was made from one end of the circuit to the other, which furnished data from which a very regular curve was drawn.
Two points deserve notice here, before we pass on to the next arrangement of the apparatus. First the automatic current interrupter that worked so beautifully in connection with the Ilertz vibrator would not function well, when the vibrator was replaced by the circuit and condenser just described. For a detailed description of this interrupter we must again refer to Mr. St. John's paper. The essential feature of the apparatus was this: the circuit was interrupted by the regular periodic lifting of a platinum plunger from a glass cup partially filled with mercury. Alcohol was poured over the surface of the mercury in order to keep it clean, and this effectually stopped the sparking when the Hertz vibrator was used. When, however, the induction coil was used to charge the large condensers, violent sparks occurred at the point where the plunger left the mercury, almost any one of which was sufficiently strong to blow the alcohol out of the cup. As the same current in the primary of the induction coil was used in the two cases, the most probable explanation of this is the following. The capacity of the condenser being considerably greater than that of the two plates in the Hertz vibrator, the current in the secondary of the induction coil in the first case must have been much greater than that in the second case. Hence the reaction of the secondary current on the primary of the induction coil, must have been much greater. This would cause a greater reaction of the secondary current on the primary of the induction coil, and would throw a greater stress on the point of rupture of the primary circuit; hence the sparking. To obviate the difficulty several methods of exciting the induction coil were tried, with more or less success. Finally, an ordinary reed interrupter with a comparatively large hammer and anvil arrangement was adopted, which gave little trouble.
The second point, and this is very important in the light of what is to follow, is the following: the insertion of a small spark gap (1mm_3mm) at the point in the secondary circuit marked-J (fig. 1) had no appreciable effect upon the position of the nodal points G and H, or of the points of maximum accumulation K and L. The form of the wave was slightly altered for a meter on each side of J, and the bolometer showed a slight accumulation in the immediate neighborhood of the spark gap. This was probably due to the charging of the spark terminals to a sufficiently high potential to break through the dielectric. The fact that the insertion of a spark gap, into a secondary circuit in the manner described has no effect upon the length of the waves set up in that circuit was tested for a number of different cases, (in none of which, however, the length of the waves was greater than in the present case) and found to be true in each one of them.
In order to determine the time of vibration we used a concave rotating mirror : and the images of the oscillating sparks were thrown on a sensitive plate. If the mirror rotated about a horizontal axis, the photographs showed bright horizontal lines perpendicular to which at their extremities extended two series of dots. The distance between the successive dots was evidently the distance on the plate through which the image of the spark gap moved during the time of a complete oscillation. Hence by determining the speed of the mirror and measuring the distances from the mirror to the plate the time of oscillation can be calculated.
The advantages of photographing the secondary spark rather than the primary are numerous. In the first place to properly photograph a spark it is necessary to use pointed terminals: but experiment has slown that the waves excited in a secondary circuit depend to a large extent upon the character of the primary spark, and that the most active sparks are those between metallic spheres with polished surfaces. It is true that waves can be produced by sparks between points, but the oscillations are not so powerful or well marked. In the second place, from the results obtained by Bjerknes one would expect the oscillations in the secondary circuit to be much less damped than those in the primary. This expectation has been fully realized. Photographs show from ten to