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The sensibility did not seem to vary in any marked degree with the size of the small electrode within the limits indicated.
The first hypothesis entertained regarding the nature of the phenomenon was that it was a true rectification, the resistance being greater for a current entering the small electrode than for one leaving it. But the following experiment seems to indicate that the direct current is due, in part at least, to some chemical action produced by the alternating current and analagous, perhaps, to the solution of Pt in H,SO, noticed by Margules and Ruer, the action being greater on the small electrode on account of the greater current density. It seems probable, however, that the cause of the phenomenon may contain more than one factor. This is indicated by the fact that a reversal of
the usual direction of the current is occasionally observed, especially with fine point electrodes and high frequencies. A cell was constructed with three electrodes, a, b, c (see fig. 2), b and F being connected to the potential wire P D with a condenser inserted in E Fand a high inductive resistance in the galvanometer circuit, the arrangement being such that most of the alternating current could be sent through either one of the electrodes of the galvanometer circuit, and very little through the other. It was found that either a or c could be made electropositive by connecting F to it, and that the direction of the current was now independent of the relative size of the electrodes, at least within wide limits.
a M. Margules: Wied. Ann. 65, p. 629, 1898; 66, p. 540, 1898.
In the accompanying curve (fig. 3) the relation is shown between the direct current produced and the alternating potential difference applied to a two-electrode cell (fig. 1), connected in series with a galvanometer having a sensibility of about 1.10-8 amp. Above 0.015 volt (not shown in figure) the curve is nearly a straight line until in the neighborhood of 0.3 volt it bends suddenly and becomes nearly parallel to the horizontal axis. The lowest voltage observable under favorable circumstances with the instrument connected directly (i. e. without a transformer) and with a sensitive galvanometer is about 0.0001 volt, the sensibility being about the same as that of a good telephone receiver. The sensitiveness is in general limited by the difficulty of keeping a steady zero. By using a small transformer the sensibility can be much increased when the external circuit is of low resistance.
I have sometimes obtained deflections of over 10 cm for 0.001 volt.
The action of the rectifier seems to be independent of frequency within the limits of ordinary laboratory practice. It responds equally well to a series of slow taps on the diaphragm of a telephone, or to a frequency of several thousand per second. It has also been found that it is sensitive to electrical waves, though less so and less regular in its action than the polarized platinum-point detector of Fessenden and Schlömilch. With a modified Blondlot apparatus it was found to be too sensitive to connect directly, but when connected to two small glass-tube condensers sliding on the wires of the secondary, as in the Rubens arrangement, the deflections increased or decreased, as the
two circuits were thrown in or out of resonance. Waves could also be detected when the receiving apparatus was moved into the next room, and about 2 meters of wire were hung up in the doorway, as an aerial, 3 or 4 meters from the source of the waves. In these experiments with waves it is necessary that the small electrode be merely a point of wire, best sealed in glass, most of the experiments being made with No. 40 wire or smaller.
To secure the best results, the small electrode must be polished every two or three days, and the solution made up with pure CuSO, and distilled water. The constancy of deflection is not by any means perfect, and unless it is found possible to keep the electrodes in a uniform condition, its main use must be as a zero instrument. I have also tested the same arrangement of a large and a small electrode of a number of other metals, not only in solutions of their own salts, but also in other electrolytes, but have found them less sensitive than copper in copper sulphate.
POSITIVE CHARGES CARRIED BY THE CANAL RAYS.
By L. W. AUSTIN.
Experiments have been made by W. Wiena, Ewers, and Villard for detecting the positive charges carried by the canal rays of Goldstein by methods similar to those of Perrind for detecting the negative charges of cathode rays. Wien and Ewers conclude that the charges observed in a Faraday cylinder placed behind a perforated cathode were carried by the canal rays, while Villard holds that they are due to the slow diffusion of positive ions into the Faraday cylinder, basing his opinion on the observation that the positive charge first appeared some time after the canal rays began entering the cylinder, and in some cases only after the discharge had ceased. As has been pointed out by J. J. Thomson, this effect is very probably due to the fact that on account of the great conductivity of the gas produced by canal rays, the charge can not accumulate in the cylinder, but after the discharge ceases the gas recovers its insulating power and the cylinder can retain any charges which diffuse into it. If this view is correct, the positive charges in the cylinder, at least when observed electrometrically, are due mainly to diffusion and not to convection by the
While almost all physicists must be satisfied from the work of W. Wien on the magnetic and electric deflections of canal rays that these rays carry a positive charge, the prominence given to the objections of Villard in recent publications make it desirable to publish the following direct experimental demonstration of the positive charges of the canal rays, which seems to be free from the objections just cited.
The plan of the experiment is somewhat similar to that of Ewers. The form of the tube is shown in figure 1. The discharge takes place
a W. Wien: Wied. Ann. 65, p. 445; 1898.
entirely in the part A, which is 7 cm long and 2.5 cm in diameter. The diaphragms d, with openings 2 mm in diameter, form the cathode, and the canal rays pass through this into the part of the tube B. This is lined with brass netting, and at its center is placed a brass cylinder c, 1.5 cm in diameter, with a 4 mm opening. This cylinder can be turned back out of the way, and is connected to earth through a galvanometer having a sensibility of 4x10- amperes. The anode e is con
FIG.1.-Plan of tube.
nected to one pole of an influence machine giving a current of about 0.0003 amp., the other pole of which, as well as the cathode d and the brass net, are connected to earth. By this arrangement practically the whole of any charge imparted to the cylinder by the canal rays which enter it passes through the galvanometer to earth, as the resistance of the gas, even when highly ionized, is always very much greater than that of the galvanometer.