The effect of change of frequency on the rate of the meters is shown in fig. 10. It is relatively small in every case but one.

These results show that with suitable precautions induction meters may be made to repeat their readings very accurately, so that precision methods may be applied in studying them. They also show that the variations due to wave form depend not only on the harmonics which are present and their magnitudes, but also on their phases.

The Bureau of Standards is now having a generating set constructed which will give all the odd harmonics up to the fifteenth and any desired combination of them with the fundamental. When this is completed it will be used to study the effects of the higher harmonics on the rate of these meters. The results given here show that for commercial purposes all the meters so far studied may be considered accurate on any ordinary wave form where only the third harmonic enters appreciably, although two meters show variations of about 3 per cent when the harmonic amounts to as much as 50 per cent of the fundamental.

DETECTOR FOR VERY SMALL ALTERNATING CURRENTS

AND ELECTRICAL WAVES.

By L. W. AUSTIN.

A number of years ago I noticed that a copper sulphate cell with copper electrodes, one of which was considerably larger than the other, showed an apparent rectifying effect for alternating currents of the order of those produced by speaking with the mouth pressed against an ordinary telephone receiver. In the original experiment the rectifying cell, the telephone, and a d'Arsonval galvanometer of moderate sensibility were connected in series. The lowest tone spoken into the telephone-produced a deflection of the galvanometer, and a louder one drove it from the scale. Recently I have again taken the matter up, desiring, if possible, to find an explanation of the rectifying effect, and also to test the usefulness of the detector in the laboratory.

G

CuSO

key was used which kept the FIG. 1.-Connections for detector (form 1). detector circuit short-circuited when not connected to the potential wire. This was necessary on account of the small electromotive forces always present in the cell, due to slight differences in the electrodes. In order to reduce these as much as possible, the electrodes were generally made from the same No. 30 wire, the small one varying from 1 mm to 3 or 4 cm in length, and the larger one from 10 to 30 cm.

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 H2SO 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 PD with a condenser inserted in E F and 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 Fto 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.

M. Ruer: ZS. f. phys. Chem. 44, p. 81; 1903.

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.

FIG. 3.-Relation between direct current and alternating P. D.

In this way 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.