« PreviousContinue »
harmonic. On the other hand, meters 1 and 2 show smaller errors due to the presence of the harmonic, but greater changes due to shifting the phase of the harmonic, both changing from slow to fast on the flat wave when the phase is shifted. Meter 4 runs faster for both peak and flat and at all phases than on a sine wave. It is the only meter of the five for which this is true.
Fig. 6 shows the effect of changing the phase of the harmonic from 0° to 90° when using a harmonic of 10 per cent, and fig. 7 shows the same for 50 per cent. Only three meters were used in these experiments.
FIG. 8. -Showing the variation in the rate of five induction meters with 10, 25, and 50 per cent of
harmonic, the phase of the harmonic in each case being 00. Fig. 8 shows the effect of changing the harmonic from 10 per cent to 25 per cent and 50 per cent, keeping the phase constant. Meters 1 and 2 show the least change in rate; meter 4 runs faster, and meters 3 and 5 run slower and show the greatest change in rate. Fig. 9 sbows for three meters the same thing as fig. 8, except that the phase of the harmonic is 90° different, Meters 1 and 2 show relatively small changes, but both run faster on the flat than on the sine. Meter 3 runs nearly 3 per cent slower on the 50 per cent harmonic than on the sine.
50 PER CENT OF HARMONIC. FIG. 9.-Showing the variation in the rate of three induction meters with 10, 25, and 50 per cent of
harmonic, the phase of the harmonic in each case being 900.
70 CYCLES PER SECOND Fig. 10.-Showing the variation in the rate of five induction meters with change of frequency.
Table V.-EFFECT OF VARYING THE PHASE OF THE HARMONIC.
Percentage of harmonic.
Fast. Slow. Fast. Slow. || Fast. Slow. Fast. Slow. || Fast. Slow. Fast. Slow. Fast. Slow. Fast. Slow.
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 1 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.
To test the sensibility of the detector for alternating currents, a known alternating current was passed through a bridge wire of about 1 ohm resistance, and from this, by means of sliding contacts, various potential differences were applied to the detector (see fig. 1). A form of R 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.