電氣學會雜誌

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1. The theory of the oscillograph given by Blondel and others shows that the amplitude and phase of the mechanical oscillation of the moving parts of oscillograph depend upon the frequency of the applied alternating force. Strictly speaking, therefore, the record of an alternating wave consisting of a fundamental and higher harmonics is not correct reproduction of the wave. But in practical instruments, the error due to these causes is made very small for ordinary frequencies.
2. The actual test on the instruments of various types gives that the general nature of the characteristic curves representing the relation between frequency and amplitude, are in accordance with the theory Remarkable discrepancy is, however, that the natural frequency of the moving system when immersed in damping oil is considerably lower than that in air. This is probably due to the fact that the viscosity of the fluid increases the effective mass of the moving parts, since the strips can not move without carrying a part of oil with them. In consequence thereof, the accuracy of an oscillograph becomes generally lower than the theoretical value based on the natural frequency in air, and obviously correction factor calculated from the same standpoint would be of no value.
3. The characteristics of the oscillograph depend largely upon the damping given to the moving parts of the instrument. The viscosity of oils, as is well known, varies largely with the temperature and so also the damping of the oscillograph. Hence for obtaining satisfactory results, too much stress can not be laid upon the choice of proper oil and its temperature. It is very desirable to change the damping oil according to the season or to provide a proper device to adjust the oil temperature. It is also necessary to place a thermometer in the oil bath so as to indicate the temperature of the oil while in use.
4. Theoretical calculation as well as experimental results show that for correctness of recording it is desirable to adjust the damping somewhat lower than the critical value. The proper damping may conveniently be judged by the movemnt of the light spot, when the steady deflecting force is suddenly applied on or removed from the vibrator. Excessive damping can be shown by a creeping of the light spot and insufficient damping by overshooting. From the theory we know that the damding is of most desirable degree when there is a slight but obvious overshoot on suddenly applying or removing a steady deflecting force. This point was also confirmed by the actual test.
5. When an oscillograph is used for research work requiring high accuracy, it is very important to know its natural frequency and damping, and to adjust the latter to a proper value. If it is used for frequencies near or above the natural frequency, the proper care should be taken in interpretating the results obtained by the instrument.

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