MEASUREMENT OF THE ABSOLUTE VALUE OF THE AMPLITUDE OF A PULSED MAGNETIC FIELD WITH A BALLISTIC GALVANOMETER

Yu. K. PETROV, V. A. VIZIR, A. V. PESHКOV

A ballistic galvanometer measures the change in the intensity of a magnetic field acting on a test coil, which is connected into the circuit of the frame of the measuring instrument. In the ballistic regime, the results of measurements do not depend on the law according to which the magnetic-field intensity changes [1]. If the time during which the measured field acts on the test coil is limited to the time interval \(\tau\) (Fig. 1), during which the field changes from zero to a maximum, then as a result of the measurements we obtain the amplitude value of the field.

The measurement problem is facilitated by the circumstance that the instant of time \(\tau\) can be determined in the course of the measurements themselves. Indeed, the readings of the galvanometer are proportional to the quantity of electricity that has flowed through its frame during one pulse. This quantity of electricity is represented in Fig. 1 by the shaded area. As is seen from the figure, when the time \(\tau\) is varied smoothly during the measurements, a maximum will be observed in the galvanometer readings, which corresponds to the amplitude value of the field.

Fig. 1.

The authors measured the amplitude of a single pulse of magnitude 12 kGs, whose duration was 0.2 sec. The effective cross section of the test coil was determined from the known number of turns and the geometrical dimensions of the coil and amounted to \(25\ \text{cm}^2\). A polarized relay RP-4 was used as the interrupter. The circuit for opening the galvanometer-frame circuit is shown in Fig. 2. Normally, the relay is closed by a direct current through one of its windings. At the moment the measured pulse arises, a triggered multivibrator is started by the zero-field sensor; it produces a rectangular pulse.

The rectangular pulse is differentiated. By the pulse associated with its trailing edge, the thyratron is extinguished; the second winding of the relay is connected into its circuit, and thus the relay opens. Switch VK serves to return the circuit to its initial state. The duration of the rectangular pulse, i.e., the time interval \(\tau\) (Fig. 1), is regulated during the measurements by means of the resistance \(R\).

When measuring the amplitude of pulses that are short in time, the mechanical interrupter may be replaced by a diode or triode limiter.

Fig. 2.

For limiting pulses of small amplitude, the authors propose a combination of two diodes, one of which is connected according to the parallel-limiting circuit, and the other according to the series-limiting circuit. The limiting level is determined from the maximum readings of the galvanometer during the measurements.

The accuracy of the field measurement is determined by the accuracy of the visual reading of the galvanometer readings, the accuracy of determining the effective cross section of the search coil, and the accuracy of calibrating the ballistic galvanometer. Calibration of the ballistic galvanometer and determination of the coil parameters can easily be carried out with an accuracy of 0.2%; therefore, the accuracy of the measurements is determined mainly by the accuracy of the visual reading of the galvanometer readings and is \(\varepsilon = 0.5\%\).

REFERENCES

1. V. O. Arutyunov. Electrical Measuring Instruments and Measurements. GEI, 1958.
2. Yu. K. Petrov, L. V. Vizir. Author’s Certificate No. 132323, 1960.