A ring shaped specimen whose dimensions are known is used for the purpose
After demagnetizing the test is started by setting the magnetising current to its lowest test vlane.
With galvanometer key K closed, the iron specimen is brought into a ‘ reproducible cyclic magnetic state’ by throwing the reversing switch S backward and forward about twenty times.
Key K is now opened and the value of flux corresponding to this value of H is measured by reversing the switch S and noting the throw of galvanometer.
The value of flux density corresponding to this H can be calculated by dividing the flux by the area of the specimen.
The above procedure is repeated for various values of H up to the maximum testing point.
The B-H curve may be plotted from the measured values of B corresponding to the various values of H.
The circuit for this test is shown in Fig.
The magnetizing winding is supplied through a potential divider having a large number of tapping.
The tappings are arranged so that the magnetizing force H may be increased, in a number of suitable steps, up to the desired maximum value. The specimen before being tested is demagnetized.
The tapping switch S is set on tapping I and the switch S is closed. The throw of the galvanometer corresponding to this increase in flux density in the specimen, form zero to some value B, is observed.
After reaching the point of maximum H i.e... when switch S is at tapping 10, the magnetizing current is next reduced, in steps to zero by moving switch 2 down through the tapping points 9, 8, 7 3, 2, 1.
After reduction of magnetizing force to zero, negative values of H are obtained by reversing the supply to potential divider and then moving the switch S up again in order 1, 2, 3 7, 8. 9, 1O.
This test is done by means of a number of steps, but the change in flux density measured at each step is the change from the maximum value + Bm down to some lower value.
But before the next step is commenced the iron specimen is passed through the remainder of the cycle of magnetization back to the flux density + Bm. Thus the cyclic state of magnetization is preserved.
The connections for the method of reversals are shown in Fig.
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