20:47 Apr-04-2009 aldo Engineering Italy, Joined Jan 2009 39
On book of NDT says the following question:When using both a circular and longitudinal magnetic field, the circular field is usually established first because:
1)It is more difficult to establish a circular field
2)It is easier to flip the domains of a circular field to a longitudinal field
3)It is easier to measure a longitudinal field
4)None of the above
I replied the number 4...but correct answer is 3..
A component magnetized in longitudinal direction behaves like a bar magnet. It has well developed north and south poles on either ends (If it is AC current these poles alternate depending on the frequency.) As you know from the Bar magnet experiments the magnetic lines of force are assumed to emanate from the north pole of a magnet, pass through surrounding space, and enter the South Pole. The lines of force then travel inside the magnet from the South Pole to the North Pole, thus completing a closed loop.
The point to be noted is that at the poles there is a strong field in the surrounding space. Hence a gauss meter placed near the ends can easily read the flux density.
Same is not true in case of circular magnetization. The lines of force are inside the components and in closed loops and hardly leave the component. As the field mostly stays inside, component it is difficult to measure. Indeed there will be leakage fields if there are structural discontinuities in the component like holes or steps but these are not the sites where you would like to measure the true field strength unless it is a research project.
Incidentally, this is the reason why before routine demagnetization in the field, we first magnetize the component in longitudinal direction with perceivably higher current than what we have given in the preceding circular magnetization. Then the component is demagnetized with a collapsing AC current so that we can easily verify the absence of residual field at the poles.
Tanks for your replay...
whereby when i misure the circular magnetic filed, the value of field which i read on Gauss Meter in not true but that true is the value the longitudinal magnetic field... Is exactly my interpretation on your replay?