User`s manual

7800 Series Hysteresisgraph System User's Manual
Theory of Operation 2-3
2.2 THEORY OF OPERATION
For an in-depth theory of the measurement of flux and flux density by a Fluxmeter or a Gaussmeter, please refer to
the respective Lake Shore Model 480 Fluxmeter and Model 450 Gaussmeter User's Manuals.
A DC permanent magnet hysteresisgraph is a device, which obtains the basic magnetic properties of a material by
determining its magnetic hysteresis loop. A magnetic hysteresis loop is a graphical representation of the induced
magnetization from a material as it responds to a changing applied magnetic field.
This applied field, or H, is created with the use of an Iron Core laboratory electromagnet (EM) and a suitable
power supply. H is usually measured in Amps/meter or Oersted. Applying current to the wound copper coils of an
EM generates the applied field, or H, in the air gap between the pole faces at the center of the EM. Reversing the
polarity of the current source will generate applied fields of opposite polarity, or –H. With a continuously variable,
bipolar power supply, we can smoothly generate a continuum of applied fields with our EM. By measuring the
applied field, H, we can generate the horizontal axis or independent variable, applied field (H), of a hysteresis
loop.
To measure this field, H, we can use either a Hall-effect probe and a Gaussmeter or a Coil of Wire and a
Fluxmeter. If we place a Hall-Effect probe in the presence of a magnetic field and apply a constant current source
to it, we will obtain a voltage proportional to the magnetic field, and, after some processing with a Gaussmeter, a
numerical representation of the field itself.
Alternatively, we may use a coil of wire to sense H. A coil of wire senses changes in magnetic flux. If we send the
output of this coil to an Integrating Voltmeter, these changes is flux can be Integrated over time and represented as
DC voltage levels. The bigger the coil’s cross-section, the greater the signal. So we normalize the coil’s voltage to
its cross section and with some further processing, we can come up with a numerical value for Applied Field
strength, H. This is the task of one of our Fluxmeters.
Now we have created our independent axis, H. To create the dependent, axis, B, we must have a specimen
permanent magnet. Our specimen magnet will have parallel surfaces where flux enters and leaves the sample; this
axis is called the preferred or ‘magnetic length’ axis. If we place these parallel surfaces so that they are in contact
with the pole faces at the center of the EM, we have created closed a magnetic circuit. By placing a coil of wire
around our sample, it will detect the changes in flux that out specimen will undergo. We are now ready to test.
When the applied field changes while our sample is present there will be a corresponding change in the magnetic
induction in the permanent magnet specimen. To measure these changes, we use the coil, which surrounds our
sample. In addition to the applied field created by the EM, this coil also senses changes in the induced flux of the
specimen. Our Fluxmeter will then process our signal (divide the output by sample’s area, number of turns on the
coil, and some other compensations) to arrive at Magnetic Flux Density, B in Tesla or Gauss.
Of course this coil is also seeing the H field created by the EM in addition to the flux that was induced by the
specimen magnet. If we take B and subtract the H applied field, we see the contribution of flux from he sample
only, or (B–H). If we plot (B–H) on the Vertical axis against H on the horizontal axis, we have what is called the
Intrinsic curve. Similarly, plotting B on the vertical axis against H on the horizontal axis gives us B vs. H or the
Normal curve.
The normal course of events in such a test is to initially apply a very large forward H field to fully magnetize, or
saturate, the sample. (This may also occur externally in a device called a pulse-discharge magnetizer, which can
achieve greater fields than our Electromagnet can.) As the H field is increased, the sample becomes magnetized as
evidenced by an increasing value for (B-H). When the sample is saturated, we have applied a sufficiently large H
field, that the contribution of flux from the sample, or the B-H value, has reached a maximum. No matter how
much additional H field is applied, (B-H) will not increase. Here all of the magnetic domains in the sample are
pointing in the same direction.
Next we return the current to zero. Because of the remanent field in the Iron of the electromagnet, this leaves us
with a slightly positive H field in the EM gap. We then start to apply negative H field. As we pass the H = 0 point,
the (B-H) and B values are equal. This axis crossing is identified as Remanence or Br.