Vibrating Sample Magnetometer (VSM)

Vibrating Sample Magnetometer (VSM) Robert Konstandelos Operation A sample is made to oscillate using a vibrational unit extended on a rod. The sample is placed between two electromagnetic pieces which are used as the applied field for this this experiment. With the sample oscillating induces a voltage between the search coils which creates a signal to determine the magnetic properties of the sample. Reference coils are used to create a reference signal such that noise generated from the signal can be filtered using a lock-in amplifier [1]. Because the signal and the reference signal are directly related through its voltage and amplitude means that precise measurements can be recorded using a voltmeter. Calibration methods are important to determine the relation between the voltages induced by the magnetic field and the sample and their magnetic properties. Calibrating the applied field is done by increasing the voltage in steps measuring the field until reaching a maximum. The system is calibrated using a nickel standard normally as a number of vo lts per unit of magnetic moment. Many materials such as types of barium ferrite or alnico materials can be placed inside to determine properties. These properties include remanence, coercivity, intrinsic coercivity and operating points once the system has been calibrated. Advantages and Disadvantages in terms of experimental facets The key advantage is the precision and accuracy of VSMs. Taking measurements at a range of angles once detection arrangements for the coils have been devised can be done. The advantage of sample vibration perpendicularly to the applied field can be found once the detection coils have been arranged appropriately. This means that there is the ability to test the sample at different angles. The positioning of the coils are done in a way to reduce the effects of sample position variation and external field variation- essentially deep into the applied field shown in figure 1. Disadvantages are that they are not well suited for determining the magnetisation loop or the hysteresis curve due to the demagnetising effects of the sample. Another problem is that, particularly for the VSM used in the third year laboratory is that temperature dependence cannot be controlled. Figure 1. A schematic layout of the VSM 2. B-H Hysteresis Loop Tracer Operation The B-H hysteresis loop tracer is essentially two coils, one with a sample and the other which is empty for comparison. The insertion of a sample into the pickup coils causes a voltage proportional to the rate of change of the vector field to occur across the difference amplifier. After passing through an integrator, a voltage proportional to the intrinsic induction is passed to the Y-amp of the oscilloscope. This voltage combined with an X-voltage representing the magnetising field generated from the solenoid without the sample results in the generation of a hysteresis loop on the oscilloscope. Calibration is through a balance and phase adjustment to establish a trace on the oscilloscope. They are done to make sure that the magnetising field is linear and that every vector corresponds to the applied field. Measurements for the magnetic properties can then be made. Advantages and disadvantages in terms of experimental facets The coils have the ability to heat the sample such that temperature variance can be observed in the way that the material behaves when influenced by a magnetic field. On the other hand, this could cause overheating of the system which could result in a failure. Using a BH-looper can give the user a more improved visualisation compared to a VSM of the way a material behaves. The values plotted on the scope are only proportional to the absolute values, therefore display yields qualitative not quantitative information about a material magnetic properties. The precision is generally low compared to a VSM. Because a hysteresis loop is viewed using an oscilloscope means that observations of whether the material is a soft or hard magnetic material. And this is why it is used in quality control testing industries like the control of ferromagnetic oxides in a magnetic tape factory. Figure 2. A schematic layout of a BH loop tracer [2]. 3(I) Difference between concepts of Vector Field B, Magnetisation M and the magnetising field H The vector field B represents the magnetic induction. Magnetisation M is the magnetic moment per unit volume of a solid. Magnetising H field is the magnetic field strength. These three quantities are related by the equation. With ÃŽ ¼0 being the permittivity of free space. To show the difference between these quantities, hysteresis loops for a magnetic material shown in figure 4 are used. One of the key differences shown is that the magnetisation saturates whereas the B field increases at a constant rate for certain values for H. The magnetisation is generated by the spin and the orbital angular momentum of electrons in the solid. H is generated outside the material by electrical currents[3]. Therefore, from the equation above, the B field is the combination of H and M which shows the difference between the quantities with the inclusion of the permittivity of free space. A way to show the difference between the 3 parameters is through the representation of a bar magnet in a magnetic field shown in figure 3. Unfortunately, due to the age of the diagram, the labels are a bit old. Hence the ‘True’ field denotes the vector field B and the Applied field represents the magnetisation M. However, the arrows represent the direction and strength of each parameter. It is clear from figure 3 that the Magnetisation is much stronger than the demagnetising field. Figure 3 An example of a magnet being demagnetised in an applied field From figure 4, the two sketches representing of B and M against H can give an understanding of other magnetic properties of the material. The curve on the left can show the saturation of the magnetic material as well as the remanence Mr the remaining magnetisation after the applied field has been turned off. The right hand diagram can show the remanent induction Br and the saturation point of the applied field. In terms of the difference between the parameters, M, B and H, they yield different properties of the material in question. Figure 4 Hysteresis loops showing (a) M and (b) B field against H 3(II) The difference between the susceptibility and relative permeability The relative permeability ÃŽ ¼r and susceptibility χ are very closely related as shown by the equation below: The relative permeability represents a characterisation of magnetic materials. Paramagnetic or diamagnetic materials have permeabilities close to the permeability of free space. However for ferromagnetic materials, the permeability is large in comparison. It represents a multiplication factor. For example, the use of an iron core with a relative permeability is 200 times greater than just an air coil used. So this is a measure of the actual magnetic field within a ferromagnetic material. Susceptibility is a measure to an extent to which a material may be magnetised in a magnetic field. It represents a ratio of how much a material is magnetised compared to the applied field on that material [4]. So the susceptibility specifies how much the relative permeability differs from one as shown in the equation above. References [1] Foner S 1959 Versatile and Sensitive Vibrating-Sample Magnetometer* Rev. Sci. Instrum. 30 548–57 [2] Howling D H 1956 Simple 60-cps Hysteresis Loop Tracer for Magnetic Materials of High or Low Permeability Rev. Sci. Instrum. 27 952 [3] Jiles D 1990 Introduction to Magnetism and Magnetic Materials (Chapman and Hall) [4] Magnetic Susceptibilty http://www.britannica.com/EBchecked/topic/357313/magnetic-susceptibility

Ethical Issues in Oroonoko: Slavery

For years, man has illustrated his willingness to perform injustices to those weaker than he. From the bully in the schoolyard, to the king and his court, it seems that man has a desire to control and dominate others. Oroonoko, by Aphra Behn, illustrates that slavery is unethical, humiliating, demoralizing, and worse than death. Oroonoko is a powerful story about the tribulations of a gallant prince named Oroonoko. Throughout the novel, he is shown to be a brave prince and a friend to many. He is given command over an army and shows his military and strategical prowess by winning his battles and conquering his enemies. In the beginning of the novel, the prince presents Imoinda, the daughter of his foster father, with a gift of slaves that had been captured in his victorious battles. For Oroonoko, slavery was accepted and part of his daily life. However, in the novel, Behn presents slavery as a controversial and central issue towards the downfall of Oroonoko. Is it ethical to enslave another human and own them like a piece of property? What does the novel say about an issue such as slavery? The book, in fact, condemns this type of horrific behavior. She illustrates this issue by showing that slavery is demoralizing and humiliating to the people who are involuntary subjected to this kind of torment. In the beginning of the story, Behn describes the native people of Surinam, a colony in the West Indies, as beautiful, respectable, and friendly. Behn says, â€Å"for those we live with in perfect amity, without daring to command ‘em; but, on the contrary, caress ‘em with all the brotherly and friendly affection in the world. † The natives are very useful to immigrants who came from other areas of the world into Surinam. Behn thought of them as being helpful because she could learn their culture and their everyday style of life. Behn finds it, â€Å"Necessary to caress ‘em as friends, and not to treat ‘em as slaves. † Here, Behn says that treating the people of Surinam as slaves is improper and immoral. She says that friends should not be enslaved because they are helpful and caring. Through this effect, Behn considers slavery as an unethical issue. In the novel, Behn displays slavery as an issue that is associated with misery and torment. This occurs when the prince encounters the English captain who deceitfully places Oroonoko and others in captivity. Oroonoko is portrayed in a state of suffering when he is enslaved, â€Å"We were no sooner arrived but we went up to the plantation to see Caesar; whom we found in a very miserable and unexpressable condition; and I have a thousand times admired how he lived in so much tormenting pain protesting our innocency of the fact, and our abhorrence of such cruelties. † While Oroonoko is at the plantation, his attitude is different from when he was the commander of the army. He is miserable, sad, and in a position with less power than he had before. In conjunction with the misery that is involved with slavery, the actions that coincide with a slave such as whippings are also condemned in the novel, â€Å"No, I would not kill myself, even after a whipping, but will be content to live with that infamy, and be pointed at by every grinning slave. † Behn depicts a slave as someone one who is mentally and physically humiliated. In general, public humiliation is and torment is an unethical issue and Behn’s portrayal of misery, humility, and torment proves that slavery is immoral Oroonoko. In Oroonoko, Behn creates a mindset that favors death over slavery. One situation in the novel where slavery is condemned is when the king threatens to send Imoinda off to another country to be sold as slaves. â€Å"He ought to have had so much value and consideration for a maid of her quality as to have nobly put her to death, and not to have sold her like a common slave; the greatest revenge, and the most disgraceful of any, and to which they a thousand times prefer death. † She states that slavery is the perfect revenge against someone who has deceived another. The king orders, that â€Å"they should be both sold off as slaves to another country, either Christian or heathen, ’twas no matter where. This cruel sentence, worse than death, they implored might be reversed. † After constantly repeating the idea of someone preferring death many times over slavery in the novel, Behn shows the reader that slavery is unethical. In conclusion, Behn shows that enslaving a friend is wrong, and that slavery brings about humiliation, torture, and grief. Behn uses the morbid effects of slavery to the extent when she categorizes slavery as being many times worse than death. Using these techniques, Behn gives an immoral connotation to slavery in Oroonoko.

History and Description of a Subordinate Group Member Essay

Throughout the history of North America, there has been one ethnic group who has given up almost everything to the European settlers. Land, home, resources, and dignity were stolen from Native Americans. The long history of the American Indian is being written, even today. Approximately forty thousand years ago, the earliest ancestors of Native Americans migrated across the Bering Strait from Asia on pack ice (Hoerder, 2005). The population rose steadily, and by the time the first substantial settlement of Europeans was established in the New World, Native Americans lived throughout the continent. In the search for more farmland, European immigrants quickly pushed the native population out of their traditional homelands. This migration began the crowding of other native bands, forcing eastern natives to move beyond the Ohio River, thus starting a series of relocations for the Native Americans that continued through the next two centuries. Less than fifty years after the end of the American Revolution, many of the tribes in the northeastern United States sold their land under pressure from the newcomers. Before 1850, these natives migrated west of the Mississippi River. If you traveled to Oklahoma today you would find the same bloodlines that once roamed the New England hills (â€Å"Indians† The Reader’s Companion to American History, 1991). Wanting to live apart from the natives and expecting them to remain controlled, reservations were established, including an Indian Territory (est. 1825) in present-day Oklahoma. The Indian Removal Act of 1830 was enacted to populate these newly established areas. President Jackson ordered the forced migration of Native Americans from multiple southeastern tribes. Approximately 4,000 Cherokee Indians perished in 1838-1839 on their 800-mile march, or during their succeeding internment. This tragic event has become known as the â€Å"Trail of Tears†. (American Indian Policy, 2002) Trying to â€Å"Americanize† instead of segregate the Indians, in 1887, Congress passed the Dawes Act, which broke up reservations and gave land to individual Indian families. The idea of the Dawes Act was to assimilate Indians by giving them land from which they could profit. What followed were laws, over the next few decades, which dissolved tribal governments and placed Native Americans completely under the jurisdiction of U.  S. laws (American Indian Policy, 2002). The reservation system is one distinctive aspect of the Native American culture that materialized from their relationship with other Americans. The United States has 310 reservations within its borders. The federal government owns 298 reservations and 12 belong to the states in which they are located. A total of 437,431 Indians resided on reservations or trust lands. That is approximately 22 percent of the Native Americans defined by the 1990 census (Shumway & Jackson, 1995). The United States has proven itself unreliable on its policies and treatment of Native Americans. The government teeters between a policy of segregation, under which Indians are treated as a self-sustaining culture, and assimilation policies, which try to integrate Indian and European cultures. The United States acknowledged Indian sovereignty and established treaties with them. Unlike foreign nations, Indians shared the continent with the quickly growing nation who needed resources, and were quick to form treaties, giving Indians land rights and territorial sovereignty but repeatedly found ways to revoke those privileges.

Keeping Lean And Healthy As A Family Essay - 1584 Words

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