Electric generators were known in simple forms from the discovery of the magnetic induction of electric current in the 1830s. In general, some form of prime mover such as an engine or the turbines described above, drives a rotating magnetic field past stationary coils of wire thereby turning mechanical energy into electricity. A very large 2000 Mw(2,682,000 horsepower) unit designed by Siemens was built for unit 3 at the Olkiluoto Nuclear Power Plant. The only commercial scale elctricity production that does not empoly a generator is solar PV.
In the pure experimental design, the independent (predictor) variable is manipulated by the researcher - that is - every participant of the research is chosen randomly from the population, and each participant chosen is assigned randomly to conditions of the independent variable. Only when this is done is it possible to certify with high probability that the reason for the differences in the outcome variables are caused by the different conditions. Therefore, researchers should choose the experimental design over other design types whenever possible. However, the nature of the independent variable does not always allow for manipulation. In those cases, researchers must be aware of not certifying about causal attribution when their design doesn't allow for it. For example, in observational designs, participants are not assigned randomly to conditions, and so if there are differences found in outcome variables between conditions, it is likely that there is something other than the differences between the conditions that causes the differences in outcomes, that is - a third variable. The same goes for studies with correlational design. (Adr & Mellenbergh, 2008).
No childhood home of Poe is still standing, including the Allan family's Moldavia estate. The oldest standing home in Richmond, the Old Stone House, is in use as the Edgar Allan Poe Museum, though Poe never lived there. The collection includes many items that Poe used during his time with the Allan family, and also features several rare first printings of Poe works. 13 West Range is the dorm room that Poe is believed to have used while studying at the University of Virginia in 1826; it is preserved and available for visits. Its upkeep is now overseen by a group of students and staff known as the Raven Society.
The earliest surviving home in which Poe lived is in Baltimore, preserved as the Edgar Allan Poe House and Museum. Poe is believed to have lived in the home at the age of 23 when he first lived with Maria Clemm and Virginia (as well as his grandmother and possibly his brother William Henry Leonard Poe). It is open to the public and is also the home of the Edgar Allan Poe Society. Of the several homes that Poe, his wife Virginia, and his mother-in-law Maria rented in Philadelphia, only the last house has survived. The Spring Garden home, where the author lived in 18431844, is today preserved by the National Park Service as the Edgar Allan Poe National Historic Site. Poe's final home is preserved as the Edgar Allan Poe Cottage in the Bronx.
In Boston, a commemorative plaque on Boylston Street is several blocks away from the actual location of Poe's birth. The house which was his birthplace at 62 Carver Street no longer exists; also, the street has since been renamed "Charles Street South". A "square" at the intersection of Broadway, Fayette, and Carver Streets had once been named in his honor, but it disappeared when the streets were rearranged. In 2009, the intersection of Charles and Boylston Streets (two blocks north of his birthplace) was designated "Edgar Allan Poe Square". In March 2014, fundraising was completed for construction of a permanent memorial sculpture at this location. The winning design by Stefanie Rocknak depicts a life-sized Poe striding against the wind, accompanied by a flying raven; his suitcase lid has fallen open, leaving a "paper trail" of literary works embedded in the sidewalk behind him. The public unveiling on October 5, 2014 was attended by former US poet laureate Robert Pinsky.
Other Poe landmarks include a building in the Upper West Side where Poe temporarily lived when he first moved to New York. A plaque suggests that Poe wrote "The Raven" here. The bar still stands where legend says that Poe was last seen drinking before his death, in Fells Point in Baltimore. The drinking establishment is now known as "The Horse You Came In On", and local lore insists that a ghost whom they call "Edgar" haunts the rooms above.