I know that the answer to the question "What was invented in the 90th year of the 19th century?" is in "In 1824, the French physicist François Arago formulated the existence of rotating magnetic fields, termed Arago's rotations, which, by manually turning switches on and off, Walter Baily demonstrated in 1879 as in effect the first primitive induction motor.  In the 1880s, many inventors were trying to develop workable AC motors because AC's advantages in long-distance high-voltage transmission were counterbalanced by the inability to operate motors on AC. The first alternating-current commutatorless induction motors were independently invented by Galileo Ferraris and Nikola Tesla, a working motor model having been demonstrated by the former in 1885 and by the latter in 1887. In 1888, the Royal Academy of Science of Turin published Ferraris' research detailing the foundations of motor operation while however concluding that "the apparatus based on that principle could not be of any commercial importance as motor." In 1888, Tesla presented his paper A New System for Alternating Current Motors and Transformers to the AIEE that described three patented two-phase four-stator-pole motor types: one with a four-pole rotor forming a non-self-starting reluctance motor, another with a wound rotor forming a self-starting induction motor, and the third a true synchronous motor with separately excited DC supply to rotor winding. One of the patents Tesla filed in 1887, however, also described a shorted-winding-rotor induction motor. George Westinghouse promptly bought Tesla's patents, employed Tesla to develop them, and assigned C. F. Scott to help Tesla, Tesla left for other pursuits in 1889.  The constant speed AC induction motor was found not to be suitable for street cars but Westinghouse engineers successfully adapted it to power a mining operation in Telluride, Colorado in 1891. Steadfast in his promotion of three-phase development, Mikhail Dolivo-Dobrovolsky invented the three-phase cage-rotor induction motor in 1889 and the three-limb transformer in 1890. This type of motor is now used for the vast majority of...". Can you tell me what it is?
Ans: the three-limb transformer

I know that the answer to the question "What is just as useful as formal education?" is in "In 2004, the United States passed an amendment to the Fair Labour Standards Act of 1938. The amendment allows certain children aged 14–18 to work in or outside a business where machinery is used to process wood. The law aims to respect the religious and cultural needs of the Amish community of the United States. The Amish believe that one effective way to educate children is on the job. The new law allows Amish children the ability to work with their families, once they are passed eighth grade in school.". Can you tell me what it is?
Ans: one effective way to educate children is on the job

I know that the answer to the question "What was the pattern of residence for those coming into area of the country known as the City of Brotherly Love?" is in "These immigrants were largely responsible for the first general strike in North America in 1835, in which workers in the city won the ten-hour workday. The city was a destination for thousands of Irish immigrants fleeing the Great Famine in the 1840s; housing for them was developed south of South Street, and was later occupied by succeeding immigrants. They established a network of Catholic churches and schools, and dominated the Catholic clergy for decades. Anti-Irish, anti-Catholic Nativist riots had erupted in Philadelphia in 1844. In the latter half of the century, immigrants from Russia, Eastern Europe and Italy; and African Americans from the southern U.S. settled in the city. Between 1880 and 1930, the African-American population of Philadelphia increased from 31,699 to 219,559. Twentieth-century black newcomers were part of the Great Migration out of the rural South to northern and midwestern industrial cities.". Can you tell me what it is?
Ans: housing for them was developed south of South Street, and was later occupied by succeeding immigrants

I know that the answer to the question "Who has little trouble studying Ashkenazi Jews?" is in "A 2006 study found Ashkenazi Jews to be a clear, homogeneous genetic subgroup. Strikingly, regardless of the place of origin, Ashkenazi Jews can be grouped in the same genetic cohort – that is, regardless of whether an Ashkenazi Jew's ancestors came from Poland, Russia, Hungary, Lithuania, or any other place with a historical Jewish population, they belong to the same ethnic group. The research demonstrates the endogamy of the Jewish population in Europe and lends further credence to the idea of Ashkenazi Jews as an ethnic group. Moreover, though intermarriage among Jews of Ashkenazi descent has become increasingly common, many Haredi Jews, particularly members of Hasidic or Hareidi sects, continue to marry exclusively fellow Ashkenazi Jews. This trend keeps Ashkenazi genes prevalent and also helps researchers further study the genes of Ashkenazi Jews with relative ease. It is noteworthy that these Haredi Jews often have extremely large families.". Can you tell me what it is?
Ans: researchers