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There was a wave of expulsions of the Jews from Spain, Germany, Austria and Hungary in XV and XVI century. Many of the Jews emigrated to Poland, where they were allowed to settle and prosper, especially during the reign of the Polish kings Zygmunt I (1506–1548) and Zygmunt II August (1548–1572). This period is often referred to as "golden age" for Jews in Poland.
The ancestors of the Epstein family emigrated to Poland either from Frankfurt or Bohemia.3) Josef's father, merchant Maximilian “Max” Epstein was born in 1828 in Lemberg (Lwów, Lviv), and the mother Ernestine Fraenkel was born in 1832 in Brody. Both of these towns (around 100 km apart) were within Poland's territory until 1795 when the area fell under Austro-Hungarian occupation. Maximilian's father Ignaz Epstein was the leading physician of the Jewish Hospital in Lemberg.
Maximilian and Ernestine Epstein moved to Leipzig in Germany.4) They had five surviving children, all born in Leipzig: Laura (born 1858), Wilhelm (1860), Josef (1862), Therese (1864) and Marie (1870). The first child was born in Lwów but died in infancy.
Maximilian was not fully satisfied with the normal German schools in Leipzig and he also secured a private tutor for his children.
Josef never married and had no children, and for many years lived in the same house with his bachelor brother Wilhelm, who only married at the age of 50. However, after the deaths of their sister Therese, her husband Gery Fraenkel and their oldest son, Josef and Wilhelm took care of their children: Marta and Ernst Fraenkel. After Josef death in 1930, because of the rise of Nazism in Germany Ernst Fraenkel emigrated to the UK and then US, and became an internationally prominent political scientist.
In the words of Ernst Loewenthal (Maries' son):5)
My uncles Wilhelm and Joseph were during the first years of my life a common notion, an inseparable pair of twins (although actually Joseph was 2 years younger than Wilhelm). Wilhelm's hair was then dark, already turning grey, while Joseph was still reddish; but otherwise their large broad figures had become very similar during their forties […]
But while Whilhelm's interest was chiefly directed towards topics of a more philosophical or even speculative nature, Joseph was really an exact physicist, always keeping very strictly to the point. I never knew a teacher who understood so well how to explain to the layman even highly complicated physical and technical problems, and although my own understanding for matters of this kind is quite limited, I always could be sure to grasp them if uncle Joseph explained them to me. I think, still more than the extraordinary teaching talent he possessed, it was his ability and readiness to place himself in the position of the other. Uncle Wilhelm would raise you to his ideas - uncle Joseph descended to you.
[Joseph's] most characteristic feature […] was his very distinct sense of fairness and justice both in public and private life. He strongly detested all kinds of political tricks wherever he met them […]
Josef studied sciences with the aim of going to public service. However, the political situation in Germany at the time was already such that as a Jew he had no chance of employment in Saxon civil service.6)
Josef studied physics and mathematics in Wurzburg and Leipzig, and fulfilled the military duty in 1882-1883. He passed the exams for the higher magisterium in December 1885. He studied under the famous philosopher Wilhelm Wundt, who was his supervisor together with Gustav Wiedemann. Epstein completed his doctorate in 1887 at University of Leipzig, with a thesis devoted to measurements of time.7)
He became a lecturer at the University of Frankfurt, where he carried out numerous lectures, courses and guided tours. These additional activities were a special attraction, not available on other courses. For his comprehensive teaching he was recognised by the Prussian Minister of Education.8)
As a preparation for the International Electrical Exhibition in Frankfurt, he organised a series of six popular lectures, which were held in the winter 1890-1891, and appeared in print under the title of “Overview of electrical engineering” (Überblick über die Elektrotechnik) in 1892. The demand for the books was so high, that the second edition was printed in 1894, and third in 1896. The six lectures included the following topics:
The Physical Association (Der Physikalische Verein) founded its Electrical Engineering Institute in 1889 and it became the first electrical engineering school in Germany (and perhaps in the world). Josef Epstein became the first head of the Institute, being just 27 years old.10)
Epstein co-organised courses (for example held during holidays), teaching electromagnetic theory to the students, as well as other lecturers who wanted to educate themselves in electrical engineering.The courses involved theory, practical experiments as well as trips and visits to industrial companies.12) He also organised special courses, like for example regarding the lightning protection principles.13)
Epstein was a member of The Physical Association and donated money regularly to the institution.14)
For 13 years (1897-1910) he left university and worked as a chief engineer for a company Felten & Guilleaume-Lahmeyerwerke Actien-Gesellschaft (which was one of the companies involved in the visits during the holiday courses led by Epstein). The company manufactured various products for the electrical industry, including one- and three-phase transformers, wires and high-voltage cables.15)
Epstein was appointed as “professor” in 1897, while already working for the company.17)
As a result of the work carried out for Felten & Guilleaume-Lahmeyerwerke AG, Epstein created the the apparatus for testing electrical steels - nowadays known as the “Epstein frame”.
After 1910 he returned to the university of Frankfurt and continued as lecturer, but remained a consultant to the industry.
Epstein's surname is recognised internationally because of Epstein frame. The technical details of this device he invented were published in 1900.18) This test apparatus is used for measurement of magnetic properties of electrical steels, and its application is nowadays standardised by international and national standards like IEC 60404-219) and ASTM A34820).
Epstein frame operates as an unloaded transformer, whose magnetic core constitutes the sample of magnetic material to be tested. The power loss of the material is measured (with a wattmeter) as the power consumed during controlled excitation of such magnetic core.
The original device used strips 500 mm long which were overlapped as in transformer (single lapping), and the sample weight exceeded 10 kg. It was later found that double-lapping is more favourable, and the size of the apparatus was reduced to a 250 mm square, with the permissible weight of the sample as little as 0.25 kg.