Since 1908, Ishiwara taught at the Army School of Artillery and Engineers, and in 1911 received the position of Assistant Professor at the College of Science of Tohoku University.

After returning to his homeland, Ishiwara received a post of professor at Tohoku University, and in 1919 for his scientific work was awarded the Imperial Prize of the Japan Academy.

After retirement, he devoted himself mainly to writing and scientific journalism (in this area he was one of the pioneers in Japan), he authored many popular books and articles on the latest achievements of science.

[5] The two-volume monograph of Ishiwara, titled "Fundamental Problems of Physics", was very popular among young scientists and specialists; he also edited the first complete collection of Einstein's works, published in a Japanese translation in 1922-1924.

[3] In 1909-1911, he studied within the framework of this theory a number of specific problems related to the dynamics of electrons, the propagation of light in moving objects and the calculation of the energy-momentum tensor of the electromagnetic field.

Assuming that the speed of light is variable and rewriting Maxwell's equations accordingly, he showed that such a representation leads to the appearance of additional terms in the energy-momentum conservation law that can be treated as a gravitational contribution.

[2] In the first paper devoted to the problems of quantum physics (1911), Ishiwara derived Planck's law and tried to substantiate the wave properties of radiation on the basis of the assumption that it consists of light quanta.

[4] On April 4, 1915, he presented to the Tokyo Mathematico-Physical Society the article "The universal meaning of the quantum of action" ("Universelle Bedeutung des Wirkungsquantums"), in which he attempted to unite the ideas of Max Planck on elementary cells in phase space, the idea of quantizing the angular momentum in the Bohr model atom and the hypothesis of Arnold Sommerfeld about the change of the action integral in quantum processes.

As a second application of the proposed hypothesis, Ishiwara considered the problem of the photoelectric effect, obtaining a linear relationship between the electron energy and the radiation frequency in accordance with the Einstein formula.

[8][9] Later in the same year Ishiwara put forward another hypothesis, according to which the product of the energy of the atom and the period of electron motion in the stationary state should be equal to the integer number of Planck constants.

The reason for the error of Ishiwara, which was manifested in the calculation of the hydrogen atom, apparently was a superfluous averaging over the number of degrees of freedom (dividing by

At the same time, his quantum condition, which differed from Sommerfeld's one in the presence of summation, allowed to obtain correct results regardless of the choice of coordinates.

This was pointed out in 1917 by Einstein, who, not knowing about the work of Ishiwara, derived the same relation and showed that in the case of separable coordinates it gives the conditions of Wilson and Sommerfeld.