RH

RH, Rh, rH, or rh can stand for:

Roentgen (unit)

The roentgen (R, also röntgen) is a legacy unit of measurement for the exposure of X-rays and gamma rays up to several megaelectronvolts. It is a measure of the ionization produced in air by X-rays or gamma radiation and it is used because air ionization can be measured directly. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays. Originating in 1908, this unit has been redefined and renamed over the years. It was last defined by the US National Institute of Standards and Technology (NIST) in 1998 as 2.58×10⁻⁴C/kg, (i.e. 1 C/kg = 3876 R) with a recommendation that the definition be given in every document where the roentgen is used. One roentgen of air kerma (kinetic energy released per unit mass) deposits 0.00877 grays (0.877 rads) of absorbed dose in dry air, or 0.0096 Gy (0.96 rad) in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from 0.01 to 0.04 Gy (1.0 to 4.0 rad) in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition. Even where the reference medium is fully defined, the ionizing energy of the calibration and target mediums are often not precisely known.

Rosh Hashanah (tractate)

Rosh Hashanah (Hebrew: ראש השנה‎) is the name of a text of Jewish law originating in the Mishnah which formed the basis of tractates in both the Babylonian Talmud and the Jerusalem Talmud of the same name. It is the eighth tractate of the order Moed. The text contains the most important rules concerning the calendar year, together with a description of the inauguration of the months, laws on the form and use of the shofar and laws related to the religious services during the Jewish holiday of Rosh Hashanah.

Contents

The Mishnah commences with an account of the four beginnings of the religious and the civil year (1:1); it speaks of the four judgement-days of the pilgrim festivals and Rosh ha-Shanah (1:2); of the six months in which the messengers of the Sanhedrin announce the month (1:3); of the two months, the beginnings of which witnesses announce to the Sanhedrin even on the Sabbath (1:4), and even if the moon is visible to every one (1:5); Gamliel even sent on the Sabbath for forty pairs of witnesses from a distance (1:6); when father and son (who as relatives may otherwise not witness together) behold the new moon they must set out for the beth din (1:7), since they do not absolutely belong to those that are legally unfit for this purpose (1:8).