. Archives of physical medicine and rehabilitation . harged particles, called ele-:-trons. In spite of the attraction ofpositive for negative, the electrons donot fall into the nucleus, probablybecause of their planetary motion aboutit. (The earth for the same reason,does not fall into the sun.) The nu-cleus, which is responsible for practi-cally the whole mass of the atom, has adifferent constitution for each atom;electrons, however, whose mass is about1/1800 of that of a hydrogen atomare identical in all atoms (See Section46 below). 1 he number of electrons,however, increases with the atomic
. Archives of physical medicine and rehabilitation . harged particles, called ele-:-trons. In spite of the attraction ofpositive for negative, the electrons donot fall into the nucleus, probablybecause of their planetary motion aboutit. (The earth for the same reason,does not fall into the sun.) The nu-cleus, which is responsible for practi-cally the whole mass of the atom, has adifferent constitution for each atom;electrons, however, whose mass is about1/1800 of that of a hydrogen atomare identical in all atoms (See Section46 below). 1 he number of electrons,however, increases with the atomicweight of an element. For example,there is much evidence to indicate thatthe hydrogen atom has only I electron,helium 2, lithium 3, and so on until wereach heavier elements such as mercurywith 80 electrons. (It is now generallyaccepted that the number of electronsin an atom is equal to the number ofthe element, when the elements are ar-ranged in the order of increasing atomicweight, the so-called atomic number.) Normally, the negative charge on ihe. r- i 1 P B 159 electrons exactly neutralizes the positivecharge on the nucleus, so that the atomin its ordinary state is electrically neu-tral. If, however, fcjj means of anionizing agent such as a flame, an elec-tron is removed from the parent atom, itshould be evident that Tvhat is left Tvillbe a particle of atomic size, rvith an ex-cess of positive electricil]), or a positiveion. The electron which has been re-moved may either remain free or at-tract to itself one or more neutral mole-cules, or atoms, thus forming what iscalled a negative ion. Later we shall see that one of themost important properties of x-rays istheir ability to ionize a gas. (It should now be evident that, toaccount for the extremely slight conduc-tivity of ordinary air we must assumethe existence in air at all times, of a tewstray ions.) Conductivity of Air at Pres-sures Less than Atmospheric 43. In this course, we are interestedparticularly in the conductivi
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