ELECTRON THEORY KLOEFFLER 243 



atom has the simplest structure. It consists of a single proton con- 

 stituting the nucleus with a single electron revolving in a surrounding 

 orbit (fig. 2). 



To get a conception of the relative physical size of this hydrogen 

 atom, all parts of the atom may be imagined to be expanded until 

 the nucleus, which is the proton, has the diameter of a baseball and 

 is placed in Dallas, Tex. Then the lone planetary electron will revolve 

 in an orbit which passes through New York City and San Francisco, 

 and the size of this expanded electron will be 300 feet in diameter, or 

 about big enough to rest comfortably in a football stadium or a baseball 

 park. This picture shows the relatively "big hole" or empty space in 

 an atom and explains why some electrons and atomic particles may 

 be projected through many atoms before being stopped. 



In order to explain radiation phenomena, Bohr assumed that there 

 are a number of possible orbits in which the revolving electrons may 

 move and that the jumping of the electron from one orbit to another 

 is due to the absorption or radiation of energy. Consequently, the 

 orbits are often referred to as energy levels. The orbits are pictured 

 as circles or ellipses and usually as a combination of both in order to 

 explain various atomic phenomena. 



At the time when many electrical engineers were in the college 

 classroom, teachers did not attempt to give any physical conception 

 of the phenomenon of electricity or of the units of measurement 

 which were employed. The scientist, the teacher, and the student of 

 former days accepted electrical phenomena as a matter of fact, and, 

 in common with the layman, said, "We do not know anything of the 

 nature of electricity." Today there are some conceptions which 

 have arisen from the discovery and measurements of the electron and 

 from our pictures of atomic structure. Thus, to take the case of the 

 phenomenon of attraction and repulsion of charged bodies, a posi- 

 tively charged body is one in which a large number of electrons have 

 been removed from the body so that the total number of positively 

 charged protons exceeds the number of electrons present. Thus the 

 body has a strong attraction for electrons and it is said to be positively 

 charged. In like manner, a negatively charged body has an excess of 

 electrons above protons and this body exerts a strong attraction for 

 protons, an excess of which may be found in a positively charged 

 body. 



ELECTRIC CURRENTS 



Two bodies oppositely charged have a difference of potential be- 

 tween them. A difference of potential or voltage is measured by the 

 work required to carry a unit positive charge from one body to another 

 against the force of attraction or repulsion. The magnitude of the 

 difference of potential depends upon the concentration of the charge 



114728—39 17 



