244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1938 



and not on the amount of the charge. Thus an excess of a billion 

 electrons on a sphere 10 centimeters in diameter would have only one- 

 tenth as much potential as it would have if concentrated on a sphere 

 1 centimeter in diameter. The 110 volts at the ends of a lighting 

 circuit are due to an enormous concentration of electrons along one 

 wire and a corresponding deficiency in the other. When a human 

 being contacts this circuit, the excess of electrons in one wire attempts 

 to pass through the body to the other wire with a result well under- 

 stood as far as one's feeling is concerned. 



If a positively charged and a negatively charged body are brought 

 in contact, electrons from the body with negative charge (excess of 

 electrons) will move over to the body having the positive charge 

 (deficiency of electrons) until an equilibrium of charge has taken 

 place. This transfer of electrons from one body to another consti- 

 tutes an electric current. Thus we can picture the phenomenon of 

 electric current as a transfer or coordinated movement of electrons 

 along a path or circuit. The protons do not move in solids because 

 their mass is 1,834 times as great as the electrons, and because of 

 their firm attachment to the atom itself. The magnitude of an 

 electric current is determined by the number of electrons which pass 

 a given point in a circuit per unit of time. To produce an electric 

 current of one ampere, 6.3 X10 18 electrons must pass a point in one 

 second. The individual electrons may move along the circuit slowly 

 or quickly. Thus in a high-voltage vacuum tube, the electrons may 

 move with a velocity approaching that of light, whereas in another 

 part of the circuit they may drift along at a "snail's pace" of only a 

 fraction of a centimeter per second. 



The direction of the movement of electrons in an electric circuit is 

 opposite to the conventional direction of current as adopted by 

 scientists many years ago. Thus the early choice of direction of 

 current was unfortunate in that it complicates conceptions and 

 explanations in electronic devices. All remarks in this discussion 

 will pertain to the direction of electron movement. 



CURRENT FLOW IN SOLIDS AND LIQUIDS 



An electric current in a solid is due to the movement of "free" 

 electrons along that solid. The "free" electrons are not "extras" or 

 those electrons above the normal number to balance the protons, but 

 rather electrons which at certain instants are free from their parent 

 atom to be moved on to another atom in a sort of relay race. Although 

 the molecules are close together within a metallic solid, they do have 

 a movement due to thermal kinetic energy and tins movement increases 

 with the temperature. The electrons, in turn, have a movement 

 about their nuclei and at certain positions, due to the molecular 

 action and the electron action, the electrons may become as close to 



