174 ELECTRICAL MEASUREMENTS 



explanations. A current can be thought of as a stream of electrons mov- 

 ing through a conductor, even if it is unlikely that any one electron 

 travels very far. Materials such as metals, in which certain electrons are 

 rather loosely bound to the atom, make good conductors because the 

 electrons can move rather easily from one atom to another. A variety of 

 other materials in which the electrons have almost no freedom to move 

 conduct very poorly. Intermediate between these two extremes are a 

 group of "semiconducting" materials. 



If electrons are to move primarily in one direction, and not at random, 

 some force must be applied. This electromotive force is a difference in 

 electrical potential, measured in volts. The ability of a material to con- 

 duct electrons is usually expressed by the inverse property, or resistance. 

 The resistance of a wire depends upon its cross-sectional area, its length, 

 and the metal of which it is rfiade. 



Voltage (E), current (I) and resistance (R) are related through Ohm's 

 law, 1 = E/R. If we know any two of these quantities, we can calculate 

 the third. Another quantity, electrical power, is the product of voltage 

 and current (W = IE) and is measured in watts. 



A current of electrons sometimes flows through a conductor in a 

 single direction. In these direct current (d-c) situations. Ohm's law 

 applies without modification. Alternating currents, to the contrary, 

 change direction with regularity and periodicity. The sine curve is 

 derived from the path traced by a point on the circumference of a circle. 

 At any one instant the voltage is positive, negative, or zero; current will 

 flow in a circuit in a forward direction, in the backward direction, or not 

 at all. Over one cycle, the voltage rises from zero to a maximal positive 

 value, falls to zero, then rises to a maximal negative value, and again 

 falls to zero (see Fig. 13-1). Alternating currents can perform work, 

 just as direct currents can, and are characterized by a frequency. 



The alternation of the current introduces some interesting and 



270" 

 Fig. 13-L Relationship of the sine curve to the circle. 



