The  ohm  is  named  for  Georg  Simon  Ohm,  a German  scientist  and  early  electrical  pioneer,  who discovered  that  there  is  a  constant  relationship between the electromotive force (E), the current (I),  and  the  resistance  (R)  in  any  electrical  circuit. This relationship is expressed in “Ohm’s law” as follows: From  the  basic  law  it  follows  that From  Ohm’s  law  you  can  (1)  determine  any one of the three values when you know the other two   and   (2)   determine   what   happens   in   the circuit when a value is varied. Suppose, for example, that the resistance (R) is  increased,  while  the  electromotive  force  (E) remains the same. It is obvious that the current (I)  must  drop  proportionately.  To  avoid  a  drop in the current, it would be necessary to increase the  electromotive  force  proportionately. When an electrical circuit is open (that is, when there  is  a  break  in  the  circuit,  such  as  an  open switch), there is no flow of electrons through the circuit.  When  the  circuit  is  closed,  however,  the current  will  begin  to  flow.  With  a  constant electromotive  force  (E),  the  rate  at  which  the current  (1)  flows  will  depend  on  the  size  of  the resistance  (R).  The  size  of  the  resistance  will increase  with  the  number  of  electrical  devices (such  as  lights,  motors,  and  the  like)  that  are placed on the circuit, and the amount of POWER each  of  these  consumes. Power may be defined as “electrical work per unit of time. ” James Watt, another early pioneer in the electrical field, discovered that there is a constant  relationship  between  the  electromotive force   (E),   the   current   (I),   and   the   power consumption (P) in a circuit. This relationship is expressed  in  the  formula  P  =  IE,  from  which  it follows  that Power is measured in units called WATTS, a watt being defined as the work done in 1 second when  1  ampere  flows  under  an  electromotive  force of  1  volt. Suppose,   now,   that   you   have   a   110-volt circuit  in  your  home.  The  constant  E  of  this circuit, then, is 110 volts. In the circuit there is probably a 15-ampere fuse. A fuse is a device that will  open  the  circuit  by  “burning  out”  if  the current  in  the  circuit  exceeds  15  amperes.  The reason for the existence of the fuse is the fact that the  wiring  in  the  circuit  is  designed  to  stand safely  a  maximum  current  of  15  amperes.  A current in excess of this amount would cause the wiring  to  become  red  hot,  eventually  to  “burn out, ”  and perhaps to start an electrical fire. Suppose  you  light  a  60-watt  bulb  on  this circuit.  Your  E  is  110  volts.  By  the  formula you know that the current in the circuit with the 60-watt  bulb  on  is or about 0.54 amperes, which is well within the margin  of  safety  of  15  amperes.  Dividing  15 amperes by 0.54 amperes you find that this fuse will  protect  a  27-lamp  circuit. But suppose now that you place on the same one-lamp circuit an electric toaster taking about 1,500  watts  (electrical  devices  are  usually  marked with the number of watts they consume) and an electrical clothes dryer taking about 1,200 watts. The total P is now 60 + 1,500 + 1,200, or 2,760 watts.  The  current  will  now  be 2,760 110 or 25 amperes. Theoretically, before it reaches this point, the 15-ampere fuse will burn out and open the  circuit. Mechanical Power Measure Mechanical   power   (such   as   that   supplied by  a  bulldozer)  is  measured  in  units  called FOOT-POUNDS  PER  SECOND  (ft-lb/sec)  or FOOT-POUND   PER   MINUTE   (ft-lb/min).   A foot-pound  is  the  amount  of  energy  required  to raise 1 lb a distance of 1 ft against the force of gravity. One  HORSEPOWER  equals  33,000  ft-lb/sec or  550  ft-lb/min.  One  horsepower  equals  about 746 watts. 1-33