If electricity is to transmit or convey energy along a wire, this energy must be imparted to the electricity from some external source, that is to say, before electricity can perform any work it must be set in motion, against more or less resistance. This involves that pressure must be applied, and to obtain this pressure, energy must be expended from some external source. Accordingly, in electrical engineering, the first principle to be grasped is that of _energy_. Without the expenditure of energy no useful work can be accomplished. Energy may be defined as _the capacity for performing work_. Although electricity is not energy, electricity under pressure is a form of energy spoken of as electrical energy. In an expenditure of energy in this form, the electricity acts simply as a transmission agent or medium to transmit the energy imparted to it in causing it to flow. In a similar manner, steam acts as a transmission agent or medium to transmit the heat energy of the coal to the steam engine, where it is converted into mechanical energy. As just stated, electricity under pressure is a form of energy, and its generation is simply a transformation of energy from one form into another. Usually, mechanical energy is converted into electrical energy, and a dynamo is employed for effecting the transformation. In transforming the mechanical energy of waterfalls into electric energy, this natural power of water due to its weight and motion is first converted into rotary motion by a turbine or water wheel, and then converted into electric energy by a dynamo, or an alternator. All dynamos are but machines for converting into electric energy the energy which is given to them by some prime mover, as a steam engine, a gas engine, by hydraulic or even by wind power. All electric motors are merely machines for reconverting the electric energy which they receive by means of the conducting wires or mains, into mechanical energy. All electric lamps are contrivances for converting into luminous energy a percentage of the electric energy that is supplied through the mains. =Potential and Kinetic Energy.=--_Potential energy_ is the capacity for performing work which a body possesses _by virtue of its position_. _Kinetic energy_ is the capacity for performing work which a body possesses _by virtue of its motion_. It must be evident that position or motion given to a body enables it to perform work. In the first instance, for example, a heavy weight at the top of a high tower possesses potential energy. A ten pound weight supported one foot above a plane has ten foot pounds of potential energy.