_Magneto-electricity_ is electricity in the form of currents flowing along wires; it is electricity derived from the motion of magnets—hence the name. _Voltaic Electricity._—This is electricity produced by the action of the voltaic cell or battery. _Electricity itself is the same thing, or phase of energy, by whatever source it is produced_, and the foregoing definitions are given only as a matter of convenience. ELECTRO-MOTIVE FORCE. The term is employed to denote that which moves or tends to move electricity from one place to another. For brevity it is written E. M. F.; it is the result of the difference of potential, and proportional to it. Just as in water pipes, a difference of level produces a pressure, and the pressure produces a flow so soon as the tap is turned on, so difference of potential produces electro-motive force, _and electro-motive force sets up a current so soon as a circuit is completed for the electricity to flow through_. Electro-motive force, therefore, may often be conveniently expressed as a difference of potential, and _vice versa_; but the reader must not forget this distinction. In ordinary acceptance among engineers and practical working electricians, electro-motive force is considered as pressure and it is measured in units called volts. The usual standard for testing and comparison is a special form voltaic cell, called the Clark cell. This is made with great care and composed of pure chemicals. The term _positive_ expresses the condition of the point having the higher electric energy or pressure, and, _negative_, the lower relative condition of the other point, and the current is forced through the circuit by the (E. M. F.) electric pressure at the generator, just as a current of steam is impelled through pipes by the generating pressure at the steam-boiler. Care must be taken not to confuse electro-motive force with electric force or electric energy, when matter is moved by a magnet, we speak rightly of magnetic force; when electricity moves matter, we may speak of electric force. But, E. M. F. is quite a different thing, not “force” at all, for it acts not on matter but on electricity, and tends to move it. THE DYNAMO, OR GENERATOR. The word dynamo, meaning power, is one transferred from the Greek to the English language, hence the primary meaning of the term signifying the electric generator is, the electric power machine. The word generator is derived from a word meaning birth giving, hence also the dynamo is the machine generating or giving birth to electricity. [Illustration: FIG. 217.—See page 251.] Again, the dynamo is a machine driven by power, generally steam or water power, and _converting the mechanical energy expended in driving it, into electrical energy of the current form_. To summarize, the dynamo-electric generator or the dynamo-electric machine, proper, consists of five principal parts, viz: 1. _The armature or revolving portion._