=Operation of a Dynamo.=--A dynamo does not create electricity, but generates or produces an _induced electromotive force_ which causes a current of electricity to flow through a circuit of conductors in much the same way as a force pump causes a current of water to flow in pipes. The electromotive force generated in the dynamo causes the current of electricity to pass from a lower to a higher potential in the machine, and from the higher back to the lower potential in the external circuit; that is, the dynamo generates electrical pressure which overcomes the _resistance_ or opposition to the current flow in the circuit. The pump produces a mechanical pressure which, for instance, may be used to force water into an elevated reservoir against the back pressure due to its weight. [Illustration: FIG. 162.--Holzer Cabot type “M” dynamo. The design of the base is such that it allows the field ring or frame to drop down, lowering the center of gravity, which gives increased stability. The pedestals are bolted directly to the base. Both front and rear pedestals are removable, so that the armature may be taken out from either end without disturbing the brushes or connections. The journals are provided with oil rings which keep the oil in continual circulation around the shaft by means of oil grooves in the journal. The pole pieces are cylindrical in shape and are fitted with shoes which retain the field coils in place and assist commutation. The field coils are former wound, the insulation being reinforced with mica. They are soaked in varnish and baked for 24 hours at 225° Fahr. The armature is wound as desired, series, shunt or compound. The armature core is of the drum type and is laminated, the discs being held by end plates locked without through bolts. The armature coils are formed of round, ribbon or bar copper, and are without joint except at the commutator; they lie in troughs of insulating material, the upper layers being insulated from the lower layers; they are retained in place by maple wedges secured by binding wires, soldered throughout their length. The commutator segments are drop forged in the smaller, and hard drawn in the larger sizes. Radial brushes are used. The efficiency of this type machine is stated by the maker at from 80% to 90%, according to size.] The point to be emphasized is that _the dynamo does not create electricity_ (nor the pump water) _but sets into motion something already existing by generating sufficient pressure to overcome the opposition to its movement_.