We have already seen that the action of the dynamo is reversible, and that just as a wire moved across a magnetic field supplies an electric current, so a wire at rest, but conducting a current across a magnetic field, will move.  The electric motor is therefore essentially a dynamo, which on being traversed by an electric current from an external source puts itself in motion.  Thus, if a current be sent through the armature of the Gramme machine, shown in figure 41, the armature will revolve, and the spindle, by means of a belt on the pulley P, can communicate its energy to another machine.

Hence the electric motor can be employed to work lathes, hoists, lifts, drive the screws of boats or the wheels of carriages, and for many other purposes.  There are numerous types of electric motor as of the dynamo in use, but they are all modifications of the simple continuous or alternating current dynamo.

Obviously, since mechanical power can be converted into electricity by the dynamo, and reconverted into mechanical power by the motor, it is sufficient to connect a dynamo and motor together by insulated wire in order to transmit mechanical power, whether it be derived from wind, water, or fuel, to any reasonable distance.

CHAPTER V.

Electrolysis.

Having seen how electricity can be generated and stored in considerable quantity, let us now turn to its practical uses.  Of these by far the most important are based on its property of developing light and heat as in the electric spark, chemical action as m the voltameter, and magnetism as in the electromagnet.

The words “current,” “pressure,” and so on point to a certain analogy between electricity and water, which helps the imagination to figure what can neither be seen nor handled, though it must not be traced too far.  ’Water, for example, runs by the force of gravity from a place of higher to a place of lower level.  The pressure of the stream is greater the more the difference of level or “head of water” The strength of the current or quantity of water flowing per second is greater the higher the pressure, and the less the resistance of its channel.  The power of the water or its rate of doing mechanical work is greater the higher the pressure and the stronger the current.  So, too, electricity flows by the electromotive force from a place of higher to a place of lower electric level or potential.  The electric pressure is greater the more the difference of potential or electromotive force.  The strength of the electric current or quantity of electricity flowing per second is greater the higher the pressure or electromotive force and the less the resistance of the circuit The power of the electricity or its rate of doing work is greater the higher the electromotive force and the stronger the current.

It follows that a small quantity of water or electricity at a high pressure will give us the same amount of energy as a large quantity at a low pressure, and our choice of one or the other will depend on the purpose we have in view.  As a rule, however, a large current at a comparatively low or moderate pressure is found the more convenient in practice.