Man's physical universe



solution, while another metal in solution accepts electrons and goes

out of solution. It is this chemical change that supplies the energy

for the production of the electric current.

A simple electric cell consists of strips of copper and zinc immersed

in sulfuric acid. These two strips of metal are called the electrodes,

while the acid solution, or any other solution that will conduct the

electric current, is called the electrolyte. It has been found that an

electric current may be produced by any of the following combinations

of electrodes and electrolytes provided that there is differential chemical



1. Any two metals

2. One metal

3. One metal

4. Different concentrations of one



in one electrolyte

in two different electrolytes

in two different concentrations of

the same electrolyte

in one electrolyte

In every case there is a chemical reaction in which a stream of electrons

is produced.

About twenty years after the invention of the voltaic cell, a discovery

was made which led to the invention of another generator of

electric current, the electric motor.

Before we take up the discoveries

which led to the electric motor, we must pause briefly to learn a little

about the electric current.

An Electric Current Is Similar to a Flow of Water in a Pipe.

In order that water shall flow in a pipe, the pipe must be of sufficient

Fig. 235.

The difference in water

level is similar to the difference in

electrical potential.

size to carry the desired quantity of water, and there must be a pressure

to maintain the flow of water.

The pressure may be supplied by a

tank filled with water at a higher

level than the water outlet.

If the water outlet is opened, the

water will run out of the tank, and

the flow of water will stop. This corresponds

to the discharge of an electrical

charge through a conductor.

If the water level is maintained

in the tank, then the water will continue

to flow through the pipe at the

same rate. This level may be maintained by a pump, which thus

supplies the energy contained by the flowing water, instead of a

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