Man's physical universe

xanabras

492 MAGNETISM AND ELECTRICITY

phenomena produced in the laboratory. FrankHn devised an ingenious

method of causing an approaching thunderstorm to announce its

presence so that he could try out further kite experiments. He hung

a small metal ball by a silk thread between two small bells. An

approaching storm induced opposite charges on the two bells. The

ball obtained a charge from one bell and was then repelled to the

other bell, which neutralized the charge and gave the ball an opposite

charge which repelled the ball again. The repeated attraction and

repulsion caused the ball to ring the bells as it swung back and forth.

It was Franklin's suggestion that lightning rods with sharp points

be used for the protection of buildings. Franklin also made the first

bifocal glasses, suggested watertight compartments for ships, and

invented the stove still known by his name.

Difference in Potential Is Essentially the Amount of Work Required to

Carry a Unit Charge from One Body to Another.

Tremendous differences of potential may be built up between the

earth and thunderstorm clouds because so much power is required to

pass an electric current through such long distances in air. The unit

of potential difTerence is the volt, often called the statvoU when referring

to static electricity.

A potential difference of about 30,000 volts is required for each

centimeter of distance in order to cause an electric discharge through

air.

There Are Several Important Applications of Static Electricity.

Two important practical applications of static electricity are the

Cottrell Process, described on p. 726, in which charged smoke particles

are attracted to oppositely charged plates in smoke stacks, and a

similar device, the precipitron, which removes a very high percentage

of dust and other suspended matter from the air. The precipitron

first induces a charge on the particles and then precipitates the particles

by attracting them to charged surfaces.

In 1926 the Norton Company discovered that the cutting property

of sandpaper could be improved by 20 to 50 per cent by causing the

sand particles to stand upright by passing them through an electrostatic

field before the adhesive is dried. This company cooperated

with the Arnold Print Works in developing a similar method to produce

simulated embroidered cloth by "printing" an adhesive on cloth

and then causing short cut fibers to cling to the adhesive in an upright

position by means of electrostatic charges.

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