Man's physical universe

xanabras

ELECTROMAGNETISM 507

of this celebration a message was sent from Glasgow to San Francisco

and returned by a. different route, a total distance of 20,000 miles, in

only seven minutes. What a tremendous development in communication

took place in the lifetime of Kelvin ! At the time of his death

there were over 225,000 miles of cables in use in the world.

Electrical Measuring Instruments Employ the Electromagnet.

The galvanometer consists of an electromagnet which is moved

under the influence of a field of a permanent horseshoe magnet when

the current passes through the coil. The coil is attached to the pointer,

which moves over the scale of the instrument. Of course, the coil

could be fixed in position and the permanent magnet moved, and

some instruments are made that way. The relative motion of the

coil and magnet is the result of the repulsion of like magnetic poles for

each other.

A very sensitive galvanometer is employed in the electrocardiograph

to study the character of the heart-beat. It is activated by a minute

electric current generated each time the heart muscles contract.

Galvanometers indicate a flow of current, but they may be so designed

and connected in a circuit as to measure either amperes or volts, in

which case they are called ammeters or voltmeters.

Some important applications of electromagnetism used in generators,

motors, and transformers will be studied in the next Section.

Defective Railway Rails Are Located by the Sperry Detector.

The Detector Car, invented by Elmer A. Sperry, made its first

commercial test on a railwoad in 1928. During the twelve years from

1928 through 1940, over 651,000 miles were tested by this device,

and 345,000 defective rails were located and replaced, thus preventing

an untold number of accidents in these days of higher train speeds

and heavier axle loads. The Sperry Detector generates a current that

passes through the rails over which it rolls. The electromagnetic field

changes as the current encounters a fissure and causes a "paint gun"

to squirt white paint on the defective rail.

STUDY QUESTIONS

1. To whom do accidental discoveries usually come?

2. How are electromagnets used today?

3. Name two discoveries of fundamental importance in putting electricity to

work. Who made these discoveries?

4. How would you construct an electromagnet?

5. How may the polarity of an electromagnet be reversed?

6. Give the principle of the first telegraph system.

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