or for hours you might
make no impression on it, but if you could knead every 100 pebbles into
a single stone, and throw these stones one per minute, you would soon
break the door in.

Any intermittent current can be transformed as regards its intensity.
You may either increase its pressure while decreasing its rate of flow,
or _amperage_; or decrease its pressure and increase its flow. In the
case that we have considered, a continuous battery current is rendered
intermittent by a mechanical contrivance. But if the current comes from
an "alternating" dynamo--that is, is already intermittent--the
contact-breaker is not needed. There will be more to say about
transformation of current in later paragraphs.

USES OF THE INDUCTION COIL.

The induction coil is used--(1.) For passing currents through glass
tubes almost exhausted of air or containing highly rarefied gases. The
luminous effects of these "Geissler" tubes are very beautiful. (2.) For
producing the now famous X or Roentgen rays. These rays accompany the
light rays given off at the negative terminal (cathode) of a vacuum
tube, and are invisible to the eye unless caught on a fluorescent
screen, which reduces their rate of vibration sufficiently for the eye
to be sensitive to them. The Roentgen rays have the peculiar property of
penetrating many substances quite opaque to light, such as metals,
stone, wood, etc., and as a consequence have proved of great use to the
surgeon in localizing or determining the nature of an internal injury.
They also have a deterrent effect upon cancerous growths. (3.) In
wireless telegraphy, to cause powerful electric oscillations in the
ether. (4.) On motor cars, for igniting the cylinder charges. (5.) For
electrical massage of the body.

[11] "What is Electricity?" p. 46.

[12] If a magnetized bar be heated to white heat and tapped with a
hammer it loses its magnetism, because the distance between the
molecules has increased, and the molecules can easily return to their
original positions.

Chapter VI.

THE ELECTRIC TELEGRAPH.

Needle instruments--Influence of current on the magnetic
needle--Method of reversing the current--Sounding
instruments--Telegraphic relays--Recording telegraphs--High-speed
telegraphy.

Take a small pocket compass and wind several turns of fine insulated
wire round the case, over the top and under the bottom. Now lay the
compass on a table, and turn it about until the c