TRANSFORMATION OF CURRENT.
The difference in the voltage of the primary and secondary currents depends on the length of the windings. If there are 100 turns of wire in the primary, and 100,000 turns in the secondary, the voltage will be increased 1,000 times; so that a 4-volt current is "stepped up" to 4,000 volts. In the largest induction coils the secondary winding absorbs 200–300 miles of wire, and the spark given may be anything up to four feet in length. Such a spark would pierce a glass plate two inches thick.
It must not be supposed that an induction coil increases the amount of current given off by a battery. It merely increases its pressure at the expense of its volume—stores up its energy, as it were, until there is enough to do what a low-tension flow could not effect. A fair comparison would be to picture the energy of the low-tension current as the momentum of a number of small pebbles thrown in succession at a door, say 100 a minute. If you went on pelting the door 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 Röntgen 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 Röntgen 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.
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 coil is on a line with the needle—in fact, covers it. Next touch the terminals of a battery with the ends of the wire. The needle at once shifts either to right or left, and remains in that position as long as the current flows. If you change the wires over, so reversing the direction of the current, the needle at once points in the other direction. It is to this conduct on the part of a magnetic needle when in a "magnetic field" that we owe the existence of the needle telegraph instrument.