Luminous Effects
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-402
2025-12-13
23
The difference of potential necessary to give a spark 1 cm. long across an air gap between two parallel plates is given by Lord Kelvin as 30,000 volts. If, however, the terminals are separated by air at a reduced pressure, the spark loses its intense brilliancy, and may be increased in length. This condition is secured by sealing platinum wires into the opposite ends of a glass tube (Fig. 1) from which nearly all the air has been removed. When this tube is attached to the secondary terminals of a Ruhmkorff coil, in a dark room, and the current is turned on, the tube will be filled with a band of violet light.
Geissler Tubes (Fig. 2) are glass tubes of various forms, supplied with platinum terminals and filled with different gases at different pressures. When placed in the current from the secondary of an induction coil, they give out many brilliant luminous effects. Commercial application of the light from a vacuum tube is made in the mercury arc lamp, in which the arc is carried by mercury vapor in a state of incandescence.
The spark from an induction coil can be used to show the conductivity of glass. If a piece of small glass tubing is drawn down to an internal diameter of about 1 mm, and then slipped over the terminals of the coil (Fig. 3), a spark can be sent through the tube. This heats the tube, and after a time the spark goes from the terminals to the glass, which conducts the current. If this is continued, the heat finally melts the glass.
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