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filled with metallic sodium. The sodium is a soft solid at room temperature, but at the operating
temperature of the valve, it melts and becomes a liquid that wets the inside surfaces of the hollow valve
stem and head. As the valve opens and closes, the sodium sloshes back and forth, picking up heat from the
head and carrying it into the stem, where it is transferred through the valve guide into the cylinder head and
is subsequently dissipated into the air.
sodium-vapor lamp. A type of lamp used for outdoor lighting, especially along highways. Metallic sodium
is vaporized inside a glass tube that contains a low-pressure gas. A voltage is placed across electrodes
inside the glass tube, and the gas between the electrodes becomes ionized and glows enough to produce an
orange light.
soft-faced hammer. A hammer whose head has one or more soft faces made of plastic, rubber, rawhide, or
a soft metal such as lead or aluminum. Soft-faced hammers are used to pound on surfaces that could be
damaged by hard hammers.
soft magnetic material. Ferromagnetic material that is easy to magnetize and demagnetize. Only a small
coercive (demagnetizing) force is needed to remove any magnetism from the material. Low-carbon steel is
a soft magnetic material.
soft solder. An alloy of tin and lead that melts at a temperature of less than 800°F (427°C). This
temperature is much lower than the melting temperature of the metal on which the solder is used. Soft
solder is normally available in the form of a hollow wire filled with a resin flux. Soft solder is melted with
a hot soldering copper, called a soldering iron, and the liquid solder wets the surface on which it is melted.
When the soldering iron is removed, the solder solidifies and forms a bond between the pieces of metal that
have been wetted.
Soft solder does not form a physically strong bond, but is used on sheet metal, over a locked seam
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 538
to make the joint liquid-proof, or on wires after they have been twisted together to make a connection
having a low electrical resistance.
soft vacuum tube. A vacuum tube that is faulty because air has leaked into the tube envelope.
software (computer operation). The package of instructions or programs used to control a computer.
Software consists of written instructions that can be easily changed by a computer programmer. The
electronic components, switches, and wiring used in computer operation are called hardware.
soft X-rays. Low-powered X-rays that have comparatively long wavelengths. Soft X-rays do not penetrate
a material nearly as deeply as the high-powered, hard X-rays.
soil cement. A material used as a base for roads and airport runways. Soil cement is made by mixing
portland cement with soil and water and then compacting the mixture.
solar cell. A silicon semiconductor device that converts light energy directly into electrical energy. These
devices are called solar cells because they are designed to convert energy from the sun directly into
electricity.
solar radiation. Electromagnetic radiation emitted by the sun.
solder. See soft solder and silver solder.
soldered splice (electrical wiring connection). A type of splice used for electrical wires in which the ends
of the wires to be joined are twisted together to give them mechanical strength. The joint is then covered
with soft solder to decrease its electrical resistance. Soldered splices are not generally recommended for use
in aircraft electrical systems.
soldering. A method of thermally joining metal parts with a molten nonferrous alloy that melts at a
temperature below 800°F (427°C). The molten alloy is pulled up between close-fitting parts by capillary
action. When the alloy cools and hardens, it forms a strong, leak-proof connection.
soldering iron. A hand-held tool used to melt solder. Modern soldering irons are electrically heated and
use a tinned copper tip (a copper tip covered with solder) to melt the solder.
soldering pistol. A special type of hand-held tool used to melt solder. The heating element is a transformer
whose secondary winding is a single turn that extends out of the transformer in the form of two connections
for the ends of a copper tip.
When the solder is ready to be melted, a trigger switch in the primary winding is pulled. When
primary current flows, a low voltage but high current is induced into the single-turn secondary. This current
flows through the copper tip and heats it enough to melt the solder. As soon as the soldered joint is made,
the trigger is released, and the tip cools off.
solderless connector (electrical wires). A crimped-on connector used to fasten a terminal to an electrical
　
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