J Channel: Metal edging used on drywall to give the edge a better finished appearance when a wall is not “wrapped.” Generally, basement stairway walls have drywall only on the stair side. J Channel is used on the vertical edge of the last drywall sheet.

Jack Hammer:  A hand held rock drill worked by compressed air.

Jack Post: A type of structural support made of metal which can be raised or lowered through a series of pins and a screw to meet the height required. Basically used as a replacement for an old supporting member in a building. See Monopost.

Jack Rafter: A rafter that spans the distance from the wall plate to a hip, or from a valley to a ridge.

Jackleg:  A pneumatic hand held percussive drill designed as a boring tool.

Jackson Turbidity Unit (JTU):  A measurement based on the light path of a suspension which just causes the image of the flame of a standard candle to disappear when viewed through the suspension.

Jamb: The side or top piece of a window or door frame.

Janitrol Furnace Filters:

Jet:  A stream of vapor, gas or liquid coming out fast from a narrow orifice.

Jet Cooling System – A jet pump is used to produce a vacuum so water or refrigerant may evaporate at relatively low temperatures. These systems usually require large condenser and have a low efficiency to remove heat. JIS:  Japanese Industrial Standard.

Jet Velocity:  The average measured velocity of air passing between the fins.

Joint(s):  An interface between elements. Joints may be needed to allow for movement of different parts of a building or assembly, or may be required to make construction sequences practical. In all cases, the functional requirements of the enclosure must be maintained the same as for the body of an enclosure element, although aesthetic requirements may be relaxed. A joint may pass through the entire enclosure assembly, in which case it is a building movement joint, or more commonly referred to as an expansion joint. Control joints are surface cuts or intentional geometric features which control the location of shrinkage cracks and are proprietary requirements. Construction joints are formed between successive building elements parts during construction work.

Joint Cement: A powder that is usually mixed with water and used for joint treatment in gypsum-wallboard finish. Often called “spackle.”

Joint Compound: A material applied to threaded connections to help prevent leaks in plumbing. Also, in carpentry, a wet gypsum material applied to sheetrock joints.

Joint Tenancy: A form of ownership in which the tenants own a property equally. If one dies, the other automatically inherits the entire property.

Joint Trench: When the electric company and telephone company dig one trench and “drop” both of their service lines in.

Joist: A horizontal wood framing member that supports a floor or ceiling.

Joist Hanger: A metal “U” shaped item used to support the end of a floor joist and attached with hardened nails to another bearing joist or beam.

Joule:  A measurement. The international unit of energy. One joule is equal to one WATT - second or 0.737 foot pounds.

Joule Effect:  The conversion of mechanical, electrical or magnetic energy into heat.

Joule Thompson Effect:  When a perfect gas flows through a throttling with constant inlet and outlet pressures, the temperature of the gas is the same before and after it flows through the throttling. There is a temperature drop, however, during the passage of gas through the throttling device itself. At this point internal energy is transformed into kinetic energy with an accompanying temperature drop. However, for real gases there is a sustained change in temperature, even though the energy content of the gas remains constant. This is called the Joule Thompson effect.

Jounce:  The compression of an air actuator (air spring) below its design height.

Journal:  That part of a shaft or axle that rotates or angularly oscillates in or against a bearing or about which a bearing rotates or angularly oscillates.

Journal Bearing:  A sliding type of bearing having either rotating or oscillatory motion and in conjunction with which a journal operates. In a full or sleeve type bearing, the bearing surface is 360 degrees in extent. In a partial bearing, the bearing surface is less than 360 degrees in extent.

Journal Crankshaft:  The part of shaft which contacts the bearing on the large end of the piston rod. 

JTPF:  Japanese tapered pipe thread (Female).

JTPM:  Japanese tapered pipe thread (Male).

Jump Duct:  A flexible, short, U-shaped duct (typically 10-inch diameter) that connects a room to a common space as a pressure balancing mechanism. Jump ducts serve the same function as transfer grilles. Used when return ducts are not located in every room.

Jumpers: Water pipe installed in a water meter pit (before the water meter is installed), or electric wire that is installed in the electric house panel meter socket before the meter is installed. This is sometimes illegal.

Junction Box:  A metal or plastic box within which electrical connections are made.