H

half cell - a probe that will generate a voltage proportional to the hydrogen content in a solution.

half duplex - see HDX

handshake - electrical lines used to establish and control communications.

hard copy - a paper based printout.

hardware - a mechanical or electrical system. The ‘functionality’ is ‘frozen’ in hardware, and often difficult to change.

HDLC (High-level Data Link Control) - an ISO standard for communications.

HDX (Half Duplex) - a two way serial connection between two computer. Unlike FDX, characters that are sent are not reflected back to the sender.

head - pressure in a liquid that is the result of gravity.

hermetic seal - an airtight seal.

hertz - a measure of frequency in cycles per second. The unit is Hz.

hex - see hexadecimal.

hexadecimal - a base 16 number system where the digits are 0 to 9 then A to F, to give a total of 16 digits. This is commonly used when providing numbers to computers.

high - another term used to describe a Boolean true, logical positive, or one.

high level language - a language that uses very powerful commands to increase programming productivity. These days almost all applications use some form of high level language (i.e., basic, fortran, pascal, C, C++, etc.).

horsepower - a unit for measuring power

host - a networked (fully functional) computer.

hot backup - a system on-line that can quickly replace a failed system.

hydraulic - 1. a study of water 2. systems that use fluids to transmit power.

hydrocarbon - a class of molecules that contain carbon and hydrogen. Examples are propane, octane.

hysteresis - a sticking or lagging phenomenon that occurs in many systems. For example, in magnetic systems this is a small amount of magnetic repolarization in a reversing field, and in friction this is an effect based on coulomb friction that reverses sticking force.

Hz - see hertz

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