1.8 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