Glossary When I use a word, it means just what I choose it to mean--neither more nor less. Humpty Dumpty, with help from Lewis Carroll, author, 1832­1898 ACE: See Area control error. Alternating current: An electrical current that changes magnitude and direction with time. In power systems, alternating current closely follows a sinusoidal pattern with a frequency of either 50 or 60 Hz. Amp: See Ampere. Ampere (often shortened to `amp'): A measure of the flow of electricity (current) along some path. Ancillary services: Services that may be provided by generating units (and sometimes energy-consuming units) that help maintain the reliability of the power system, distinct from the simple delivery or generation of electric energy over recognized operating periods. Such services include regulation, voltage and reactance support,