In addition to the types already described (see _Ammeter_), there is a class depending on the mutual action of current-carrying conductors placed near one another. This type is largely used in alternating-current work. It is also specially suitable for power measurements, and practically all _wattmeters_ work on this principle. The Siemens Dynamometer was the first instrument of this type.
The Kelvin Standard Balance is a special form of dynamometer, in which the mechanical turning-moment due to weights on a beam is balanced by the electrical turning-moment due to currents in fixed coils and in coils attached to the ends of the beam. The electrostatic voltmeter mentioned above is the only instrument which is operated by a P.D. instead of a current. In it a moving vane is attracted into a fixed pair of quadrants; or a set of vanes is attracted into a set of quadrant cells.
The majority of electricity meters are of the motor type, i.e. a disc is driven by motor action at a speed which is proportional to the power passing through the meter. The disc spindle engages with gearing which drives the pointers on a set of dials recording the energy units. There are also meters depending on electrolytic action (Wright meter); or on the difference in period between two pendulums, one of which is controlled by the load current (Aron meter).
ELECTRIC BATTERY, a group of primary or secondary cells, suitably arranged for the purpose of producing an electric current. Primary batteries consisting of a few cells are commonly used for intermittent work where a relatively small current is required, e.g. for electric bells. If a larger current is necessary, especially if it has to be maintained over a considerable period, a battery of secondary cells is used. Such batteries are commonly used for country house lighting. Very large batteries, used either alone or in conjunction with automatic reversible boosters, are frequently employed in public electric supply systems.
The name _electric battery_ was originally given to an arrangement of Leyden jars (see _Leyden Jar_), but is now applied only to cells, the Leyden-jar arrangement being called a Leyden-jar battery.
ELECTRICITY, the name given to the ultimate cause of electrical phenomena. The laws governing these phenomena are well known, but the actual nature of electricity has not yet been fully revealed, although much light has been thrown on the subject by recent researches. (See _Electron_.) Although the practical applications of electrical phenomena have all been developed within the last fifty years, the production of an electric charge by friction, as demonstrated by the power of rubbed amber to attract light bodies, was observed by a Greek philosopher as long ago as 600 B.C. The Greek name for amber, [Greek: êlektron] (electron), is the root from which our word electricity is derived. Friction was the only artificial source of electricity known until Galvani, near the close of the eighteenth century, accidently obtained it by the contact of two metals with the limbs of a frog; and Volta, developing Galvani's discovery, invented the first galvanic or voltaic battery.