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VOLTMETER 253 VOLUNTEERS addition called II Masteo, was long used as a penitentiary. In the Palazzo Com- munale is the Museo Civico, founded in 1208, and containing a valuable collection of Etruscan antiquities and a library. Volterra has also remains of baths and an amphitheater. Large alabaster works are the main industry of the town. At Saline are important salt works and borax springs. Near Volterra are also several Etruscan burying places and a subterranean labyrinth. The ancient Volaterras (Etruscan Velathras), Vol- terra, was one of the 12 great Etruscan cities. The Romans took it in 474 B. c, and it became an important municipium, resisting a siege by Sulla for two years. Pop. (commune) about 17,500. VOLTMETER, in electricity, any in- strument for measuring the pressure, electromotive force, or difference of po- tentials at the ends of an electric cur- rent. The gold-leaf electroscope is a kind of voltmeter, but will only measure large differences of potential. If the terminals are connected with flat plates arranged parallel to each other, one of which is movable, the attractive force between the plates at a given small dis- tance will be a voltmeter. This method is too coarse for ordinary currents, but a modification of it is employed in Thompson's quadrant electrometer. In Cardew's voltmeter the heating effect of the current in a wire, which varies with the electromotive force, and is measured by the expansion produced, is employed. In the majority of instruments the elec- tro-magnetic action is employed in some form of galvanometer. These are more usually wound to act as ammeters, and if wound with very thin wire the high resistance allows the electromotive force required to drive a certain current through them to be calibrated and de- noted in volts. Such instruments are ad- justed or calibrated by comparison with a "standard" voltaic cell or voltameter. VOLTTJRNO, a river of Italy, rising in the province of Campobasso; flows S. E. to its junction with the Galore, and then W. past Capua into the Medi- terranean, 20 miles S. E. of Gaeta; course, about 100 miles. VOLUME, the space occupied by a body; dimensions in length, breadth, and depth; compass, mass, bulk. In chem- istry, the volume of an irregular body may be found from its weight and specific gravity; that is to say, the weight of a unit volume. If w be the weight of the body in grammes, and s w its specific gravity, the fraction — gives s Cyc its volume in cubic centimeters. The capacities of vessels are determined by filling them with water or mercury from a measuring tube, or other vessel whose capacity is previously known; or, when very great accuracy is required by de- termining the weight of water or mer- cury, at the standard temperature, which fills the vessel. In music, a term applied to the power and quality of the tone of a voice or instrument, or of a combina- tion of sounds. In physics, the volume of a body may be real or apparent; the former is the portion of space actually occupied by the matter of which the body is composed, the latter is the sum of its real volume and the total volume of its pores. The real volume is invariable, the apparent volume can be altered in various ways; for instance, it diminishes as a rule on the solidification of the body. A unit of volume is the volume of the cube constructed on the unit of length. VOLUNTEERS, citizens who of their own accord offer the state their services in a military capacity without the stipu- lation of a substantial reward, and with- out being attached to the regular army. The oldest volunteer force in Great Britain is the Honorable Artillery Com- pany of the city of London, which re- ceived its charter of incorporation from Henry VIII. In 1794, and again in 1803. when the ambition and threats of France agitated England, the govern- ment reckoned on having nearly 500,000 efficient volunteers in arms. The num- ber soon declined, and in 1815 the force almost ceased to exist. About 1857 a feeling of insecurity began to manifest itself in consequence of the alleged in- sufficiency of the national defense, and the Victoria Rifles and one or two other corps were formed. In the course of two or three years many thousands of volunteer riflemen were enrolled throughout the kingdom. Acts of Par- liament relating to the force were passed in 1863, and 1869, and these with the Regulation of the Forces Act of 1871, the Volunteer Act of 1896, and orders in council issued from time to time, con- stitute the law relating to volunteers. There are now also corps of artillery- men, engineers, light horse, and mounted rifles as part of the volunteer force. In connection with the volunteer movement a National Rifle Association was formed in 1860, which inaugurated those great annual meetings held till 1889 at Wim- bleton (since removed to Bisley, near Woking), where many thousands annu- ally compete for prizes. This was fol- lowed by a National Artillery Associa- tion, whose annual meetings are held at Shoeburyness, 17 Vol. X