=64. Electrical Units.= In order to measure electricity for experimental or commercial purposes, standards or units are just as necessary as the inch or foot for measuring distances. =65. Potential; Electromotive Force.= If water in a tall tank be allowed to squirt from two holes, one near the bottom, the other near the top, it is evident that the force of the water that comes from the hole at the bottom will be the greater. The pressure at the bottom is greater than that near the top, because the "head" is greater. When a spark of static electricity jumps a long distance, we say that the charge has a high _potential_; that is, it has a high electrical pressure. Potential, for electricity, means the same as pressure, for water. The greater the potential, or _electromotive force_ (E.M.F.) of a cell, the greater its power to push a current through wires. (See "Study," § 296 to 305, with experiments.) =66. Unit of E.M.F.; the Volt.=--In speaking of water, we say that its pressure is so many pounds to the square inch, or that it has a fall, or head, of so many feet. We speak of a current as having so many volts; for example, we say that a wire is carrying a 110-volt current. The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F. of about one volt. This name was given in honor of Volta. =67. Measurement of Electromotive Force.= There are several ways by which the E.M.F. of a cell, for example, can be measured. It is usually measured _relatively_, by comparison with the E. M. F. of some standard cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by comparison with the two-fluid cell.) [Illustration: Fig. 62.] _Voltmeters_ are instruments by means of which E. M. F. can be read on a printed scale. They are a variety of galvanometer, and are made with coils of such high resistance, compared with the resistance of a cell or dynamo, that the E. M. F. can be read direct. The reason for this will be seen by referring to Ohm's law ("Study," § 356); the resistance is so great that the strength of the current depends entirely upon the E. M. F. [Illustration: Fig. 63.] Voltmeters measure electrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeter with illuminated dial. An electrical bulb behind the instrument furnishes light so that the readings can be easily taken. =68. Electrical Resistance.= Did you ever ride down hill on a hand-sled? How easily the sled glides over the snow! What happens, though, when you strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easily over bare ground or ashes? Snow offers very little _resistance_ to the sled, while ashes offer a great resistance. [Illustration: Fig. 64.]