63. Binding-Posts are used to make connections between two pieces of apparatus, between two or more wires, between a wire and any apparatus, etc., etc. They allow the wires to be quickly fastened or unfastened to the apparatus. A large part of the apparatus shown in this book has binding-posts attached. Fig. 61 shows a few of the common forms used. (See "Apparatus Book," Chapter V., for home-made binding-posts.)
CHAPTER VI.
UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.
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.)