ELECTRICAL MEASUREMENTS.

The Edison Meter is shown in Fig. 51. A glass bottle contains two zinc plates, immersed in a standardized solution of sulphate of zinc. The electric current enters the bottle from the terminal of one plate, passes through the solution and goes out through the terminal of the other plate. In so doing it disintegrates from the "losing plate" particies of zinc and deposits on the "gaining plate" their exact equivalent. The amonnt thus transferred is in exact proportion to the amount of current passing through the bottle, according to a well-known physical law. The quantity of current

shunted through the bottle is a definite proportion of the quantity going directly into the house, "resistances" within the meter fixing the exact proportions. The exactness of this proportion is verified before the meter is placed on the system, and can be checked at any time by electrical tests on the consumer's premises.

Each three wire meter contains two pairs of bottles, one pair on the positive, the other on the negative side of the system. The same amount of current passes through each bottle of the pair and by weighing the "gaining plate" in one bottle and the "losing plate" in the other, a double check is obtained.

NOTE.-After a meter is installed on a customer's premises it is then sealed until the next round of the "meter-wagon." The company's representative unseals the meter, removes the bottles, replacing them by others whose plate-weights have been carefully recorded, and brings the four bottles in a four-part box back to the Meter Department. Each meter has its proper number; each bottle-box has an identifying label which bears the record of the bottle-changing and meter-inspection; and the plates within the bottles are also specifically numbered.

ELECTRICAL RESISTANCE.

Measurements of Resistance.—Unlike the frictional resistance of water in pipes, the frictional resistance of the wire is readily computed and measured, making the discussion of the internal action of the electric system much simpler than one of the hydraulic system would be. In fact, we may measure the resistance power in a certain definite unit.

The resistances of wires and circuits are measured in practice by comparing them with certain standard "resistance coils," sets of which are often employed arranged in "resistance boxes;" the particular instruments employed in making the comparison being of two kinds, namely, the differential galvanometers and the Wheatstone's bridge.

These resistance coils require great accuracy in their measurement, in the insulation of the wire and in the mounting of the coils. The wires must be carefully selected and tested. The insulation must be such as will withstand the highest temperature to which it is subjected without change. Silk thread is extensively used for the insulation. The wire is usually wound on spools or in coils so as to occupy as little room as possible, and are mounted in a box, which protects them from injury and places them in convenient form to be carried.

The ends of the coils are connected to plates or binding posts in the cover. This, also, must be carefully constructed so that the resistance at the point of conract will be as low as possible. A single coil is sometimes placed in an ebony case, or any number, according as the work for which it is to be used seems to require. When a large number is placed in one

ELECTRICAL RESISTANCE.

box the ends of the wires are usually connected to metal blocks, placed at such a distance apart that a metal plug will make a good connection between any two. The resistance coils being uniform in size, the entire resistance or any part may be used.

The Mariner's Compass.-The practical interest of magnetism (electricity) applied to navigation need not be enlarged upon. The mariner's compass usually consists of a flat circular card, on the under surface of which are secured four to eight light magnetic needles. The card swings in a compass-box" on a pivot placed at its centre, the box having a pointer corresponding to the direction of the ship's head-the box is supported on gimbols—an arrangement for preserving it horizontal while the ship is pitching and tossing. The card is divided into thirty-two points by a star engraved on it, Fig. 52, and it is by these points the course is steered.

SYMBOLS, ABBREVIATIONS AND
DEFINITIONS.

It is well for the reader to memorize the following ques tions and answers in the same way that the letters of the alphabet or the multiplication are committed to memory, as more or less of them are used upon every page of printed matter relating to electricity.

QUES. What is the meaning of the letters E. M. F.?

ANS. Electro-motive force (abbreviated E. M. F.) is the name given to the force or cause which produces an electric current, see page 45.

The fundamental property of matter is, that it cannot impart motion to itself. The inability of matter to put itself in motion is called inertia-that which causes motion is force and that which causes the electric current is electro-motive force (E. M. F.)

QUES. What are the “powers of ten”?

ANS. This system of notation is sometimes called "Index Notation." This consists in using some power of ten as a multiplier in order to avoid the use of long rows of cyphers.

SYMBOLS, ABBREVIATIONS AND DEFINITIONS.

The following examples will show the convenience of the system. The resistance of selenium is about 40,000,000,000, or 4 X 1010 times as great as that of copper; that of air is about 1026, or

100,000,000,000,000,000,000,000,000

times as great. The velocity of light is about 30,000,000,000 centimetres per second, or 3 X 1010 times.

QUES. What is the C. G. S. system of notation?

ANS. The C. G. S. unit represents the work of a body equal in weight to one gramme, through a space equal to one centimetre in one second. This is the absolute or fundamental unit of electrical work or energy and denominated an erg.

QUES. What is the definition of function?

ANS. This word is derived from a Latin word meaning to perform-hence its general sense is "performance."

NOTE.-The system of units adopted by almost universal consent is the so-called "Centimetre-Gramme-Second" system, in which the fundamental units are:

The Centimetre as a unit of length;

The Gramme as a unit of mass;

The Second as a unit of time.

The Centimetre is equal to 0.3937 inch in length.

432

The Gramme is equal to 15.1000 grains.

The Second is 1-60th of one minute.

All physical quantities, such as electricity, force, velocity, etc., can be expressed in terms of these fundamental quantities: length, mass, and time. Each of these quantities must be measured in terms of its own units.