The U tube is 3/4-inch diameter, and 8 inches long. Starting with it empty the tap A is opened, and the whole U tube filled with zinc sulphate solution, and the tap A is closed. The zinc rod usually kept in the tube L is put in place, tightly corking up its end of the U tube.

The c.o.c.k C is opened, which lowers the level of the solution in the right-hand limb of the U tube only. The tap B is opened and the copper sulphate solution is run in, preserving the line of separation of the two solutions. The copper rod is taken out of its tube M, and is put in place. India rubber corks are used for both rods. As the liquids begin to mix the mixture can be drawn off at C and the sharp line of demarcation re-established. In Dr. Sloane's standard cell two test tubes are employed for the solutions and a syphon is used to connect them.

Oxidation of the zinc lowers the E. M. F.; oxidation of the copper raises it. With solutions of equal sp. gr. the E. M. F. is 1.104 volts.

If the copper sulphate solution is 1.100 sp. gr. and the zinc sulphate solution 1.400 sp. gr., both at 15? C. (59?F.), the E. M. F. will be 1.074 volt. Clean pure zinc and freshly electrolyzed copper should be used.

Fig. 79 STANDARD DANIELL CELL--FLEMING'S FORM.

110 STANDARD ELECTRICAL DICTIONARY.

Cell, Standard Voltaic, Latimer Clark's.

A mercury and zinc electrode couple with mercurous sulphate as excitant and depolarizer. The positive element is an amalgam of zinc, the negative is pure mercury. Each element, in a representative form, the H form, is contained in a separate vessel which communicate by a tube.

Over the pure mercury some mercurous sulphate is placed. Both vessels are filled to above the level of the connecting tube with zinc sulphate solution, and kept saturated. It is tightly closed or corked. The E. M.

F. at 15? C (59? F.) is 1.438.

Temperature correction

(1 - (.00077 *(t - 15? C) ) )

t being expressed in degrees centigrade (Rayleigh). A diminution in specific gravity of the zinc solution increases the E. M. F. The cell polarizes rapidly and the temperature coefficient is considered too high.

Fig. 80. LATIMER CLARK'S STANDARD CELL.

111 STANDARD ELECTRICAL DICTIONARY.

Cements, Electrical.

A few cements find their use in electrical work. Marine glue, Chatterton's compound, and sealing wax may be cited.

Centi-.

Employed as a prefix to indicate one-hundredth, as centimeter, the one-hundredth of a meter; centi-ampere, the one-hundredth of an ampere.

Centigrade-scale.

A thermometer scale in use by scientists of all countries and in general use in many. The temperature of melting ice is 0?; the temperature of condensing steam is 100? ; the degrees are all of equal length. To reduce to Fahrenheit degrees multiply by 9 and divide by 5, and add 32 algebraically, treating all readings below 0? as minus quant.i.ties. For its relations to the Reamur scale, see Reamur Scale. Its abbreviation is C., as 10? C., meaning ten degrees centigrade.

Centimeter.

A metric system unit of length; one-hundredth of a meter; 0.3937 inch.

The absolute or c. g. s. unit of length.

Centimeter-gram-second System.

The accepted fundamental or absolute system of units, called the C. G.

S. system. It embraces units of size, weight, time, in mechanics, physics, electricity and other branches. It is also called the absolute system of units. It admits of the formation of new units as required by increased scope or cla.s.sification. The following are basic units of the system :

Of length, centimeter; of ma.s.s, gram; of time, second: of force, dyne: of work or energy, erg.

See Dyne, Erg., and other units in general.

112 STANDARD ELECTRICAL DICTIONARY.

Central Station Distribution or Supply.

The system of supplying electric energy in current form from a main generating plant to a district of a number of houses, factories, etc. It is in contrast with the isolated plant system in which each house or factory has its own separate generating installment, batteries or dynamos.

Centre of Gravity.

A point so situated with respect to any particular body, that the resultant of the parallel attracting forces between the earth and the several molecules of the body always pa.s.ses through it. These are resultants of the relative moments of the molecules. If a body is suspended, as by a string, the centre of gravity always lies vertically under its point of suspension. By two trials the point of intersection of plumb lines from the point of suspension being determined the centre of gravity is known. The vertical from the point of support coincides with the line of direction.

Centre of Gyration.

The centre of gyration with respect to the axis of a rotating body is a point at which if the entire ma.s.s of the body were concentrated its moment of inertia would remain unchanged. The distance of this point from the axis is the radius of gyration.

Centre of Oscillation.

The point referred to in a body, suspended or mounted to swing like a pendulum, at which if all the ma.s.s were concentrated, 1t would complete its oscillations in the same time. The distance from the axis of support to this point gives the virtual length of the pendulum which the body represents.

Centre of Percussion.

The point in a suspended body, one free to swing like a pendulum, at which an impulse may be applied, perpendicular to the plane through the axis of the body and through the axis of support without shock to the axis. It is identical with the centre of oscillation, q. v., when such lies within the body.

Centrifugal Force.

The force which draws a body constrained to move in a curved path away from the centre of rotation. It is really due to a tangential impulse and by some physicists is called the centrifugal component of tangential velocity. It has to be provided against in generator and motor armatures, by winding them with wire or bands to prevent the coils of wire from spreading or leaving their bed upon the core.

113 STANDARD ELECTRICAL DICTIONARY.

Centrifugal Governor.

The usual type of steam-engine governor. The motion of the engine rotates a system of weights, which are forced outward by centrifugal force, and are drawn inwards by gravity or by springs. Moving outwards they shut off steam, and moving inwards they admit it, thus keeping the engine at approximately a constant speed. The connections between them and the steam supply and the general construction vary widely in different governors.

C. G. S.

Abbreviation or symbol for Centimeter-gram-second, as the C. G. S.

system. (See Centimeter-gram-second System.) It is sometimes expressed in capitals, as above, and sometimes in small letters, as the c. g. s.

unit of resistance.

Chamber of Incandescent Lamp.

The interior of the bulb of an incandescent lamp. (See Lamp, Incandescent.)