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The metric standards of length and mass are kept at the International Bureau of Weights and Measures in the Pavilion de Breteuil, Sevres, near Paris. The Bureau is jointly maintained by the principal civilized governments as members of the Metric Convention. The use of metric weights and measures was legalized in the United Kingdom in 1897. LENGTH Unitthe centimetre, 1/100 of the international metre, which is the distance, at the melting-point of ice, between the centres of two lines engraved upon the polished " neutral web " surface of a platinum-iridium bar of a nearly X-shaped section, called the International Prototype Metre. DERIVED C.G.S. UNITS Velocity :-Unit-\ cm. per second. Angular Velocity :-Units-I radian (57°-2C;6) per sec. ; I revolution per sec. Acceleration :-Time rate of alteration of velocity. Unit-(i cm. per sec.) per sec. Angular Acceleration :-Units-I radian per sec. 2 ; i revolution per sec. 2 Momentum :-Mass multiplied by velocity. Uniti gm. cm. sec.-1. Moment of Momentum :-Momentum multiplied by distance from axis of reference. Unit-I cm. 2 gm. sec.-1. Moment of Inertia :-~s ,md?,where m is the mass of any particle of a body, and d its distance from the axis of reference. Unit-i cm. 2 gm. (see p. 16). Angular Momentum :-Moment of inertia multiplied by angular velocity round axis of reference. Unit-I cm. 2 gm. sec.-1. Porce :-Measured by-the acceleration it produces in unit mass. Unitthe dyne = cm. gm./sec. 2 Gravitational unitthe weight of I gram = g dynes. Couple, Torque, Turning Moment :-Force multiplied by distance from point of reference. Uniti dyne cm. Work :-Force multiplied by distance through which point of application of force moves in direction of force. Unitthe erg = I dyne cm.; I joule = lo 7 ergs. [i calorie = 4-18 joules]. Gravitational unitweight of I gm. x i cm. =^dyne cms. = g ergs. Energy :-Measured by the work a body can do by reason of either (i) its motion-Kinetic Energy (= mv 2 /2) or (2) its position-Potential Energy. Unit-the erg. (See "Work.") 1 Board of Trade Unit = 1 kilowatt hour = 3'6 x io 8 watt-sees. Power :-Work per unit time. Unit-1 erg per sec. 1 watt = io T ergs per sec. = 1 joule per sec. = 1 volt-ampere. 1 kilowatt = 1*34 horsepower. Pressure, Stress :-Force per unit area. Unit-1 dyne per cm. 2 1 megabar = io 6 dynes per cm. 2 =; 750 * mm. mercury at 0°C, lat. 45 , and sea-level (g = 980-6). 1 atmosphere = 760 mm. mercury at 0°C, lat. 45°, and sea-level = 759/4 mm. mercury at o°C. in London = J/0132 x io 6 dynes per cm. 2 = 147 lbs. per inch 2 = o-94 ton per foot 2. »^,. t. ... 7T tr • • Correct to I part in 5000. Elasticity :-Ratio of stress to resulting strain. Unit-1 dyne per cm. 2 , since the dimensions of a strain are zero. HEAT UNITS Temperature :-The melting-point of pure ice under 1 atmosphere is defined as o°C, and the boiling-point of water under 1 atmosphere as ioo°C. This fundamental interval is divided into 100 parts by use of the constant-volume hydrogen thermometer (see p. 44) ; each part is a degree Centigrade. Dimensions of temperature ...

Many metals have been found to exhibit evidences of volatility at temperatures considerably below their melting points. As long ago as 1872, Merget demonstrated that frozen mercury volatilised perceptibly in air in course of time. Demarçay, in 1882, conducted similar experiments in vacuo and found that cadmium evaporated sensibly at as low a temperature as 160°, zinc at 184°, and lead and tin at 360° C. In 1887, Zenghelio obtained evidence of the volatility of lead, copper, zinc, etc., even at room temperatures. Spring, in 1894, working at atmospheric pressure, showed that zinc was appreciably volatile at 300°, and cadmium and copper'at 500°. Roberts-Austen and Merrett, in some unpublished experiments at the Royal Mint, in 1896, detected the volatility of cadmium and zinc at 100° in vacuo , while Krafft in 1903 and 1905 investigated in some detail the volatilisation of a number of metals at low pressures. Rosenhain, at the National Physical Laboratory, has obtained beautiful crystals of sublimed zinc by heating a piece of zinc to 300° C. for some weeks in a glass tube containing hydrogen at atmospheric pressure. The notable phenomena of the interdiffusion of metals, with which Roberts-Austen’s name is associated, provide, of course, additional evidence of the vapour pressure that solid metals exert even at ordinary temperatures.

A knowledge of thermal conductivities has now become of importance in a variety of industries; for example, in the designing of electrical machinery and in the lagging of refrigerating and high-temperature plants. While the literature on thermal conductivity is fairly extensive, little attention has hitherto been paid to the design of apparatus for accelerated precision tests. For example, the ingenious methods of experiment devised by Prof. Lees in the course of his classical and extensive investigations, while admirably adapted for the particular object he had in view, would be somewhat laborious if the number of specimens to be studied were considerable. Thermal conductivity is a physical constant not capable of ready determination by simple appliances, and in the design of the apparatus described later our aim has been to produce an equipment which will give the desired data with the minimum expenditure of time and labour.

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