82 THE TRANSATLANTIC LONGITUDE. 



In each cable the conductor is formed by six copper wires twisted around a 

 seventh one. It has a diameter of 0.147 inch or 3.7 millimeters; and weighs 300 

 pounds to the nautical mile, or 73.476 grams to the meter. The copper was 

 guaranteed by the manufacturers to have a chemical purity of 85 per cent., and its 

 specific conducting power (that of pure copper being 100) was found by test to be 

 93.1 for the cable of 1865, and 94.6 for that of 1866. Its specific gravity as deter- 

 mined by Mr. Willoughby Smith was 8.90. 



The electrical tests of the cables, after they were laid and in complete working 

 order, had been made by Mr. Latimer Clark, a short time previous. They gave 

 the following values, expressed in terms of the standard units,^ adopted by the 

 British Association for the Advancement o£ Science, and which promise to become 

 generally accepted, as a peculiarly convenient system of electrical measurement. 



The cable of 1865 gave^ a resistance of 4.01 ohms to the knot; the "insulation,'' 

 or resistance of the coating, being 2945 megohms to the knot, and the electrostatic 

 capacity 0.3535 farad to the knot, or about one farad to each 3^ statute miles. 



» This excellent system of measures is derived from the absolute eleetrodynamic units of Weber, 

 by multiplying them by such powers of 10 as shall refer them to a convenient scale. 



The unit of force /is that force which, acting during 1 mean second upon a mass weighing 1 gram, 

 would impress upon it a velocity of 1 meter in 1 second. It differs from the meter-gram, 

 which is the force requisite for lifting a gram through a meter in a second, and is 9.80868 f. 

 The unit of current c is that current which acting through 1 meter, at 1 meter distance exerts the 

 force /upon a similar current. It decomposes about 92 milligrams of water in each cell in a 

 second, consuming about one-third of a gram of zinc. 

 The unit of resistance r is the resistance of the conductor which transmits the current c in 1 



second. , 



The unit of electromotive force e is the tension which maintains the current c with the resistance r. 

 The unit of quantity q is that amount of electricity which flows in the current c during 1 second. 

 These measures, 'absolute' in so far as they depend only upon the gram, the meter, and the 

 second, are referred to convenient scales in the British Association's system ; the measures adopted 

 being named in honor of eminent discoverers in electrical science, in accordance with a suggestion 

 of Mr. Clark. 



The measure of electromotive force is 10*/, or one hundred thousand times the absolute unit. 



This has about 0.92T the tension of a Daniell's cell, and is called a volt. 

 The measure of resistance is 10^ r, or ten million times the absolute unit. 



This is about 1.0456 times the unit adopted by Siemens, and is called an ohm. 

 The measure of quantity is lO"' q, or the hundred millionth part of the absolute unit. 



This is called a farad. 

 Consequently, with a tension of one volt, and a resistance of one million ohms, the quantity of 

 electricity would be one farad in each second. 



Moreover, since the volt-farad is 10"' /. q, we have 1000 volt-farads = the absolute unit of work; or 

 9808.08 volt-farads per second = the meter-gram. 



One million of ohms is conveniently designated as a megohm; and one million of farads as a 



^ In the manufactory, the resistances found in each knot, at the temperature 75° Fahr. were 4.27 

 and 4.20 ohms, for the two cables respectively ; and the respective insulating capacity of the cover- 

 ings, 349 and 342 millions of ohms to the knot. These data show an increase of conducting power 

 by 6 per cent, for the cable of 1855, and 8 per cent, for that of 1866 ; while the insulation had been 

 increased in the ratios of 8.44 and 7.13. Hence, we may roughly infer the average temperature of 

 the cables to be not far from 5° Centigrade in their ocean bed. 



