SUBMARINE TELEGRAPHY 201 



Thomson, published in the ' Proceedings of the Royal Society,' 

 June 15, 1857: 



It has only to be remarked that a submarine telegraph, con- 

 structed with copper wire of the quality of the manufacture A, of 

 only ^\ of an inch in diameter, covered with gutta-percha to a dia- 

 meter of a quarter of an inch, would with the same electrical power, 

 and the same instruments, do more telegraphic work than one con- 

 structed with copper wire of the quality D, of ^ of an inch diameter, 

 covered with gutta-percha to a diameter of a third of an inch, to 

 show how important it is to shareholders in Submarine Telegraph 

 Companies that only the best copper wire should be admitted for 

 their use. 



As soon as it came to be understood that the value of a cable 

 might be enhanced forty per cent, by a judicious selection of 

 the copper employed, tests were adopted which should not only 

 show that the conductor would transmit a current, but also 

 that it was the best conductor which could be procured of the 

 dimensions and material chosen. In other words, the resistance 

 of the conductor was measured. 



Measurement implies comparison with some unit. The 

 resistance of some special piece of wire at a given temperature 

 may be taken as a standard ' one unit,' and the resistance of all 

 other wires or conduccors may be referred to this unit. This 

 comparison was rendered possible by the discoveries of Ohm, 

 published in 1827 ; measurements were made by him and his 

 followers, Lenz and Fechner, in terms of arbitrary units, and 

 Professor Wheatstone in 1843 published an elegant method of 

 making these measurements, and then proposed the adoption of 

 a fixed standard or unit of resistance. When, therefore, it was 

 found desirable to measure the resistance of conductors, the 

 means were not wanting, and were soon very generally adopted. 

 For these measurements ' resistance coils ' are required ; these 

 consist in a graduated series of fine wires of known resistance, 

 which can be combined at will so as to give any multiple of the 

 standard or unit that may be required ; they are arranged in 

 boxes, and fitted with stops, slides, or handles, so that the 

 required additions or subtractions of resistance may be easily 

 made. As early as 1847 or 1848, the Electric and International 

 Telegraph Company in England, and Dr. Siemens in Berlin, 



