200 APPLIED SCIENCE 



may still be heard to sigh for the good old times when a cabls 

 was good if a needle stood upright, and bad if it leant to one 

 side ; when there were neither complications nor calculations to 

 perplex or mislead any one. 



These simple tests, when applied to long cables, had serious 

 defects. Sir W. Thomson was the first to insist on the import- 

 ance of ascertaining not only that some current would pass 

 through the conductor, but that the greatest possible current 

 did pass which could be expected with a conductor of given 

 dimensions and material. The current which a given battery will 

 produce depends not only on the length and size of the con- 

 ductor, but on the material of which it is composed ; roughly 

 speaking, a given battery will produce a six-fold greater current 

 in a long wire of good copper than it will in an equally long 

 wire of iron of the same diameter. The property of the conductor, 

 determining the amount of current which will pass through it 

 under given constant circumstances, is termed its resistance. 

 The greater the resistance the less the current, and vice versa. 

 Each metal and each alloy has its specific resistance, from which 

 the resistance of any given wire may easily be calculated. It 

 further happens that various specimens of commercial copper 

 differ exceedingly in this electrical property, so that one copper 

 wire will transmit double the current transmitted by a second, in 

 similar circumstances, although to the eye the two wires do not 

 differ. To this fact Sir W. Thomson drew attention in 1857. 

 It might seem of little importance what the resistance of a 

 conductor is, since the current can always be increased by 

 increasing the power of the batteries employed; but Sir W. 

 Thomson pointed out that the rapidity with which a succession 

 of distinct currents, such as are required to produce signals, 

 could be made to follow one another through a long submarine 

 cable, was, cceteris paribus, inversely proportional to the resist- 

 ance of its conductor, so that the commercial value of that 

 cable as a speaking instrument depended on this resistance, 

 which could be diminished only by (at increased cost) increasing 

 the dimensions of the conductor and insulator, or, without any 

 sensible increase of cost, by simply selecting that copper which 

 possessed the smallest specific resistance. This point is clearly 

 explained in the following extract from a paper by Sir W. 



