OF AKTS AND SCIENCES. 



129 



be connected by a pole line or cable, or both, the product of whose 

 total resistance by its total capacity is less than 2,000 if transmitters 

 of the Blake type be used, and less than 4,500 if transmitters of the 

 Hunnings type be used." 



This rule is purely the result of experiment. It applies to a 

 single conductor joining together the two telephones, and is equally 

 applicable whether the return circuit is completed through the earth 

 or by means of a second wire similar and parallel to the first. It 

 supposes the line to be ordinarily free from extraneous noises, and, 

 as in positions especially liable to extraneous noises metallic cir- 

 cuits would naturally be used, it is in all cases a perfectly safe 

 working rule. 



Having thus determined the general rule above annunciated, we 

 must be able to apply it to specific cases ; as, for example, to write a 

 specification for a line which is to connect two cities one hundred 

 miles apart, and which is to pass from the centre of each city under- 

 ground, two miles, to the suburbs. 



In order to do this, we need to know the resistance per mile of 

 various sizes of wire, whether of iron or copper, and excellent tables 

 are published in various text-books giving these figures. I have, for 

 convenience, printed the resistances of various sizes of iron and copper 

 wire in the annexed Tables V. and VI. 



TABLE V. — Iron Wire. 



VOL. XXIII. (N. S. XV.) 



