SCIENCE AND PRACTICE. 



421 



From several determinations with specimens of gutta-percha, 

 kindly given us by Mr. Willoughby Smith, we have found 

 the mean value of s, the specific gravity of this material at 

 4 C. to be 0,9693 .... The value of a-, the specific gravity 

 of telegraph copper may be taken at 8,899 



When the conductor is a strand of seven wires, the value 

 of r, which enters into the formula for calculating the speed, 

 is obtained by taking five per cent, from the value of 1*5 

 times the diameter of a single wire. 



The following table, extracted from a more detailed one by 

 Professor Thomson, who was the first to put it in this form, 

 gives the relative speeds of working in similar lengths of 



cables, with different values of . 



" It is easy to understand/' says Professor Thomson, " why 

 there should be a particular diameter of copper which will 

 give a maximum rate of signalling with a stated outer 

 diameter of gutta-percha, since if the copper wire is too 

 small there is a loss of speed owing to a too large resistance 

 not compensated by the smallness of the electro- static capacity; 

 while on the other hand, if the diameter of the conductor be 



