OF ARTS AND SCIENCES. 3 



actual telephone currents. The values might perhaps be called the 

 " telephonic specific inductive capacity," leaving the expression " tele- 

 graphic specific inductive capacity " to indicate values measured in the 

 old-fashioned way. 



Table op Specific Inductive Capacities, measured by Telephone 



curkents. 



Petroleum (Brooks Cable) 1.6 



Solid paraffine 2.0 



Cotton saturated with paraffine in vacuum (Faraday Cable) . . 2.0 



Cotton boiled in paraffine (Patterson Cable) 2.6 



India-rubber 3.7 



Artificial gutta-percha (Gwynn) 3.9 



Gutta-percha 4.2 



Glass 4.6 



Water 6.3 



An inspection of this table shows that, so far as capacity is con- 

 cerned, petroleum is the best substance to be used, and doubtless this 

 would be the case were it possible to keep it free from water ; but 

 water, we see from the table, has a specific inductive capacity of 6.3, 

 80 that its presence in the petroleum raises the capacity from the 

 lowest to the highest in the list. 



This observation is borne out by actual experience with " Brooks " 

 cables in telephony. When new, and the petroleum dry, it works 

 excellently ; but as water finds its way in, the cable rapidly loses its 

 efficiency for telephonic work. 



This action of the water is quite different from its action as a con- 

 ductor to produce leakage, for the loss of electricity due to leakage in 

 a Brooks cable that has lost its efficiency from the presence of moist- 

 ure is entirely insufficient to account for the deterioration. 



Next to petroleum, solid paraflSne is seen to be the best substance 

 to use ; but on account of mechanical difficulties it has never been 

 found practicable to coat wires directly with solid paraffine. 



If the wires are wound with cotton and then boiled in paraffine, as 

 they are in making "Patterson" cables, the specific inductive ca- 

 pacity is raised to 2.6, an increase of 30%, which we have seen is a 

 very great detriment. 



If, however, the wires are wound with cotton, and the air and 

 moisture removed by the aid of heat and a vacuum, and they are 

 then boiled in paraffine, from which the air and moisture have also 

 been removed by heat and vacuum, the specific inductive capacity 



