284 



THE ELECTRIC TELEGRAPH. 



From these results Messrs. Bright and Clark find that the 

 curve between the resistance and temperature is a logarithmic 

 one, and have arrived at the empirical formula, 



E^ = R, (0,8944) <1- ; 



expressing the resistance R^ , at a temperature ^, as equal 

 to the product of the resistance R^ at some lower tempera- 

 ture, t, and the (^ t) power of a constant hase, 0,8944. 

 That this empirical formula is only an approximation to 



Tumperatures. 

 Fig. 132. 



the true expression is obvious from the curves shown 

 graphically in Fig. 132, the black line representing the 

 values obtained from the foregoing table, and the dotted line 

 those given by the above formula. The agreement is never- 

 theless sufficient to enable us to calculate, by means of the 

 given formula, the resistance of a gutta-percha covered wire 

 for any temperature between 0C. and 38 0., with very slight 

 error, from the resistance at any given temperature. Hence 

 it becomes needless to test the cores of cables at the gutta- 

 percha works at an uniform temperature of 24 C., as has been 

 customary with cables hitherto made, since the resistances 

 may be reduced to a standard temperature, and thus save the 

 expense and trouble of keeping up warming tanks. 



The difference of temperature, in combination with the 



