412 THE ELECTRIC TELEGRAPH. 



Dr. Siemens has applied the laws of heat in conductors to 

 electric induction, expressing the capacity K of an insulated 

 wire, with the help of the formula developed for the resist- 

 ance of cylinders, 



in which I, the inductive capacity of the material, replaces 

 C, the conducting power in the resistance formula, and |3 a 

 constant factor. 



In Mr. de Sauty's method, as well as in that of Mr. 

 Yarley, the quantity K is measured by comparison with the 

 capacity of a jar or condenser of unit surface, or of known 

 value in the capacity of an unit of length of some good 

 cable. 



Dr. Siemens' experiments were made with a galvanometer, 

 the needle of which was deflected by the charge- current 

 entering a cable. He supposes that when the deflection is 

 caused by a current of very short duration, the quantity of 

 electricity K passing through the galvanometer is propor- 

 tional to half the sine of the angle of deflection, which is true 

 with the single needles of tangent galvanometers when the 

 angle is not too great ; or, a being the angle of the thread, 

 and t half the time of a complete oscillation of the needle 

 under the influence of the earth's magnetism, 



1) . . K = t sin. 

 2 



To compare the charges K and K L of two different wires, 

 having obtained, with the aid of a tangent galvanometer, the 

 angles a and a lt we have, therefore, the proportion, 



2) . . : K, : : sin. ~ : sin. -^- 



while the comparison of the charges by calculation with the 

 dimensions, &c., would give 



<0 IT IT' n ^ W L_JL_ 



) . Jv . IY . . . - -r 



UK-! 



log. e - log., - 



