230 



VARIABLE STATE. 



We may then write 



3V 



- \a' 



4y 



The curve A (Fig. 61) represents as a function of the time and 

 taking the final strength as unity, the curve of the current produced 

 at that end of a wire which is in connection with the earth, when 

 a constant potential is established at the other end. 



Til 



\ 



\ 



Fig. 61. 



238. MOMENTARY CONTACTS. In order to obtain the strength 

 corresponding to the case in which the wire is connected with a 

 source of constant potential V , for the time r, it is sufficient, if, as 

 in the case of the insulated wire, we calculate the expression 



or construct geometrically the curve, the ordinate of which is equal 

 at each point to the difference of the ordinates of the two curves 

 F (/)andF (/-T). 



Curves II, III, IV, V, VI, VII (Fig. 61) represent thus the 

 currents arising from contacts whose durations are respectively 

 equal to 20', 30', ..... , 70'. The phenomenon appears as an 

 electrical wave, or a momentary impulse at the end of the wire. 



If the time of contact is infinitely short, the arrival curve of the 

 current / is represented by the equation 



d d du 



