V//. 1 .VN.U/.s'.S'/OAT OF l'\vr. /,'/>')' ALTERNATING CURRENT 397 



168. Horn Gaps. For high voltages, tho horn gap, Fig. 361, 

 is often used. The gap consists of two horns, each mounted on an 

 insulator, and the gap itself is located between the lower parts 

 of the horns. One horn is connected directly to the line to be 

 protected and the other is connected through a resistance, usually 

 and a choke coil to ground. The gap is so set that ordi- 

 nary operating voltages cannot jump it. When the voltage 

 rises so that it is from 150 to 200 per cent, of its normal value. 

 it jumps the gap and the disturbance passes to ground. The 



linhtiiiii: 



UK* and choke coil limit the current and so prevent the line 



imded by the arc. The function of tin- horns is to 



the arc. An an hud- to rise because of its heat, and also 



because of the well known law that a current tends to form as 



op as possible, in order to make the permeance of the 



Circuit a maximum. (See Vol. I. p. 12, Par. 



Horn gaps are not altogether satisfactory, because they 



often are over unner.->-anly ; the protection which they afford is 



insufficient In nd choke coil, and they do 



always suppress the dynamic arc which follows the 



