628 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK v. 



contact,, rest two smaller plates, A, B, connected on one side with the 

 line wires, L, L 2 , and on the other with the apparatus F, F 2 . The 

 galvanic currents have not sufficient intensity to overcome the resist- 

 ance arising from the distance of the conductors from the plate in 

 connection with the earth, but in the case of a storm, the atmospheric 

 electricity takes this latter path and the instruments are preserved. 



The lightning-conductor on the Belgian lines (Fig. 413) also 

 consists of metallic plates, pp, df, separated by an insulating sheet of 

 thin paper. The line wires of the two neighbouring stations on the 

 right and left terminate in L and L', and those of the apparatus of the 

 station itself in F and F' ; T communicates with the button I and with 

 the earth. Four holes, 1, 2, 3, 4, in the socket of wood which carries 

 the plates are made to receive a metallic plug, c, which puts the 



FIG. 412. Siemens' s and Halske's lightning- 

 conductor. 



FIG. 413. Lightning-conductor on the Belgian 

 lines. 



various parts of the lightning-conductor in connection with each 

 other. In ordinary times the plug is in the position marked in the 

 figure, and then the neighbouring stations can communicate with the 

 apparatus in this one, and the correspondence on both sides is free. 

 If a storm appears on the right, the plug is placed in the hole No. 2, 

 and the atmospheric electricity passes to the earth by the plate and 

 wire T. If the disturbance occurs on the left, the plug is placed in 

 hole No. 1 ; and lastly, it is placed in No. 4, if both sides are 

 threatened at the same time. The hole No. 3, serves to establish 

 direct communication between the two lines, so that the apparatus 

 is at the same time a commutator as well as a lightning-conductor. 



