CHAP, v.] TELEGRAPHIC LINES. 627 



the station is interrupted by this very fusion of the wire, and the 

 electricity of the storm passes to the earth. If the storm has been 

 foreseen, the clerk can put the commutator in connection with the 

 earth wire, and all communication with the station is then cut off. 



Fig. 411 represents another arrangement of the lightning-con- 

 ductor which is also based on the power of points, and on the 

 different behaviour of electricity according as it arises from galvanic 

 currents or is due to an atmospheric disturbance. The commutator 

 is provided with three branches. When the middle one is on the 

 button d, as shown in the figure, the current from the line goes 

 directly to the station, its course being easily seen by following the 

 dotted lines which mark the electrical connections of the various 

 parts of the apparatus. From the end L of the line wire, the current 

 passes through the commutator and thence by r to the station, with- 

 out passing through the wire / at all. In the event of a storm, the 

 middle branch is placed on the button b, and then the current crosses 

 the pointed plates and the wire before reaching the station. And 

 further, if the storm be violent, the commutator is put opposite the 

 letter T, with its middle branch on the button c. Then all the 

 currents pass directly to the earth without any communication 

 whatever with the station, which is thus preserved from all danger. 



Bianchi's lightning-conductor is also founded on the. power of 

 points. When the electricity of a storm comes from the line, it 

 passes away by a series of points arranged upon a glass bowl all 

 round a metallic sphere, which, by a metal ring is in permanent 

 communication with the earth. If the glass bowl is exhausted the 

 passage is quicker, but this precaution is not absolutely necessary. 



The lightning-conductors represented in Figs. 412 and 413 are 

 not based on the power of points, but simply on the inequality of 

 electrical tension between the regular line currents and those of 

 atmospheric or storm cloud electricity. While the first is stopped 

 by an insulating sheet and enters the apparatus, the other crosses to 

 the large conductor offered to it, in spite of the interposition of the 

 insulating body. It can thus easily pass on to the earth without causing 

 any disturbance or damage in the station. 



Siemens's and Halske's lightning- conductor (Fig. 412) is com- 

 posed of a plate of cast iron in communication with the ground ; upon 

 this, and as near as possible to it without being in actual metallic 



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