208 HISTORY AND PROGRESS. 



however, insulated from each other and from the coupling 

 screws by the latter being contained in cylinders of ivory, 

 and by the insertion of insulating rings of gutta-percha one- 

 eighth of a line thick between the plates, outside each of the 

 ivory cylinders. The coupling screws serve also to fasten the 

 two plates to a base board, which is nailed or screwed against 

 the wall. 



The end of the line wire L is attached to the corner of 

 the upper plate A, by means of a binding screw. From the 

 opposite corner a finer wire, I, goes to one end of the wire 

 forming the coils of the receiving apparatus ; the other end of 

 the coils is connected by the wire E with the terminal b on 

 the plate B, which is put in communication with earth by 

 means of a thick wire, e. The two thin wires / and E are 

 covered with silk and are carried from the lightning discharger 

 to the board twisted round each other. Should the tension 

 electricity in the line, therefore, escape by any chance a 

 passage across the plates A B, it will certainly pass from the 

 wire I through the silk covering to the wire L, before it 

 reaches the apparatus. 



The galvanic currents from the sending station arriving 

 by L, cross over the plate A to the wire /, by which they reach 

 the apparatus ; from the apparatus they come to the plate B, 

 through the wire E, and, after crossing over B, go to earth 

 by e. The intervening stratum of air between the plates offers 

 an infinite resistance to the galvanic currents, which are there- 

 fore not weakened by the lightning guards in the circuit ; but 

 electricity of greater tension finds this air resistance infinitely 

 small in comparison with that of the wire of the coil, and, 

 therefore, on its arrival at A by the line L, it immediately 

 springs over to B, and goes to earth through the thick 

 wire e. 



107. Siemens and Halske's Plate Lightning Discharger.* 

 Fig. 114 gives a perspective view of a lightning- guard com- 

 monly supplied by Messrs. Siemens and Halske. Over a 

 cast-iron plate, a, called an earth-plate, are placed as near as 

 possible, but without making metallic contact, two smaller 

 plates, b , called conductors. Each plate has two screws for 



