HISTORY AND PROGRESS. 201 



1 millimetre thick and 2 -25 square decimeters of coated 

 surface, it was found that a length of 1 metre of the sus- 

 pended wire had the same capacity as a plate of glass 

 1 millimetre thick, with 100 square millimetres or O'OOOl 

 square metre coated surface; or that an English mile of 

 wire would have a jar-capacity equal to that of a glass plate 

 condenser of 1 millimetre thickness, with a coated surface of 

 the 0*16 part of a square metre ; or of such a plate, 1 metre 

 long and 0'16 metre broad. Although the wire in this 

 experiment was suspended much higher than is usual with 

 line wires, the result cannot be far short of the truth, as the 

 place where the experiment was made is in the immediate 

 neighbourhood of tall buildings and trees, which also played 

 their role in the phenomenon. 



The Earth- plate. A proper earth connection is as essential 

 to the working of a telegraph line as the line itself. The 

 earth connection is customarily obtained by a plate some 

 six or eight square feet of sheet copper, buried in the earth 

 at a depth that will insure it being always damp ; it is con- 

 nected with the apparatus by a stout insulated copper wire. 

 In a large station it is well to employ several different earths 

 parallel to each other. For instance, a wire soldered to the 

 gas-pipe gives an excellent earth; a wire soldered to the 

 water-pipe is still better. Both these give earth-plates of 

 large surface, being connected with the gas and water mains 

 of the town. 



The want of a good earth connection can cause serious 

 interruptions in the service. 



In temporary stations, as for instance those in military 

 service, difficulty has sometimes been found in burying the 

 plates, and in getting other means of earth. Some of the 

 Prussian and other military stations have therefore been 

 supplied with earth posts which are more portable, and are 

 said in the end to be cheaper than buried plates of metal. 



The earth-post consists of a wr ought-iron tube, 12 feet 

 long and 1 inch outside diameter. The lower portion of 

 the iron is covered with a copper tube, soldered to it to pre- 

 vent an insulating coat of oxide being formed. To the 



