2 4 3 PLEASANT WAYS IN SCIENCE. 



to produce the more striking phenomena of frictional elec- 

 tricity. Let us, however, consider how it would operate at 

 a distance. 



The current will pass along any length of conducting 

 substance properly insulated. Suppose, then, an insulated 

 wire passes from the positive pole of a battery at a station 

 A to a station B, and thence back to the negative pole at 

 the station A. Then the current passes along it, and this 

 can be indicated at B by some action such as electricity 01 

 low intensity can produce. If now the continuity of the 

 wire be interrupted close by the positive pole at A, the cur- 

 rent ceases and the action is no longer produced. The 

 observer at B knows then that the continuity of the wire has 

 been interrupted; he has been, in fact, signalled to that 

 effect. 



But, as I have said, the electrical phenomena which can 

 be produced by the current along a wire connecting the 

 positive and negative poles of a galvanic battery are not 

 striking. They do not afford effective signals when the 

 distance traversed is very great and the battery not excep- 

 tionally strong. Thus, at first, galvanic electricity was not 

 more successful in practice than frictional electricity. 



It was not until the effect of the galvanic current on the 

 magnetic needle had been discovered that electricity became 

 practically available in telegraphy. 



Oersted discovered in 1820 that a magnetic needle poised 

 horizontally is deflected when the galvanic current passes 

 above it (parallel to the needle's length) or below it If the 

 current passes above it, the north end of the needle turns 

 towards the east when the current travels from north to 

 south, but towards the west when the current travels from 

 south to north ; on the other hand, if the current passes 

 below the needle, the north end turns towards the west when 

 the current travels from south to north, and towards the east 

 when the current travels from north to south. The deflection 

 will be greater or less according to the power of the current It 

 would be very slight indeed in the case of a needle, 



