ELECTRO-TELEGRAPHY 239 



generated in a long coil of very fine wire in order to have electro-motive force sufficient 

 to overcome tho resistance opposed to it. In like manner the electro-magnets of the 

 instrument D, in which it is received at the far-off station, have the same multiplying 

 characteristics. The magneto-electric current exists only during the motion of the 

 electro-magnet in front of the steel magnets, and this motion must be rather brisk, or 

 the change of place is slow and the current feeble ; but the current ceases with the 

 motion. Tho needle, however, remains deflected from causes to which we have already 

 referred, and if the hand is raised gently, so that the coils return slowly to their 

 normal position, the needle will remain deflected ; but, if the hand is so removed that 

 the coils return quickly from the region of greatest to one of lesser magnetic force, a 

 reverse current of lesser force than the original is generated, which releases the needle 

 from its deflected position and restores it to its normal place, ready for making the 

 next signal. In a recent form of this instrument Mr. Henley has obviated the neces- 

 sity of moving the electro-magnets, still retaining the same fundamental principles. 

 He uses a set of large U-shaped permanent magnets, and places the electro-magnet in 

 the space between the branches of the permanent magnet, and so that the four poles 

 of the two magnets, the permanent and the electro, shall be flush with each other or 

 in the same plane. A couple of iron armatures are mounted on a disc in front of the 

 magnets. The disc has a motion on a centre ; the armatures are curved or crescent- 

 shaped. Their form is so adjusted to the relative positions of the poles of the res- 

 pective magnets that, in their normal or ordinary position, one crescent connects the 

 N. pole of the magnet with one, say tho upper pole of the electro-magnet, and the 

 other crescent connects the S. pole of the permanent magnet with the lower pole of the 

 electro-magnet. On pressing a key the disc moves, and the armatures so change in 

 position that the N. pole of the magnet is connected with the lower, and the S. pole 

 with the upper poles of the electro-magnet. By this arrangement the polarity of the 

 electro-magnet is reversed at pleasure ; and in its transition from being a magnet with 

 poles in one direction, to becoming a magnet with poles in the reverse direction, an 

 electric current is generated in tho wire with which it is wound, and the direction of 

 the current is this way or that according as the transition is from this direction of 

 polarity to that. This form of magneto-electric machine allows of larger electro- 

 magnetic coils being used, and gives the manipulator comparatively very little weight 

 to move in signalling. 



We have shown how an electric current generates magnetism, and how magnetism 

 generates another electric current ; it would follow logically that one electric current 

 should therefore generate another electric current ; for tho magnetism produced by a 

 current circulating in one wire, must have all the properties of magnetism, and among 

 them, that of producing another current in another wire ; and so it is. A few convolu- 

 tions of a large-sized wire are coiled round an iron rod ; and outride the larger wire 

 is a very great length of finer wire. The current from the battery is called the primary 

 current in this arrangement ; and the moment it begins to circulate in the large wire, 

 it magnetises the iron and generates a current, called secondary, in the fine wire, 

 which is able to penetrate to a very great distance. When the primary current 

 ceases, magnetisation ceases, the lines of magnetic force disappear, and a reverse 

 secondary current is produced. This was the method proposed for obtaining the 

 secondary current for traversing the Atlantic Ocean from Ireland to Newfoundland. 

 The large wire is not necessarily first coiled on ; in the coils for the Transatlantic 

 telegraph it was coiled outside. Nor is the presence of iron essential to obtaining 

 secondary currents. 



It will have been noticed in all the arrangements which have hitherto been described, 

 that tho signals are produced by motions that the electric current on reaching the 

 far station is multiplied by being directed through many convolutions of wire, and is 

 made to act upon either a piece of soft iron 

 or a piece of magnetised steel, and to move 

 them, the motion being turned to account 

 directly, or by the intervention of mechanism. 

 We have yet another property of electricity, 

 that has been very successfully applied to 

 the production of telegraphic signals by 

 Mr. Bain, in his electro-chemical telegraph. 

 If a current of electricity is led into a com- 

 pound fluid body, say into water, by one wire 

 and out of it by another wire, the body is 

 decomposed into its constituent elements, 

 one of which, the oxygen in the case in 

 question, makes its appearance at one wire, and the other, the hydrogen, makes its 

 appearance at the other wire. The same holds good with bodies of a more complex 



