238 ELECTRO-TELEGRAPHY 



there is no reacting spring ; and in which tho movement or signal is produced by 

 the joint action of attraction and repulsion ; and tho return to its normal state by 

 the same joint action. Each pole of Henley's electro-magnet has a double inftoftd 

 of the single termination that wo have been considering in all preceding cases. A 

 piece of soft iron, like a crescent, is screwed upon each of the poles ; tho horns or 

 cusps of the respective crescents are facing and near to each other; and a magnetised 

 steel needlo is balanced between them. This arrangement is somewhat like the 

 following ( I ) So long as no current is circulating in the coils of tho electro- 

 magnet, the crescents are impassive soft iron, and no one point of either of them 

 has moro tendency than nvv other point to attract either end of tho magnetised needle 

 that is between them, liut while a current is circulating, one of the crescents is 

 endowed with north magnetic polarity, which is especially developed at its horns, 

 and the other with south polarity. Suppose tho horns of tho right-hand crescent 

 are north poles, those of the left south poles, and tho top end of tho needle is 

 north. Four forces will conspire to move the needle to the left. Its top will bo 

 attracted by tho left-hand crescent and repelled by the right; its bottom will be 

 repelled by the loft, and attracted by the right. When this current ceases to cir- 

 culate, tho simple attraction between the magnetised needle and tho soft iron of the 

 crescent tends to retain it in a deflected position. This tendency is increased by 

 a little residual magnetism, that is apt to remain in the best iron, notwithstanding 

 every care in its preparation. In order, therefore, to restore the needle to its normal 

 position, a short quick current in the reverse direction is given. These instruments 

 are single or double. Only one kind of deflection of the needlo is available for 

 actual signals ; the other motion being merely the return to the normal state. The 

 single-needle alphabet is composed of deflections of a short or a long duration ; these 

 are produced by holding on the current for an instant or for more than an instant ; 

 and the various combinations of short and long correspond to Morse's dot-and-dash 

 system. The double-needle alphabet consists of combinations of the deflection of 

 either or both needles. 

 Fig. 812 shows Henley's instrument ; and, in completing the description of it, we 



812 



have to describe another source of electric current to which no allusion has been 

 hitherto made. The electricity here employed is obtained neither by friction nor by 

 chemical action, but by means of magnetism and motion. If a piece of metal is moved 

 in the presence of a magnet, or a magnet is moved in presence of a piece of metal, a 

 current of electricity is generated in the metal. Tho results are multiplied when the 

 metal is a coil of covered wire; so that we have here tho converse of the deetro- 

 magnet ; in the one case electricity had produced magnetism, in the other magnetism 

 produces electricity; hence the name magneto-electric telegraph. Wo have here a 

 powerful set of steel magnets A A, all the north ends pointing in one direction, ami 

 bound together with a plate of iron, and all the south ends similarly arranged in the 

 other direction. Facing each end, but not quite in front when at rest, is an electro- 

 magnet proper, B u, consisting of the U-shaped iron rod and the coil of covered wire, 

 as described in/y. 807. Each electro-magnet is mounted upon an axis, c is a short 

 lever or key ; on depressing this the electro-magnet moves from its normal position 

 in a region of lesser magnetic force, into a new position in the region of greatest m.-itr- 

 netic force, and thus is the double condition, enunciated above, complied with; tho 

 copper-wire is moved in tho presence of a magnet, and this under the most favourable 

 conditions ; and the U-iron, rising from a feeble to a strong magnet, its lines of mag- 

 netic force move in presence of the copper wire. Just as a current, coming from a 

 long distance, had to be received in Morse's arrangement (fig. 808) in an electro- 

 magnet of a long coil of fine w ire, so as to bo much multiplied in order to do its work, 

 so here a magneto-electric current, that has to be sent to a long distance, must bo 



