ELECTRO-TELEGRAPHY 



233 



various mechanical arrangements, which aro the keys, commutators, or handles of the 

 various telegraph instruments, and are often the only parts presenting any complexity 

 about them. In fig. 804, the source of electricity, E, we have already described ; the 

 test-instrument for the abnormal state of the wire, that is to say, the telegraph proper, 

 is the part A. The complex part, consisting of springs, cylinders, and studs, shown 

 below A, is nothing more than the necessary mechanical arrangement for directing at 

 pleasure the current from the battery E, in either direction through the wire, and 

 through the part A. By following the letters in the order here given, the course of 

 the current may be traced from its leaving, say the positive or copper end of the 

 battery, till its return to the zinc or negative end ; c </ D ww u A z' b B z. If a com- 

 panion instrument were in any part of the circuit of the wire w w, it would correspond 

 in its signals with the home instrument, fig. 804. 



In 1820 Oersted of Copenhagen made the capital discovery that a magnetic needle 

 suspended in the neighbourhood of a wire, through which a current of electricity was 

 passing, was deflected from its position of rest, the angle of deflection varying with 

 the amount of electricity passing along the wire. Ampere found the law by which 

 this influence was governed, numerous investigators continued the inquiry with more 

 or less success, and eventually Michael Faraday, by a series of the most perfect induc- 

 tive experiments which adorn the annals of British science, made the two discoveries 

 of Volta-electric induction and of magneto-electricity. These completed the series 

 which were necessary for the full development of electro-telegraphy, and it only re- 

 mains for us now to deal with the practical application of those great laws to which 

 we have alluded. 



One of the properties possessed by a wire, during the discharge of a voltaic battery 

 is to deflect a magnetised needle. If the needle and the wire are parallel in the nor- 

 mal state of the wire, the needle is deflected this way or that, when the wire is in the 

 abnormal state ; and if the needle is very delicate, and a large enough amount of elec- 

 tricity is circulating through the wire, the needle will reach the maximum deflection 

 of 00. This is an extreme case, and cannot be approached in practice. Indeed, the 

 deflection of any ordinary needle, under the action of an ordinary telegraph wire, 

 would not be appreciable. But, as every foot of the wire has the same amount of reac- 

 tion, we have merely so to arrange things that many feet shall be made to react upon 

 the needle at the same time, and thus the effect is multiplied in proportion to the 

 length of wire so concentrated. This is managed by covering a considerable quantity 

 of fine wire with silk or cotton, and winding it on a frame A (fig. 804), suspending the 

 needle within the frame. Such an instrument is called, from its properties, a multi- 

 plier. It is seen at a glance that the 

 wire of the multiplier is an addition 

 over and above the length of the 

 actual telegraph wire required for 

 reaching the distant station, and thus 

 it practically increases the distance- to 

 be traversed. The multipliers com- 

 monly used add a resistance equal to 

 six or seven miles of telegraph wire. 



Let us now turn to the face of the 

 instrument. Here we have a dial and 

 an index, which is on the same axis as 

 the magnetised needle above described, 

 capable of being deflected to the right 

 or left, and limited in its motion by 

 ivory pins. "We have a handle for 

 working the mechanical part so con- 

 nected that, as it moves to the right, 

 it directs a current into the wire such 

 that the needle moves to the right, 

 and vice versa. An alphabet is con- 

 structed from the combination of these 

 two elementary motions, one or more 

 of either or both kinds of deflection 

 being used for the various letters, as 

 shown on the engraved dial. This 

 is Cooke- and Wheatstone's single- 

 needle instrument, fig. 805. 



The form and character of their 

 double-needle instrument is shown in 

 fig. 806. It is precisely a duplicate of the former : two handles, and their respective 



