236 



ELECTRO-TELEGRAPHY 



it consists essentially of a magnetic needle, which is deflected by an electric current 

 pissing through considerable lengths of wire carefully coiled around it. Several 

 attempts have been made to render these instruments in the highest degree sensitive 

 to electrical influences. Professor "Weber employed a galvanometer the magnetic 

 needle of which is a circular steel mirror reflecting the divisions of an illuminated 

 scale placed at some distance from it, into a telescope through which the observer 

 reads off the deflections of the mirror. Sir William Thomson has greatly improved 

 upon "Weber's instrument. He employs a mirror whoso weight does not exceed a few 

 grains, and he dispenses with the telescope by throwing a spot of light directly upon 

 the scale and lessoning its distance. For all measurements in whicli the instrument 

 serves as a galvanoscope, and when the readings are not very different in value, Sir W. 

 Thomson's instrument is superior to Weber's. 



Mr. Sabine, in his work on the Electric Telegraph, remarks : ' Mr. Becker has given 

 it a very convenient form, by fixing the coil in the back of a brass barrel or cylinder, 

 in the front of which a glass plate enables the interior to be seen from before, and 

 prevents dust and currents of air getting to the needle. 



' In the centre of the coil is suspended by a fine cocoon fibre, in a frame, a small 

 silvered mirror J of microscope glass, between and of an inch diameter. A little 

 magnet, made of a piece of thin watch-spring, is fastened to the back or silvered side 

 of the glass, and, being magnetised, operates as the needle of the system. Above tlio 

 brass barrel a vertical rod carries a curved permanent adjusting magnet, and a rack 

 and pinion enables the latter to be turned horizontally to bring the point of light to 

 any part of the scale which may be desired. 



'The adjusting magnet is elevated or depressed on the vertical rod for the purpose of 

 increasing or decreasing the directing force upon the magnet needle. When the ends 

 of the adjiisting magnet coincide with the poles of the earth's magnetism, it adds to 

 the directive force of the latter, and the instrument becomes proportionally unsensi- 

 tive. The magnet may, however, be turned round, so as to oppose the directive force 

 of the earth, and in this position be lowered towards the mirror, until it very nearly 

 neutralises the earth's directive force. The instrument so placed has its maximum 

 sensibility. 



' The scale, divided from the middle towards the ends into equal parts, is fixed upon 

 a wooden stand, at a distance of two or three feet from the mirror. Behind the scalo 

 is a paraffin lamp, whose light falls through an adjustable slit underneath on to the 

 mirror, which reflects it back upon the scale ; and, in order that the point of light 

 shall be as well defined as possible, a small plano-convex lens is placed before the 



mirror, through which the rays converge into a focus, 

 throwing a sharp image of the slit upon the scale. 



' Mr. Darley has made some of these instruments 

 for the measurements of the Atlantic cable, and has 

 substituted a plano-convex lens silvered on the 

 curved side, for the mirror in Professor Thomson's 

 instrument dispensing of course with the lens in 

 front.' 



We may here refer to the case of another bell or 

 alarum, in which the magnetic attraction derived 

 from the current that arrives, is not equal to the 

 mechanical work of striking a blow and sounding a 

 boll; but which is able to raise a detent, that h.-ul 

 restrained a train of wheels ; and so allow the 

 mechanism of tho latter to do the work required. 

 This arrangement is shown in Cooko and Wheat- 

 stone's alarum, Jig. 809 ; t is tho bell ; m m, is the 

 double-headed hammer, which is in fact the pni- 

 dulum, attached to the pallets /, which work in a 

 scape-wheel hidden in the figure, and in gear in tho 

 usual way with a coiled spring in the box b. l.y 

 the train r t , r t , r 3 , r v The electro-magnetic part 

 here, as in other instruments, is simple enough ; a c 

 is a lever moving on a centre above I, having at ono 

 end an armature a, facing tho poles of tho electro-magnet c ; and at the other end c, 



1 The process of depositing metallic silver upon plass is as follows : (A) Dissolve 10 part* of 

 nitrate of silver in 50 parts of water, and neutralise with (about) f. pnrts of liquor ammonia : :i <1<1 t<> 

 this a solution (B) of 1 part of tartaric acid In 4J parts of wat-r, and dilute the whole (A 4- P) with 

 500 parts of water. The things to be silvered should be placed conveniently in a vessel, the solution 

 poured in, and then put away in a quiet place for a few hours, at a temperature of from 40 t 

 When silvered they may be washed by a gentle stream of water, dried, and varnished with a solution 

 of amber in chloroform. 



809 



